Download REPORT of the INDIAN OCEAN SYNTHESIS GROUP on the

JGOFS REPORT No. 35
REPORT of the INDIAN OCEAN SYNTHESIS GROUP
on the ARABIAN SEA PROCESS STUDY
Published in Bergen, Norway, January 2002 by:
Scientific Committee on Oceanic Research
Department of Earth and Planetary Sciences
The Johns Hopkins University
Baltimore, MD 21218
USA
and
JGOFS International Project Office
Centre for Studies of Environment and Resources
University of Bergen
5020 Bergen
NORWAY
The Joint Global Ocean Flux Study of the Scientific Committee on Oceanic Research (SCOR) is
a Core Project of the International Geosphere-Biosphere Programme (IGBP). It is planned by a
SCOR/IGBP Scientific Steering Committee. In addition to funds from the JGOFS sponsors,
SCOR and IGBP, support is provided for international JGOFS planning and synthesis activities
by several agencies and organizations. These are gratefully acknowledged and include the US
National Science Foundation, the International Council of Scientific Unions (by funds from the
United Nations Education, Scientific and Cultural Organization), the Intergovernmental
Oceanographic Commission, the Research Council of Norway and the University of Bergen,
Norway.
Citation:
Report of the Indian Ocean Synthesis Group on the Arabian Sea Process Study.
January 2002
ISSN:
1016-7331
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University of Bergen
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JOINT GLOBAL OCEAN FLUX STUDY
– JGOFS –
REPORT No. 35
REPORT of the INDIAN OCEAN SYNTHESIS GROUP
on the ARABIAN SEA PROCESS STUDY
Editorial Group
Louisa Watts
Peer Review Committee Secretary - Terrestrial Sciences
Terrestrial and Freshwater Sciences Team
Natural Environment Research Council
Polaris House
North Star Avenue
Swindon, SN2 1EU
UNITED KINGDOM
Peter Burkill
Plymouth Marine Laboratory
Natural Environment Research Council
Prospect Place
West Hoe
Plymouth, PL1 3DH
UNITED KINGDOM
Sharon Smith
Division of Marine Biology and Fisheries
Rosenstiel School of Marine and Atmospheric Science
University of Miami
4600 Rickenbacker Causeway
Miami, FL 33149-1098
United States of America
i
ABSTRACT
The JGOFS Process Studies of the Arabian Sea have resulted in more than 120 publications that
address different aspects of the biogeochemistry and physical forcing of this fascinating region.
These publications arose from an integrated international study involving ca. 80 research cruises
carried out by seven countries in the Arabian Sea.
Our understanding of the region has improved considerably from such disciplinary publications
and suggest, for instance
• Seasonal monsoonal forcing results in levels of primary production that are as high during
the weaker NE Monsoon as they are during the stronger SW Monsoon;
• Bacterial production would seem to be less strongly coupled to monsoonal forcing than
primary production with an amplification of ca. 2, considerably lower than primary
production;
• While microzooplankton grazing continues at high levels throughout all seasons, they have
little impact on diatoms that bloom during the SW Monsoon;
• The fate of these diatoms depends on the trophic coupling to herbivorous crustaceans, such
as Calanoides carinatus;
• Export flux to the deep ocean has been shown to be strongly seasonal and spatially variable.
These and other findings are presented in specialist disciplinary chapters in this report. A
bibliography of research papers and other seminal publications including eight Special Issues of
Deep-Sea Research has been compiled at the end of this report.
The next step is to focus on syntheses of the primary literature and data by scientific teams
working across national boundaries and aiming for interdisciplinary publications in the peerreviewed literature. A time-line for this, led by the JGOFS Indian Ocean Synthesis Group
(IOSG), is laid out in this report.
ii
CONTENTS
1. INTRODUCTION
1
2. SPECIFIC TOPICS
2.1
The Surface Circulation of the Northern Arabian Sea
3
2.2
Hydrography
9
2.3
Air-Sea Interactions and Exchange
13
2.4
Primary Productivity in the Arabian Sea
26
2.5
Remote Sensing of Ocean Colour in the Arabian Sea
33
2.6
Microbial Dynamics
39
2.7
Zooplankton
54
2.8
The Role of the Oxygen Minimum Zone in Biogeochemical Cycles
60
2.9
Sedimentation
68
2.10
Arabian Sea Benthic Dynamics
77
2.11
Modelling Physical-Biogeochemical Interactions
82
3. MAJOR ACHIEVEMENTS, RECOMMENDATIONS AND FUTURE TIME-LINE
3.1
Major Achievements
88
3.2
Recommendations
89
3.3
Future Time-line and Research
90
4. APPENDICES
APPENDIX 4.1: JGOFS IOSG Membership
91
APPENDIX 4.2: Reference List from Section 2
92
APPENDIX 4.3 Seminal Literature: 1990-Present
115
iii
1. INTRODUCTION
During the last decade, an intense study of the biogeochemistry of the Arabian Sea has taken
place. This has involved scientists from Canada, Germany, India, Netherlands, Oman, Pakistan,
UK and USA, who participated in Process Studies run as part of the JGOFS Programme. Studies
in the Arabian Sea were co-ordinated by the JGOFS Indian Ocean Planning Group (IOPG), who
produced a Science Plan (SCOR 1995) that identified the nature of a co-ordinated,
multidisciplinary study. The SCOR (1995) report also served as an Implementation Plan that
identified the measurements to be taken by each nation on cruises, the PI’s responsible, and their
contact details.
When the Process Studies ended in 1997, more than 80 research cruises had been carried out in
the Indian Ocean, with most of these in the Arabian Sea over a 6-year period. During this time,
the cruises mounted by Germany, India, Pakistan, UK and USA focused on the northern Arabian
Sea during 1992/97 while Netherlands JGOFS cruises tackled the Somali system during 1992/93
(Figure 1).
Figure 1: JGOFS cruises mounted by
Germany, India, Pakistan, UK and USA
focused upon coverage of the northern Arabian
Sea during 1992/97, while Netherlands JGOFS
cruises tackled the Somali system during
1992/93.
Details of each cruise are given on
http://ads.smr.uib.no/jgofs/inventory/index.htm
National participation in the Arabian Sea Process Study and other regions of the Indian Ocean
are given in Table 1. A detailed list of cruises is given on the Internet at:
http://ads.smr.uib.no/jgofs/inventory/index.htm.
On completion of field studies in 1997, publication of the primary scientific papers began. This
has been achieved largely through publication of Special Issues of Deep-Sea Research, Part II. A
list of primary scientific papers, the Special Issues and other publications are given in Chapter 3.
The next step in developing our understanding of Arabian Sea biogeochemistry involves
synthesis of the primary literature to produce coherent integrated understanding of the key
pathways, their interactions and controls. This was discussed by the JGOFS Indian Ocean
Synthesis Group (IOSG) at a meeting held in Bangalore in 19991; IOSG concluded that it should
produce a report that “summarised advances in understanding of biogeochemical cycling in the
Arabian Sea derived from JGOFS Process Cruises and other associated activities”. Funds
1
A list of IOSG Committee Members is given in APPENDIX 4.1
1
released by JGOFS in 2000/01 allowed three scientists to meet and produce this report in June
2001. This is based on statements of recent advances in scientific understanding viewed by
recognised experts in some dozen disciplinary fields (Chapter 2). However, we recognise that
this report is an important, but not defining, step of Arabian Sea biogeochemical synthesis.
Further international integration of syntheses will be required with peer-reviewed publications in
the future. The time-line considered appropriate by IOSG is outlined in Chapter 3.
Table 1: A summary of cruises carried out in the Arabian Sea and other regions of the Indian
Ocean.
Country
Australia
France
Germany
India
Japan
Netherlands
Pakistan
South Africa
UK
USA
Region
SW Indian Ocean
W Indian Ocean
Arabian Sea
Arabian Sea
Indian Ocean
Arabian Sea
Arabian Sea
SW Indian Ocean
Arabian Sea
Arabian Sea
Total
Cruises
4
1
12
7
4
11
5
5
4
28
81
Years
1992/95
1992
1995/97
1992/97
1994/96
1992/93
1992/94
1990/93
1994
1994/96
2
2. SPECIFIC TOPICS
2.1 The Surface Circulation of the Northern Arabian Sea
John C. Kindle
Oceanography Division, Naval Research Laboratory
Stennis Space Center, MS 39529-5004
USA
Introduction
Prior to the Arabian Sea Expedition of 1994-1995, our understanding of the northern Arabian
Sea’s response to the seasonally reversing monsoon cycle was based on climatological ship drift
reports (Cutler and Swallow, 1984). Although occasional expeditions to the region provided new
insights (e.g., Elliot and Savidge, 1990; Bauer et al., 1991) an integrated understanding of the
mesoscale response to the monsoon cycle relative to the mean seasonal circulation was lacking.
That the best estimates of the surface forcing resulted from monthly composites of ship wind
observations complicated the efforts to understand this highly variable region. Hence, it is not
surprising that prominent among the recent discoveries are 1) the pronounced spatial and
temporal structure of the monsoon atmospheric forcing and 2) the importance of mesoscale
variability in the associated ocean response. Among the important mesoscale features are coastal
jets and filaments along the Oman coast during the Southwest (SW) Monsoon that are capable of
exporting nutrient rich upwelled water hundreds of kilometres offshore. This paper reviews
observational and modelling studies of the seasonal response of the northern Arabian Sea
circulation to the monsoon cycle with a focus on results from the Joint Global Ocean Flux Study
(JGOFS) Arabian Sea Expedition of 1994-95. Emphasis is placed on the circulation features
north of approximately 10°N. An excellent review of the basin-wide circulation features,
including results from the World Ocean Circulation Experiment (WOCE) efforts of 1994-96 can
be found the recent article by Schott and McCreary (2001).
Atmospheric Forcing
The basin-scale circulation of the Arabian Sea is governed by the response to a seasonally
reversing monsoon cycle, characterised by upwelling-favourable, south-westerly winds during
the SW Monsoon, reversing to north-easterly (albeit weaker) winds during the Northeast (NE)
Monsoon. Meteorological measurements from a moored array centred at approximately 15.5°N,
61.5°E and deployed from October 1994 to October 1995, provide the first high frequency (i.e.,
sub-monthly) direct observations of the monsoon cycle in the interior of the Arabian Sea
(Rudnick et al., 1997; Weller et al., 1998). Based on these measurements, the four seasons for
the 1994-95 monsoon year were defined as: September 16 to October 31 for the Fall
Intermonsoon; November 1 to February 15 for the NE Monsoon; February 16 to May 31 for the
Spring Intermonsoon and June 1 to September 15 for the SW Monsoon. The excellent agreement
between these observations and results from operational atmospheric prediction models (Weller
et al., 1998) (Figure 1) suggest that the atmospheric models may be used to describe the
essential characteristics of the spatial and temporal variability of the of the atmospheric forcing
throughout the basin. Likewise, Halpern et al., (1998) discovered excellent agreement between
the observed winds from the mooring and the satellite-derived winds from the ERS-1
scatterometer.
3
Figure 1. Time-series of observed wind magnitude (dark line) versus atmospheric model winds
(light line) at 15.5°N, 61.5°E for the period October 1994 to October 1995. The
atmospheric model winds are from the U.S. Navy’s Operational Global Atmospheric
Prediction System (NOGAPS).
Both the atmospheric model winds (Weller et al., 1998) and the satellite-derived winds (Halpern
et al., 1998) reveal significant regional differences in the timing of the onset and relaxation of
the winds within a season. For example, Weller et al., (1998) used the operational wind product
from the U.S. Navy to reveal a 7-10 day delayed onset of the 1995 SW Monsoon at the mooring
site, whereas the onset near the Oman coast occurred normally. However, it should be noted that
upwelling-favourable winds along the coast of Oman generally begin in April-May (Weller et
al., 1998), even prior to the reversal of the winds in the interior of the Arabian Sea. This is
consistent with the analysis of ship reports by Fieux and Stommel (1977) indicating that the
onset of upwelling-favourable winds along the Oman coast may begin in April. While the winds
along the Oman coast in April-May are not as strong as during the SW Monsoon period, they
appear to be sufficient to generate a coastal upwelling response as detected in AVHRR imagery
(Rixen et al., 1996; Arnone et al., 2001). The potential roles of this weak-to-moderate coastal
upwelling during April-May in setting the stage for the physical and biogeochemical responses
to the primary SW Monsoon onset in June warrant further study.
Ocean Response to the Monsoon Cycle
The Southwest Monsoon
Historical ship drift records show the existence of the Oman Coastal Current (OCC) north of
approximately 14°N by early May (Cutler and Swallow, 1984). In their analysis of the ship drift
records, Elliot and Savidge (1990) showed a north-eastward flowing OCC of around 0.4 m sec-1
in magnitude and extending to 200 km offshore during the SW Monsoon. The current turned
abruptly to the east off Ras al Hadd (Figure 2). However, direct measurements of the coastal
flow from ADCP instruments during this season in 1987 showed that the north-eastward coastal
flow weakened toward the southwest from an estimated transport of ~10 Sv. near Ras al Hadd to
weak and variable flow at ~ 17°N-18°N (Elliot and Savidge, 1990). Additionally, the ADCP data
analysed by Flagg and Kim (1998) for the 1995 SW Monsoon displayed no mean coastal current
4
to the northeast, but rather the presence of variable flow characterised by current reversals over
relatively small distances. Flagg and Kim (1998) hypothesised that the differences between the
direct observations of the OCC and the historical ship drift data may be accounted for by a
systematic bias in the ship observations due to the persistent nature of the high winds and seastate during this period. Clearly, questions remain about the nature of the Oman Coastal Current:
During the SW Monsoon, is the OCC comprised of several distinct flows of limited alongshore
extent or does it exist in the mean but disrupted by the pronounced mesoscale variability?
Moreover, while we know the fate of the coastal flow after it leaves the coast at Ras al Hadd, we
know little of its origins. The southeastern-most extent of the OCC has not been documented.
Figure 2. AVHRR image from 06 August 1994 showing major filaments extending offshore
from Ras ash Sharbatat, Ras al Madrakah and Ras al Hadd (Image courtesy of R. Arnone).
Among the most prominent SW Monsoon features along the Oman coast are filaments and jets
(Figure 2) which are capable of exporting cool, nutrient rich, upwelled coastal waters hundreds
of kilometres offshore (Young and Kindle, 1994; Brink et al., 1998). The first direct
measurements of these features were described by Elliot and Savidge (1990) who discovered
plumes of cold water extending offshore from the coast in the region between Ras ash Sharbatat
and Ras al Madrakah. This is in approximately the same region as a major filament that was
observed during the 1995 SW Monsoon (Brink et al., 1998; Flagg and Kim, 1998; Arnone et al.,
2000; Lee et al., 2000; and Manghnani et al., 1998). Some properties of the filaments have been
described as similar to those off the U.S. Pacific west coast (Elliot and Savidge, 1990; Brink et
al., 1998). However, Flagg and Kim (1998) view the offshore-directed plumes as part of a major
anticyclonic feature that remains essentially in place for approximately six months, thereby
extending through the SW Monsoon into the winter season. Manghnani et al. (1998)
hypothesised that the plume was not part of the offshore deflection of the coastal current, but
rather resulted from an interaction between the wind field and anticyclonic mesoscale features
that may have existed prior to the onset of the SW Monsoon.
When the Oman Coastal Current reaches Ras al Hadd, the flow turns offshore to the northeast
and east to form the Ras al Hadd Jet (Figure 2). This feature is also referred to as the Ras al
Hadd Front because it forms the seasonal boundary between the northern Arabian Sea and the
Gulf of Oman. The measurements of Elliot and Savidge (1990) of the OCC just prior to reaching
5
Ras al Hadd, revealed a transport of ~ 10 Sv. within 100 km off the coast. Flagg and Kim (1998)
discovered that the Ras al Hadd Jet intensified in August 1995 following the reversal of the flow
along the northeastern Oman coast from northward to southward, thereby adding to the flow
along the Ras al Hadd Front. They speculated that the reversal of the flow along the northeastern
Oman coast in August was related to the intensification and/or propagation of a cyclonic eddy in
the Gulf of Oman during this period. Similarly, Baker et al., (1996) suggested that such an eddy
plays a role in the dynamics of the Ras al Hadd Jet: They hypothesize that the OCC at Ras al
Hadd forms a double vortex as it extends offshore during the Fall Intermonsoon period. It is
speculated that to the south, an anticyclonic vortex (or eddy) forms while to the north, in the
Gulf of Oman, a cyclonic eddy forms, both of which are driven by the extension of the Ras al
Hadd Jet into open waters. Böhm et al. (1999) provided an extensive description of the Ras al
Hadd Jet and the associated cyclonic and anticyclonic eddies based on ADCP measurements and
remotely sensed observations in 1994 and 1995. They estimated that the transport of the jet
varied from 2-8 Sv. in magnitude and exhibited a maximum flow in September. Böhm et al.
(1999) suggested that, while the counter-rotating eddies may interact with the jet, they may not
necessarily be generated by this feature. The relationship(s) between the Ras al Hadd Jet and the
respective anticyclonic and cyclonic eddies to the south and north deserves further attention.
Among the key results of the ADCP analysis by Flagg and Kim (1998) is that the eddy kinetic
energy (EKE) in the northern Arabian Sea dominates that of the mean and is enhanced in the
near coastal region. In an extension of the ADCP-based study, Kim et al. (2001) utilised
satellite-derived sea level anomalies from the TOPEX/Poseidon altimeter to provide additional
analyses of the EKE properties in the northern Arabian Sea. The results were in agreement with
the ADCP study in affirming the importance of eddy activity in this region. The seasonal
variation of EKE was such that it increased in magnitude and areal extent during the SW
Monsoon. Horizontal scales of the eddies varied from ~200-500 km nearshore to ~100-200 km
in the interior. Interestingly, they also discovered that the primary area of reduced eddy presence
coincided with the core of the Oxygen Minimum Zone (OMZ). Furthermore, the study revealed
that eddy activity within the Arabian Sea basin was confined primarily to the region north of
15°N. The generating mechanisms for the eddy field remain to be determined.
The behaviour of the surface mixed-layer during this season was addressed in both observational
and modelling studies. Lee et al., (2000) used high-resolution observations from a towed profiler
during 1994 and 1995, as well as climatological records of surface forcing and mixed-layer
depths, to examine the seasonal behaviour of the surface mixed-layer in the northern Arabian
Sea. Their analysis suggests that during the SW Monsoon, the shallow mixed-layer inshore of
the wind maximum is maintained by a balance between wind-driven entrainment and the
combined effects of horizontal advection and Ekman pumping. Offshore of the wind maximum,
Lee et al., (2000) show that Ekman pumping and wind-driven entrainment act together to deepen
the mixed-layer with minimal influence from horizontal advection. Fischer (2000) utilised data
from the Woods Hole Oceanographic Institute (WHOI) mooring at ~15.5°N, 61.5°E, a location
that was chosen to approximately coincide with the climatological axis of the wind stress
maximum during the SW Monsoon, and a three-dimensional model to examine the mixed-layer
response during this season. His results indicated that, at this location, local forcing dominated
the mixed-layer response except during the latter stages of the SW Monsoon, when horizontal
advection played an important role. Similarly, Rochford et al., (2000), in a modelling study
using a three-dimensional numerical model forced by 12-hourly atmospheric model winds,
suggested that local forcing dominated the mixed-layer response at the mooring site during the
first half of the SW Monsoon, but horizontal advection was also an important component during
the second half of the summer (SW) Monsoon period. Finally, the modelling studies of Fischer
(2000) and McCreary et al. (2000) found that rectification to the diurnally forced signal needs to
6
be included to produce the most realistic mixed-layer response. This is particularly true for
coupled bio-physical models attempting to simulate the seasonal surface chlorophyll distribution
in the interior of the northern Arabian Sea (McCreary et al., 2000).
The Fall Intermonsoon
Prominent features along the Oman coast generated during the SW Monsoon tend to linger in
place during the Fall Intermonsoon, as noted above for the cyclonic and anticyclonic eddies
associated with the Ras al Had Jet (Arnone et al., 2001). Flagg and Kim (1998) noted the
continued presence of the Ras al Hadd Jet, which appeared to maintain its intensity during the
Fall Intermonsoon period due to the intensification of the southward flow along the northeast
coast of Oman. Shi et al. (2000) reported that the cold surface water upwelled during the SW
Monsoon lingered for nearly a month following the end of the monsoon in bays along the Oman
coast.
The Northeast Monsoon
The transition from the Fall Intermonsoon to the NE (winter) Monsoon occurs in November. In
the northern Arabian Sea, the primary circulation response to the onset of the northeasterly
winds is the reversal of the Oman Coastal Current to southeastward flow, thereby yielding a
continuous southward current that extends along the northeast coast of Oman, turns the corner at
Ras al Hadd and continues southward along the coast until it is entrained into offshore directed
squirts and jets south of ~ 20°N (Flagg and Kim (1998). Anticyclonic features that were once
directly connected to the coastal circulation during the SW Monsoon and Fall Intermonsoon
periods evolve into separated eddies that exhibit a tendency to propagate southward along the
coast and may occasionally directly impact the coastal circulation. Along the northeast coast of
Oman, the southward flow decays and reverses to weak northward flow that persists from
January to July (Flagg and Kim, 1998).
The mixed-layer behaviour during the NE Monsoon is characterized by increased deepening
with distance offshore and is dominated by convective overturning (Lee et al., 2000). This is
consistent with the earlier work of Weller et al., (1998) who utilised observations from the
WHOI mooring to demonstrate the importance of convective overturning to the mixed-layer
response in the central Arabian Sea during this season. Wiggert et al. (2000) used observations
from the mooring and a one-dimensional coupled bio-physical model to show the importance of
diurnal forcing to the surface chlorophyll a distributions. They also hypothesised that the
interannual variation of this mechanism may be an important factor in the observed interannual
variability of chlorophyll a distributions during this monsoon season. Such interannual
variability was also examined by Kumar et al. (2001) who discovered deeper mixed-layer
depths, colder SST values and elevated chlorophyll concentrations during the NE Monsoon of
1997 relative to that of 1995.
Recommendations
1. To perform retrospective synthesis studies that combine in situ and remotely sensed data
together with data-assimilative model simulations capable of representing observed
features during the Arabian Sea Expedition of 1994-95. Such simulations would be able
to significantly aid the investigation of the relative importance of physical forcing
mechanisms governing the biological response during the monsoon cycle. For example,
such studies are needed to help interpret the statistical relationships between the sediment
flux measurements and surface forcing.
7
2. To conduct additional field work just prior to and following the onset of the SW
Monsoon to investigate such features as 1) the role of the upwelling along the Oman
coast during April-May prior to the onset of the SW Monsoon, 2) the generating
mechanisms for the pronounced filaments observed during the SW Monsoon with a focus
on examining the role(s) of pre-existing coastal features in the development of coastal
jets and filaments, 3) the intriguing hypothesis of the role(s) diapausing copepods play in
the timing of diatom blooms relative to the onset of the SW Monsoon.
3. To conduct additional fieldwork during the NE Monsoon to better understand and to test
emerging hypotheses to explain the pronounced interannual variation of phytoplankton
blooms during this season.
8
2.2 Hydrography
John M. Morrison
Department of Marine, Earth and Atmospheric Sciences
North Carolina State University, Raleigh, NC 27695
USA
Introduction
The semi-annual reversal in wind stress associated with the monsoon, water mass intrusions
from marginal seas and other oceans, geographical effects (the northern Indian Ocean being
blocked by the Eurasian landmass) and therefore no subtropical convergence or deep water
formation, give the upper waters of the Arabian Sea a unique thermohaline structure and
circulation (Wyrtki, 1971; Morrison and Olson, 1992; You and Tomczak, 1993; Morrison et al.,
1998). The complex water mass structure (Figure 1) is due, in part, to advection and interleaving
of water masses, and, in part, to formation of high-salinity waters in the Red Sea, Persian Gulf
and northern portion of the basin (Arabian Sea Water) which sink to moderate depths in the
central basin. Wyrtki (1971) provides an overall view of water mass structure for the Indian
Ocean. The yearly cycle in the Arabian Sea is normally divided into a Northeast (NE) Monsoon,
characterised by moderate winds from the northeast during the late fall and winter, a Southwest
(SW) Monsoon, characterised by strong winds from the southwest during the late spring and
summer, and Fall and Spring Intermonsoons. As Düing (1970) notes, the transition periods can
vary from year to year and can occur at different times from region to region. Off Somalia, for
example, SW Monsoon conditions begin to “spin-up” in May, but in the central Arabian Sea,
May is a period of relatively weak winds and lowest Coastal Zone Color Scanner (CZCS)
chlorophyll levels (Smith et al., 1991). Winds during the peak of the SW Monsoon are amongst
the strongest tropospheric winds on the planet (Smith et al., 1991) and are associated with
extremely high inputs of dust (Prospero, 1996), coastal upwelling, and, in the canonical
descriptions (e.g., Smith and Bottero, 1977), offshore upwelling.
The canonical description of the wind-driven upwelling regime during the SW Monsoon
involves coastal upwelling driven by the wind stress and an offshore upwelling regime driven by
the curl of the wind stress (e.g., Smith and Bottero, 1977) that occurs inshore of the high velocity
core (the Findlater Jet) of the SW Monsoon’s wind field. Observations made on sub-mesoscale
sampling grids, in combination with satellite sea surface temperature (SST) data, suggest that
high surface nutrients in the offshore regions of the Arabian Sea during the SW Monsoon are
associated with complex dynamics in the upwelling zone that include cool filaments, similar to
those formed in other upwelling regions (Abbott and Barksdale, 1995). These observations also
suggest that it is difficult for the wind regime to produce organised upwelling circulations
offshore (Brink et al., 1998). The hypothesis is that any coherent offshore upwelling northwest
of the Findlater Jet quickly becomes unstable and collapses to an eddy field, as shown in the
acoustic Doppler current profiler data by Flagg and Kim (1998). Offshore upwelling, however,
has been suggested for other portions of the Arabian Sea (Muraleedharan and Kumar, 1996).
Recently, the northern Arabian Sea was extensively studied, as part of the Joint Global Ocean
Flux Study (JGOFS) during 1994-1996. The observations consisted of repeated ship surveys;
special frontal surveys in the upwelling regions; and satellite sea surface temperature (SST) and
altimetry data. For the SW (summer) Monsoon circulation off the Arabian peninsula, it was
determined that a northeastward coastal jet develops along the south coast of Oman in early May
and persists throughout the SW Monsoon, called the Ras al Hadd Jet by Böhm et al. (1999). It
flows past Ras al Hadd at the southeast corner of the Gulf of Oman and meanders into the
interior, transporting cooler, coastally upwelled waters into the warmer environment of the Gulf
9
of Oman and northern Arabian Sea, and therefore is easily identified in satellite SST
observations. The jet is associated with an anticyclonic gyre just south of its offshore turning
point and a cyclonic one to its north (Flagg and Kim, 1998; Böhm et al., 1999). Shi et al. (1999)
determined that the cause of this alongshore, northeastward flow is an onshore pressure gradient
(established by the upwelling) that reduces sea level near the coast by about 30 cm. During the
Northeast Monsoon, warm water flows southward past Ras al Hadd into the open Arabian Sea,
so that the coastal jet is fed from the north, rather than the west as during the SW Monsoon.
Along the rim of the northern Arabian Sea, the mean flow during the NE Monsoon is westward
with downwelling along the coast and an offshore pressure gradient (Shi et al., 2000).
Other circulation cells exist elsewhere along the Oman coast, their locations apparently linked to
protruding capes of the Arabian Peninsula. Between these cells, upwelled waters can be
transported offshore in filaments or squirts, which extend as far as 400 km offshore and can be
detected in satellite SST and colour imagery. In these filaments with widths of up to 100 km
offshore, currents in excess of 50 cm s-1 have been observed (Brink et al., 1998).
Water Mass Formation in the Upper Ocean
Because of this intense variability in the circulation associated with the annual monsoon cycle,
water mass structure in the upper kilometre of the Arabian Sea is extremely complex (Figure 1).
Low salinity waters of southern origin are advected into the region by the Somali and North
Equatorial Currents. In addition, during the SW Monsoon, the waters that upwell along the
Arabian Coast have relatively low salinities as a consequence of poleward advection associated
with the Somali Current. Precipitation and runoff from rivers in the eastern Arabian Sea must
contribute to the local formation of low salinity waters, although salinities in the upper several
hundred metres are always greater than 35.0 and generally greater than 35.5.
During the NE Monsoon, the cold and dry winter winds from Asia, combined with Ekman
pumping, cause subduction of high-salinity surface waters in the interior of the northern Arabian
Sea (Morrison, 1997; Schott and Fischer, 2000). This generates the widespread Arabian Sea
Water (ASW) salinity maximum (Figure 1) just underneath the mixed-layer with a potential
density (σθ) of ~25 (Morrison, 1997). It should be noted that Donguy and Meyers (1996) define
a surface water mass with salinity between 35.5-36.5 and temperatures greater than 22°C as
Arabian Sea Water. The most dense ASW as defined by Donguy and Meyers (1996) would have
a σθ of ~25.35 kg m-3 which would mean that it could overlap with the ASW. However, we
prefer the definition that associates this water mass with a salinity maximum at a σθ of ~25.0 kg
m-3. The formation of ASW is a complex process involving seasonal heating, cooling, and
evaporation, and advection of low salinity waters. Morrison (1997) describes the formation of
ASW and its associated salinity. Briefly, waters with ASW salinity and temperature
characteristics can be formed at the sea surface in the northern Arabian Sea by cooling and
evaporation during the NE Monsoon. This relatively dense water can then be capped by lower
density surface waters advected into the region or arising from heating during the Intermonsoons
and the SW Monsoon. A major cause of the ASW salinity maximum (when it is present) is the
advection of low salinity water from the Oman upwelling region in the Ras al Hadd Jet during
the SW Monsoon. This upwelled water can sink beneath the warm surface waters and lie above
ASW, thereby contributing to the salinity maximum that is associated with ASW.
During the Indian JGOFS programme, observations during February-March 1995 suggest the
presence of a relatively cool, high salinity water mass (Arabian Sea High Salinity Water ASHSW) that has been formed due to the action of the NE Monsoon winds and reduced
insulation (Kumar and Prasad, l996). The mode of formation of this water mass is similar to that
for ASW, and the main distinction is that its density tends to be lower than the 25.0 kg m-3σθ
value that is often used to characterise the ASW salinity maximum. Kumar and Li (1996)
10
suggest a characteristic temperature, salinity, and σθ for this water mass of 26.345°C, 36.387,
and 24.082 kg m-3, respectively. Nevertheless, the denser fractions of ASHSW have σθ’s > 25.0
kg m-3. It might be appropriate, therefore, to think of ASW as a relatively dense fraction of
ASHSW.
Intermediate Water Masses
At intermediate depths, You and Tomczak (1993) suggest that Indian Central Water (ICW) and
an “aged” version of this water mass that they call North Indian Central Water (NICW) are the
main source for thermocline ventilation for the northern Indian Ocean with only minor
contributions from the Red Sea and the Persian Gulf outflows. The conversion of ICW to NICW
is a slow process, accompanied by a significant fall in oxygen values due to consumption of
organic matter and a corresponding increase of nutrient concentrations (You, 1997). They
suggest a transition path of ICW: subduction in the South Indian Ocean at ~ 40-45 °S, advection
with the subtropical gyre, and exit to the northern Indian Ocean as part of the western boundary
currents. The northward spreading of ICW into the equatorial and northern Indian Ocean is by
way of the Somali Current centred at 300-400 m with σθ of ~26.7 during the SW Monsoon and
by way of the Equatorial Countercurrent during the NE Monsoon (You, 1997). This is consistent
with the indications of Warren et al. (1996) that northward-moving Subtropical Subsurface
Water which corresponds to the ICW of You and Tomczak (1993) appears to penetrate well
north of the equator in the Somali Basin. Kumar and Li (1996) call ICW Central Indian Water
and suggest a core σθ of ~27.5 kg m-3. They agree that it is of major importance in ventilating
the thermocline of the northern Indian Ocean, but they consider it less important for supplying
waters to the thermocline of the western basin (Arabian Sea). They suggest that evaporation in
the Arabian Sea, Red Sea, and Persian Gulf contributes to the formation of North Indian
11
Intermediate Water (NIIW). This is a high salinity water mass with a characteristic σθ of ~27.1
kg m-3 that is not very different from Wyrtki’s RSW (see below).
The northern marginal seas, the Red Sea and Persian Gulf (sometimes referred to as the Arabian
Gulf), inject water masses into the Indian Ocean that are identifiable over long distances by their
high salinities at core densities. The dynamics of both marginal seas are quite different, with the
Red Sea being deep and separated from the Gulf of Aden by a shallow sill, similar to the
situation of the Mediterranean Sea, while the Persian Gulf shallows northward from its outflow
channel. Salinity maxima (Figure 1) arising from Persian Gulf (PGW) and Red Sea (RSW)
waters are centred at 26.6 kg m-3 and 27.2 kg m-3 σθ, respectively (Wyrtki, 1971). Persian Gulf
Water has two spreading paths into the Arabian Sea. First, a narrow subsurface jet of PGW flows
to the south as a western boundary undercurrent under the upwelling zone where it spreads
eastward into the central basin (Morrison, 1997). Second, PGW spreads to the east in the
northern extremes of the Arabian Sea and down the coast of India during NE Monsoon and
advects to the west into the central basin during SW Monsoon (Morrison, 1997). PGW is present
throughout the entire northern Arabian Sea, identified by it core salinity maximum, but it rapidly
looses its identity further to the south.
A seasonal cycle of the outflow from the Red Sea through the Gulf of Aden and into the Indian
Ocean with a winter maximum has been documented (Murray and Johns, 1997). Its preferred
spreading route into the Indian Ocean is through the passage between Socotra Island and the
African continent (Schott and Fischer, 2000) with a second branch of this undercurrent going
around Socotra (Schott and Fischer, 2000). This water mass is observed as far south as the
Mozambique Channel (Beal et al., 2000a). Water mass properties show that the lower part of
this undercurrent is the main supplier of RSW out of the Gulf of Aden into the Indian Ocean.
Meanwhile, in the central basin, RSW is a rapidly mixed away with the spread of RSW out of
the Gulf of Aden being very inhomogeneous in the Arabian Sea due to the intense eddy activity
(Quadfasel and Schott, 1982). RSW is almost mixed away by 64° E in the central basin and has
little effect on the water masses in the central Arabian Sea (Morrison, 1997). You and Tomczak
(1993) found that the influence of RSW in the upper thermocline is mainly restricted to the
Arabian Sea, where it contributes up to 30% to the thermocline water. In contrast, Olson et al.
(1973), using chlorofluorocarbon (CFC) data, found an even smaller contribution of RSW
(<10%). Meanwhile, Antarctic Intermediate Water (AAIW) is spreading northward into the
Arabian Sea on essentially the same density horizon as RSW (core density of 27.1- 27.3 kg m-3
σθ; Figure 1). AAIW has entered the Indian Ocean basin from the Antarctic in the southeastern
region (Fine, 1993). It is generated by subduction in the subpolar frontal zone, and is marked by
low salinities due to the excess of precipitation over evaporation in that area and oxygen
maximum.
Deep Ocean Water Masses
As noted above, the Indian Ocean has no northern sources of deep water. Therefore, deep water
can only be supplied from the south. The deep water has the cold, fresh, oxygen-rich
characteristics of Lower Circumpolar Water (CDW) which flows northward in the Madagascar
and West Australian basins (Toole and Warren, 1993), but it cannot directly enter the central
Indian Basin because of obstruction by the deep ridges. The western pathway of this water mass
into the Northern Hemisphere is through the Somali Basin (Johnson et al., 1991b. Beal et al.,
2000b) where it must disappear through gradual upwelling into the overlying Deep Water to
form Indian Deep Water (IDW) which is specific to the northern Indian Ocean (Figure 1). This
water eventually enters the Arabian Basin. The depth range of IDW in the Arabian Basin is from
3,800 m upward to about 1,500 m (Johnson et al., 1991a). Like the flow of Bottom Water, the
flow of Indian Deep Water is northward and concentrated in western boundary currents (Warren
and Johnson, 1992).
12
2.3 Air-Sea Interactions and Exchanges
S.W.A. Naqvi
National Institute of Oceanography
Dona Paula, Goa 403 004
India
Introduction
At the time of planning JGOFS Process Studies, it was recognised that the Arabian Sea makes a
significant contribution to biogeochemical ocean-atmosphere transfer processes. However, the
magnitudes of these exchanges were not well quantified even for constituents such as carbon
dioxide (CO2), nitrous oxide (N2O) and methane (CH4), for which some data were available. For
others like ammonia (NH3) and dimethylsulphide (DMS), which had not been measured
previously, new measurements were planned. The work carried out during the JGOFS and
related cruises on these climatically important atmospheric constituents, has greatly expanded
the database, allowing for improved estimates of their air-sea exchange rates.
Carbon Dioxide (CO2)
At the time of JGOFS planning, the Indian Ocean north of the Hydrochemical Front at ~10°S
was believed to serve as a source of CO2 to the atmosphere. In this respect there were several
crucial questions concerning carbon cycling in the region that the JGOFS programme aimed to
address. It was thought that since the upwelled water at the surface had very high pCO2,
upwelling in conjunction with a wind-induced enhancement of the air-sea gas exchange
coefficient would result in a large efflux of CO2 to the atmosphere from the northwestern Indian
Ocean. This effect would be aided by the secretion of CaCO3 by organisms (which raises pCO2),
but opposed by the high rate of photosynthetic production (which lowers pCO2), sustained by
copious nutrients in the mixed-layer. The relative importance of these processes was not known.
Also, as the physical forcings associated with the NE and SW Monsoons are highly variable in
space and time, large changes in the air-sea exchange of CO2 were expected.
Extensive surveys made under the JGOFS, WOCE (World Ocean Circulation Experiment) and,
to a smaller extent, OACES (Ocean-Atmosphere Carbon Exchange Study) programmes have
greatly increased the existing carbon database for the Indian Ocean. This has enabled a much
more reliable quantification of the air-sea CO2 exchange and an evaluation of its spatial and
temporal variations in the Indian Ocean, particularly in the Arabian Sea. These data were
generated through both continuous underway pCO2 measurements and discrete water sampling.
Results of these studies have conclusively established that the Arabian Sea is indeed a source of
CO2 to the atmosphere at almost all places and during all seasons, but with very large seasonal
and geographical variability. The surface-water pCO2 levels are the highest in coastal upwelling
zones: values reaching up to 700 µatm being observed off Oman (Figure 1) during the
Southwest (SW) Monsoon season (Körtzinger et al., 1997; Goyet et al., 1998; Mintrop et al.,
1999). In the northern Arabian Sea, convective mixing during the NE Monsoon can elevate the
pCO2 level to 500 µatm even in offshore areas (Sarma et al., submitted). pCO2 approaching 700
µatm was also recorded in the SW Monsoon upwelling zone off western India; however, as the
upwelled water in this region is often overlain by a low-salinity layer formed as a result of
intense rainfall in the coastal zone, the surface water pCO2 (minimum 266 µatm) may sometimes
be below saturation (Sarma et al., 2000). Ignoring these rare instances of pCO2 undersaturation
caused by a combination of physical and biological processes (a similar phenomenon has also
13
been reported from the Bay of Bengal; see Kumar et al., 1996), it would appear that the
increases in pCO2, through upwelling and vertical mixing of subsurface waters greatly
overwhelm its biological removal in the northwestern Indian Ocean. Interestingly, the
atmospheric pCO2, which also exhibits considerable seasonal variability, is the lowest (~340
µatm) during or just after the SW Monsoon (Goyet et al., 1998) accentuating the sea-air CO2
gradient (∆pCO2 = pCO2sea - pCO2air), while the exchange coefficient is the highest due to strong
monsoon winds. The combination of these effects leads to some of the most intense emissions of
CO2 (up to 119 mmol m-2 d-1, Körtzinger et al., 1997) from the sea surface.
Figure 1. Partial pressure of CO2 in surface water (bold curves) and in the atmosphere (dots on
dashed curves) and water temperature (thin curve) along two transects of the upwelling zone off
Oman during July-August, 1995 (from Körtzinger et al., 1997).
In the Arabian Sea, seasonal contrasts in pCO2 and in the air-sea flux are best marked in coastal
waters, but large seasonality in the open ocean is also observed in the Southern Indian Ocean
(Figure 2; Louanchi et al., 1996; Sabine et al., 2000). The amplitude of seasonal changes in the
open Arabian Sea is somewhat uncertain. The U.S. JGOFS observations in the central and
western Arabian Sea over a complete annual cycle, involving continuous, automated
measurements, revealed substantial seasonal changes in the offshore region, with the lowest
∆pCO2 (<32 µatm) occurring during the non-Southwest Monsoon periods (Goyet et al., 1998).
14
Figure 2. Season-wise distribution of surface water pCO2 (µatm) in the Indian Ocean (from
Sabine et al., 2000).
In comparison, the Indian JGOFS data, mostly based on pH-TCO2 or pH-alkalinity
measurements (Sarma et al., 1998), indicated smaller variability and higher ∆pCO2 (up to 124
µatm). Accordingly, estimates of annual CO2 flux to the atmosphere from the Arabian Sea also
differ significantly (7 TgC by Goyet et al., 1998, and 79 TgC by Sarma et al., 1998). The latter
estimate is close to that [30-76 TgC (3 months)-1] based on observations during the SW
Monsoon season as a part of the German JGOFS programme (Körtzinger et al., 1997). While
some of these differences may arise from different techniques used for measurements, data
interpolation and computations, they probably reflect the large temporal (both seasonal and
interannual) and spatial variations that characterize air-sea CO2 exchange in this region.
The WOCE-JGOFS data combined with earlier measurements made during the GEOSECS and
INDIGO expeditions have been used to evaluate the extent of anthropogenic CO2 penetration in
the Indian Ocean (Sabine et al., 1999). The highest concentrations and the deepest penetrations
(>1 km) were found to occur in the region of the subtropical convergence (30-40°S) evidently
through the formation of intermediate waters, while these were the least in the Antarctic region.
The northwestern Indian Ocean, which receives outflows from the Persian Gulf and the Red Sea,
appears to hold more anthropogenic CO2 than the equatorial Indian Ocean (Sabine et al., 199;
Figure 3A). The total inventory of anthropogenic CO2 in the Indian Ocean north of 35°S has
been estimated to be 13.6 PgC (1 Pg = 1015 g) while the increase over the 18-year interval
between the GEOSECS and WOCE-JGOFS expeditions works out to be 4.1 Pg C. An interesting
aspect of these results is that the observed trends of anthropogenic CO2 distribution (Sabine et
al., 199; Figure 3A) show some important differences from those predicted by the Princeton
15
ocean biogeochemistry model (Sabine et al., 1999; Figure 3B) with the databased penetration
estimate being higher in the northern Indian Ocean and lower in the Antarctic sector.
Figure 3. Distribution of vertically integrated anthropogenic CO2 (mol m-2) based on (A) data
and (B) model estimates (from Sabine et al., 1999).
Evaluation of anthropogenic CO2 penetration in the northern Indian Ocean has also been made
following another approach that takes into account contributions from various source water
masses. This work is documented in Goyet et al. (1999). The WOCE data used for this study
were collected along three transects, the first in the Gulf of Aden parallel to the Yemeni coast,
the second across the Arabian Sea along 9°N latitude, and the third across the Bay of Bengal
(including the Andaman Sea) along 10°N latitude. Surface waters in these regions were
computed to contain over 40 µmol kg-1 of anthropogenic CO2. The depth of penetration of the
anthropogenic signal was noticed to decrease from ~1500 m in the Gulf of Aden (which receives
freshly-formed intermediate waters from the Red Sea) to <1000 m in the Bay of Bengal.
Underway pCO2 analysis carried out during the same expedition (Figure 4) revealed fairly large
supersaturations in the Gulf of Aden and in the western and central parts of the Arabian Sea.
This was presumably due to the aforementioned upwelling and warming of CO2-rich deeper
waters. Lower pCO2 values in the easternmost portion of the Arabian Sea transect and along
most of the Bay of Bengal transect were associated with low-salinity surface water masses. It
may be noted that the ∆pCO2 reached a minimum of ~-30 µatm in the Andaman Sea that
receives large runoff from the Irrawaddy and Salween rivers. This further reinforces the view
that freshwater discharge in the northeastern Indian Ocean makes this region a net sink for
atmospheric CO2 unlike the northwestern Indian Ocean (Kumar et al., 1996).
Regional variability of pCO2 in surface waters and of the causative processes (thermodynamics,
vertical mixing, biological activity and air-sea exchange) has also been demonstrated through
modelling by Louanchi et al. (1996). The one-dimensional model employed by these authors in
different provinces of the Indian Ocean north of 50°S latitude, yielded a spatio-temporal
distribution that closely matched with the observations. However, some discrepancies between
the model results and observations were noticed. For example, the model-predicted distribution
for the Arabian Sea showed two peaks separated by a minimum during the SW Monsoon (JulyAugust) that is not consistent with the data as described above.
16
Figure 4. Distributions of pCO2 in surface water (circles) and in the atmosphere (triangles)
during August-October, 1995, along three WOCE transects: (a) from Gulf of Aden to the
Arabian Sea; (b) along Lat. 9°N in the Arabian Sea; and (c) along Lat. 10°N in the Bay of
Bengal and the Andaman Sea (from Goyet et al., 1999).
This indicates a possible overestimation of the loss (through the biological pump and air-sea
exchange) or an underestimation of supply (entrapment from deeper water). Also, the seasonal
contrast in surface pCO2 was generally greater in the eastern equatorial Indian Ocean than in the
northwest Indian Ocean. The model results enabled the identification of four different provinces
based on their characteristic surface pCO2 fields. While the region south of 35°S latitude appears
to be a net sink (~500 TgC y-1) for atmospheric CO2 during all months (with a maximum during
the Austral winter), that north of 10°S latitude is a net source (~150 TgC y-1 with maximal
emission during the SW Monsoon). Between these two extremes lie the southern subtropical
region and the transition zone bounded by the latitudes 20°S and 10°S; the former serves as a
moderate sink (~170 TgC y-1) whereas the latter is neither a sink nor a source on an annual basis.
Sabine et al. (2000) have recently utilized 240,000 surface pCO2 measurements made during
1995-96 under JGOFS, WOCE and OACES to establish its relationships with other parameters
such as the sea surface temperature (SST), salinity and location. These relationships are then
used with the NCEP/NCAR reanalysis, SSTs, winds and atmospheric pressures and the salinity
values derived from Levitus et al., (1994) to compute basin-wide pCO2 and air-sea fluxes for
17
each month. Their results show a larger seasonal cycle for the tropical and subtropical Indian
Ocean than simulated by Louanchi et al. (1996). Also, unlike the Pacific and the Atlantic
Oceans, the equatorial Indian Ocean appears to be a relatively small source of CO2 to the
atmosphere. The net annual flux of CO2 for the region lying north of 36°S latitude was 150 TgC,
and it was directed from the atmosphere to the ocean. This is at variance with the results of
Louanchi et al. (1996) who concluded that the Indian Ocean north of 35°S latitude serves as a
weak source of atmospheric CO2 (22 TgC y-1). The estimate of Sabine et al. (2000) is in
agreement with that of Takahashi et al. (1999), although the regional breakdowns of the
estimates are different because of different techniques used for data processing. As expected, the
air-sea fluxes exhibit large geographical and seasonal changes. The Indian Ocean as a whole
(north of 36°N) oscillates between a weak net source (22 Tg) during January-March and a net
sink (87 Tg) during July-September. Interestingly the period of maximal net absorption is the
same as that of maximal SW Monsoon-forced emissions from the Arabian Sea reflecting the
dominance of removal by CO2-undersaturated waters, largely due to biological processes, in
determining the net CO2 uptake/emission balance in the region. The study of Sabine et al. (2000)
did not cover the frontal zone, which is an even stronger sink of atmospheric CO2. For example,
Metzl et al. (1997) estimated that the sub-Antarctic waters (40-50°S) in the Indian sector absorb
420 TgC annually; this compares favourably with the model-derived estimate of Louanchi et al.
(1996) for the region south of 35°S. Thus, it may be concluded that the Indian Ocean, including
the sub-Antarctic Zone, on balance absorbs a minimum of 0.5 Pg CO2-C from the atmosphere
annually, a quarter of the total oceanic CO2 uptake (Houghton et al., 1996). It may be noted that
the average annual increase (4.1/18 = 0.23 Pg C y-1) in anthropogenic CO2 inventory north of
35°S estimated by Sabine et al. (1999) is of the same order.
Molecular Nitrogen and Nitrous Oxide
The Arabian Sea is one of three major oceanic water-column denitrification sites. Current
estimates of denitrification rates in this region vary from 12 TgN y-1 (Mantoura et al., 1993) to
33 TgN y-1 (Naqvi and Shailaja, 1993). The high-quality nutrient data collected during the
JGOFS cruises were expected to narrow this gap. Howell et al. (1997) combined the nitrate and
chlorofluorocarbon (CFC-11) data collected during the WOCE programme to arrive at a
denitrification rate of 21 TgN y-1. This is slightly lower than most other estimates (the most
recent one being 33 TgN y-1 by Bange et al., 2000). Thus, it would appear that the Arabian Sea
accounts for at least one-third of the global oceanic water column denitrification (Naqvi, 1987).
This high rate of molecular nitrogen (N2) production is countered by substantial N-fixation in the
region as evident from recent measurements of natural abundance of nitrogen isotopes in
dissolved nitrate (NO3-) (Brandes et al., 1998). Brandes et al. (1998) observed that near-surface
NO3- in the Arabian Sea suboxic zone has a nitrogen isotopic composition close to the oceanic
average (δ15N ≈ 6 per mil) in spite of the huge denitrification-related enrichment of 15N (δ15N
>11 per mil) in subsurface waters (>200 m). This has been attributed to the fixation of light
nitrogen at an estimated basin-wide rate of 6 Tg y-1. This implies a net loss of combined nitrogen
and production of N2 at a rate of ~15-27 TgN y-1.
Apart from denitrification and nitrogen fixation, the other important term in the nitrogen budget
to which the Arabian Sea makes a significant contribution, is the efflux of N2O from the sea
surface. Of the total inputs of N2O to the atmosphere from all natural sources (~12 TgN-N2O y1
), at least one-third seems to come from the oceans (Nevison et al., 1995). However, emissions
of N2O are not uniformly distributed over the oceanic surface with the eastern-boundary
upwelling zones underlain by O2-deficient waters making disproportionately large contributions
(Codispoti and Christensen, 1985; Suntharalingam et al., 2000). Studies carried out under
JGOFS have led to a better quantification of this efflux. These studies (e.g., Figure 5) revealed
18
very high surface saturations (>300%) in the upwelling zones off Oman, Somalia and
southwestern India (Bange et al., 1996; de Wilde and Helder, 1997; Patra et al., 1999).
Figure 5. N2O of equilibrated seawater along the same transect as in Figure 1 (lower panel)
(from Bange et al., 1996).
The sea-to-air fluxes of N2O from these regions are also similarly high (maximum of ~500 µmol
m-2 d-1). In the open ocean, surface N2O saturations and its fluxes to the atmosphere are
generally lower (Figure 5) than those reported previously (Law and Owens, 1990; Naqvi and
Noronha, 1991). These also show smaller temporal variability except for the zones of offshore
upwelling and winter mixing (Bange et al., 1996; de Wilde and Helder, 1997; Lal and Patra,
1998). However, even in coastal upwelling zones, seasonal changes in the mixed layer N2O
saturations may sometimes be small, as observed by Upstill-Goddard et al. (1999) in the western
Arabian Sea, indicating a balance between the processes of gas exchange, net production and
supply/removal by advection and vertical mixing. Nevertheless, large changes in air-sea
exchange can still occur due to variations in the wind speed (Upstill-Goddard et al., 1999).
However, the highest globally recorded N2O concentrations in surface water (up to 436 nM or
8,250% saturation) were measured within the seasonally-suboxic zone off western India during a
three-year (1997-99) study. This study was conducted as part of the Indian LOICZ (Land-Ocean
Interactions in the Coastal Zone) programme (Naqvi et al., 2000). These extreme concentrations
are higher than those observed (62.5 nM or 953% saturation) during a more limited survey made
during the SW Monsoon of 1995 (Naqvi et al., 1998). Interestingly, the highest concentrations
were found in waters that were apparently affected by denitrification, as suggested by high
nitrite concentrations (see Figure 6), a process that generally results in a net consumption of
N2O (Codispoti and Christensen, 1985).
This is consistent with the results of incubation experiments (Naqvi et al., 2000), which
indicated that in shallow, denitrifying systems with high rates of denitrification and experiencing
frequent aeration due to turbulence, the N2O-reductase activity may be suppressed to such a
degree that a large fraction of the nitrogen undergoing bacterial reduction ends up as N2O (Naqvi
et al., 2000).
19
16
NO2- (µM)
12
8
4
0
0
200
400
600
N2O (nM)
Figure 6. Plot of nitrite (NO2-) versus nitrous oxide (N2O) in the shallow (open circles) and deep
(closed circles) suboxic zones of the Arabian Sea (from Naqvi et al., 2000).
Estimates of the annually integrated N2O-flux from the Arabian Sea, based on the JGOFS data,
range between 0.16 and 1.5 Tg N2O y-1. These estimates are strongly dependent on the choice of
model used to compute the air-sea exchange coefficient and the interpolation scheme. Recently,
Bange et al. (submitted) have pooled all the data generated between 1977 and 1997 to compute
high resolution (1°x1°) seasonal and annual N2O concentration fields in the surface layer. The
annual emission rate, estimated to range between 0.33 and 0.70 Tg N2O, seems to be dominated
by fluxes from coastal regions during the SW Monsoon. However, this data set does not include
the anomalously high N2O concentrations in the inner shelf off the Indian west coast. In this
region high oxygen demand coupled with strong stratification causes severe oxygen depletion
below the shallow pycnocline. But while the causative processes (upwelling of nutrient-rich
waters and the outflow of freshwater from land resulting from intense rainfall in the coastal
zone) are primarily natural, it is quite likely that the coastal hypoxic system has further
intensified in recent years. Such intensification, evident from the previously unreported
occurrence of hydrogen sulphide, might arise from an increase in productivity fuelled by a
higher flux of nutrients from land. This could have contributed to the enormous N2O
accumulation observed in the region. But if the N2O concentrations in the region were high even
before this intensification, then the above estimate of “natural” N2O emissions from this region
would be too low. In any case, if we take into account the seasonal flux of N2O from the shelf
(0.06-0.39 Tg N2O; Naqvi et al., 2000), the current sea-to-air flux of N2O from this region could
be as much as 1 Tg N2O y-1, about one quarter of the total global oceanic emissions (Houghton
et al., 1996).
Ammonia and Methylamines
Under the UK JGOFS programme, measurements of ammonia/ammonium (NH3/NH4+) and
methylamines (MAs) [monomethylamine (MMA), dimethylamine (DMA) and trimethylamine
(TMA)] were made in seawater, atmosphere and rainwater in the central and western Arabian
Sea during two seasons (August-October and November-December, 1994) (Gibb et al., 1999).
20
Concentrations of all species were generally elevated in the coastal upwelling zone off Oman
relative to those measured in the central Arabian Sea, with NH3/NH4+ levels being up to two
orders of magnitude higher than those of the MAs. MMA was the most abundant MA, being
present in concentrations reaching up to 66 nM, while TMA levels did not exceed 4 nM (see also
Burkill, this volume). Both the coastal and offshore waters of the Arabian Sea appeared to emit
NH3 to the atmosphere with large temporal variations. However, the air-sea exchange probably
represented a small proportion of the aquatic turnover of NH3 and MAs, which seem to be
controlled mainly by the structure, activity and seasonality of the microbiological cycle. Most of
the removal of NH3 and MAs appeared to occur through wet deposition (Figure 7), contributing
little (<1%) to the new production. However, by virtue of being the dominant atmospheric bases,
NH3 and MAs may play an important role in tropospheric chemistry and climatic modification.
Figure 7. Schematic diagram of the cycling of NH3+NH4+ in the remote Arabian Sea in which
pools (boxes, nmol m-3) and processes (arrows, nmol m-3 d-1) are represented in broad
approximation to their relative magnitudes (from Gibb et al., 1999).
Nitrate and Lead-210
As a part of the Indian JGOFS programme, seasonal distributions of NO3- and lead-210 (210Pb)
have been studied in the aerosols over the central-eastern Arabian Sea for investigating the
sources of NO3- and quantifying its deposition flux (Sarin et al., 1999). Higher NO3concentrations (0.4 to 4.1 µg m-3) were observed during the Spring Intermonsoon and NE
Monsoon as compared to the SW Monsoon (0.2-0.8 µg m-3). Using a NO3-/210Pb ratio of 43 µg
dpm-1 and the measured 210Pb deposition flux of 20 dpm m-2 d-1, the atmospheric deposition of
NO3- was estimated to be 1 mg m-2 d-1. These results suggest that that the atmospheric NO3deposition plays an insignificant role in fuelling new production in the region.
Methane
Prior to the JGOFS programme, there had been only one report on CH4 distribution in the
Arabian Sea (Owens et al., 1991). High surface-water concentrations and consequently largerthan-the-oceanic-average CH4 fluxes observed in this study prompted additional investigations
on CH4 by several groups, the results of which have been published recently (Patra et al., 1998;
21
Bange et al., 1998; Upstill-Goddard et al.¸ 1999; Jayakumar et al., 2001). In agreement with the
findings of Owens et al. (1991), Patra et al. (1998) reported high surface saturations, averaging
140, 173 and 200% during the Northeast Monsoon, Spring Intermonsoon and SW Monsoon
periods in the central and eastern Arabian Sea. Upstill-Goddard et al. (1999) observed moderate
saturations in the Omani upwelling zone both during the late SW Monsoon (170±55%) and
Spring Intermonsoon (179±15%) seasons; the corresponding open ocean values were 130±5%
and 115±2%. In contrast to these data, Bange et al. (1998) showed near-saturation (103-107%)
in the central Arabian Sea, rising to a maximum of 156% in the upwelled water near the coast of
Oman (Figure 8). The annual emission rate of CH4 from the Arabian Sea was estimated at
0.032-0.053 Tg CH4 y-1 by Patra et al. (1998), 0.011-0.020 Tg CH4 y-1 by Bange et al. (1998)
and 0.1-0.18 Tg CH4 y-1 by Upstill-Goddard et al. (1999). Whether the order-of-magnitude
differences in the published estimates of CH4 emissions from the Arabian Sea represent natural
variations or they arise, at least in part, from systematic measurement errors is still not clear.
However, even the highest of these estimates is probably still <2% of the estimated emission rate
from the oceans as a whole (Upstill-Goddard et al., 1999), which itself is at the most 4% of the
atmospheric flux of CH4 from all natural and anthropogenic sources (Crutzen, 1991). Evidently,
unlike N2O, CH4 emissions from the Arabian Sea are not globally significant.
It should be noted that most of the CH4 data collected in the Arabian Sea have come from
offshore areas. As pointed out by Bange et al. (1994), the contribution of the coastal zone to the
oceanic CH4 budget may be disproportionately large. This is also supported by the results of a
recent study (Jayakumar et al., 2001), which yielded CH4 saturations comparable to those
observed by other workers in the open ocean, but very large supersaturations (up to ~2500%) in
the coastal waters affected by monsoonal runoff from land. Still higher supersaturations (up to
~13,000%) were recorded in the estuarine surface water pointing to a source of CH4 in coastal
wetlands. Elevated (CH4) concentrations (~50 nM) have also been found in the seasonal oxygendepleted zone, when the sub-pycnocline waters turned fully anoxic (D.A. Jayakumar,
unpublished data). However, the impact of these processes on the CH4 budget of the Arabian Sea
still remains to be evaluated fully (see also Burkill, this volume).
Dimethylsulphide
As part of the UK and German JGOFS cruises and a related programme in India, measurements
of dimethylsulphide (DMS) have been made for the first time in the Arabian Sea. Although the
complete data sets generated during these studies have not yet been made available, large spatial
and temporal changes in the concentrations of DMS, and its precursor and oxidation product,
dimethylsulphoniopropionate (DMSP) and dimethylsulphoxide (DMSO), respectively, appear to
occur in this region. For example, in the productive surface waters of the Omani upwelling zone,
sampled by Hatton et al. (1999), marked increases in the concentrations of DMSP (on average
from 21.5 to 37.5 nM), DMSO (from 2.9 to 7.7 nM), and DMS (from 1.3 to 3.3 nM) were
observed over an interval of three weeks during the transition from SW Monsoon to Fall
Intermonsoon, presumably caused by a shift in the dominant phytoplankton groups (from
diatoms to prymnesiophytes). The concentrations were relatively low (21.0, 4.3 and 2.1 nM,
respectively) in the oligotrophic surface waters of the central Arabian Sea. DMSO occurred in
fairly high and rather uniform concentrations throughout the water column, and although its
production through photo-oxidation in the surface layer probably represented an important sink
of DMS, significant amounts of the latter should be emitted to the atmosphere from this region.
This flux remains to be quantified, and for this purpose representative data from different
biogeochemical provinces (particularly those which support the growth of the DMS-producing
organisms such as coccolithophores and Phaeocystis) are needed.
22
Figure 8. CH4 saturations (solid curves) and sea surface temperature (dashed curves) in the
western Arabian Sea during June-July 1997. The transect ABC was located offshore outside the
upwelling zone while the drift stations experienced upwelling (from Bange et al., 1998).
Recommendations
Due to the delicate biogeochemical balance that exists in the Arabian Sea, minor
climatic/environmental perturbations can have potentially large impacts on chemical fluxes,
especially of greenhouse gases in the region. It is for this reason that the Arabian Sea is regarded
as a sensitive ocean scale barometer of global climate change (Mantoura et al., 1993). Recent
results suggest that the human activities may already be modifying such fluxes, even though it
cannot be established whether the effects observed are due to local processes (eutrophication)
alone or caused by slight modifications of physical forcing functions (circulation and freshwater
runoff) (Naqvi et al., 2000). Nonetheless, since these fluxes are globally significant, we must
keep a close watch on both the coastal and open ocean oxygen-deficient zones, fully understand
the mechanisms of biogeochemical transformations using modern tools (for example, stable
isotopes and molecular probes), identify the emerging trends and develop capability to predict
future changes. In spite of the recent focus on this region under several international
programmes, there still exist some serious deficiencies in the geographical and temporal
coverage that need to be remedied. For example, the Bay of Bengal still remains to be
adequately sampled (especially along the Burmese and Bangladeshi coasts). Also, the
contribution of benthic cycling to greenhouse gas fluxes is still largely unknown. As another
major international programme is unlikely to be launched in this region in near future, serious
efforts should be made towards capacity building such that the less-developed coastal states can
initiate studies in their coastal zones.
23
2.4 Primary Productivity in the Arabian Sea
John Marra
Lamont-Doherty Earth Observatory
Columbia University, Palisades, NY 10964
USA
Introduction
Between 1994 and early 1996, the JGOFS programme conducted a comprehensive study of the
Arabian Sea not seen since the International Indian Ocean Expedition of the early 1960’s (see
Krey and Babenerd, 1976). Combining all the primary production observations, the Arabian Sea
is now one of the better-sampled regions of the ocean in terms of organic productivity. The data
collected during the various JGOFS programmes consist of photosynthesis versus irradiance
response curves (Sathyendranath et al., 1999; Johnson et al., in review), standard primary
productivity measurements (assimilation of 14C) (Mahdupratap et al., 1996; Barber et al., 2001;
Savidge and Gilpin, 1999), community production measurements (light-dark bottle O2)
(Robinson and Williams, 1999; Dickson et al., 2001), and gross production estimates (Dickson
et al., 2001). The data sets also include measurements that further support the productivity
studies. These include phytoplankton absorption cross-sections (Marra et al., 2000), the variety
of phytoplankton pigments (Latasa and Bidigare, 1998), the variety of phytoplankton taxa
(Garrison et al., 1998, 2000), and on certain of the cruises, spectral irradiance as a function of
depth (Pinkerton et al., 1999; Marra et al., 2000). All the observations are spatially extensive in
the eastern and central parts of the Arabian Sea, and span August 1994 through December 1995
at nearly monthly time intervals.
The JGOFS programmes have uncovered new findings about the Arabian Sea, as well as
reinforced some of the previous suppositions about the effects of the monsoons on the
phytoplankton and their rates of production. In this short report I focus on the measurements of
productivity from 14C assimilation, productivity as estimated from variables measured from
moored sensors, and the mixed layer depth as an indicator of the extent of mixing in the water
column. For the comparison of shipboard measurements, I compiled data that was readily
available in the published literature or tabulated on web sites.
Prior Work
Our sense of progress can be appreciated by examining what we thought we knew about the
productivity cycles in the Arabian Sea before embarking on the JGOFS programmes in 1994 and
1995. Sharon Smith compiled what was then known into a report (U.S. JGOFS, 1991). That the
Arabian Sea is productive, by virtue of the semi-annual monsoons and important fisheries, was
not in question. The theoretical basis (e.g., Bauer et al., 1991), supported by a small number of
cruises to the region, was that the summertime Southwest (SW) Monsoon, sometimes called the
Findlater Jet (Findlater, 1971), blowing toward Asia, would generate upwelling near the coast of
Oman. Further offshore, the changing curl of the wind-stress on either side of the Findlater Jet
would produce an Ekman divergence region and upwelling to the north of its axis. South of the
Jet axis, however, the curl of the wind-stress would be such as to produce a convergence,
downwelling, and deep mixed layers. The effect of the winter Northeast (NE) Monsoon, coming
off the Asian continent, was believed to be minor, perhaps driving upwelling near the coasts of
India and Pakistan.
According to the earlier literature, the SW Monsoon dominates seasonality in the Arabian Sea
almost entirely. What we knew of the biology seemed to reinforce this idea. The ocean colour
24
data from CZCS show large increases during August and September, a few months following the
onset of the SW Monsoon in late May and June, (Banse, 1987; Brock et al., 1991). Because of
cloudiness (and perhaps dust), however, there are no satellite images for most of June and July,
thus the oceanic response to the SW Monsoon was almost unknown. Brock et al. (1993) model
the seasonal change in primary production, extrapolating both from a chlorophyll profile during
the Spring Intermonsoon, and from the composite ocean colour images for each of the seasons.
They predict a seasonal productivity cycle with a maximum in August and September, and at
over 2 g C m-2 d-1, four- or five-fold greater than the rest of the year. Yoder et al. (1993)
completed a more comprehensive analysis of the CZCS data for the tropics in general, and also
found a peak in chlorophyll a biomass dominating the Indian Ocean at the end of summer (SW
Monsoon). The primary productivity estimated by the satellite imagery appears to be a multiple
of the seasonal variation in chlorophyll biomass (Brock et al., 1993; Howard and Yoder, 1997).
The geographic and seasonal variability in primary production is, to a large extent, driven by
variations in water column mixing, as indicated by the depth of the mixed layer. Water column
mixing controls the availability of irradiance and affects the flux nutrients to the phytoplankton
at the surface. Like other areas of the world ocean, the re-stratification and vertical mixing of the
water column is central to the seasonal cycle in the Arabian Sea (Sathyendranath and Platt 1994,
1999; McCreary et al., 1996). The relationship between productivity and mixed layer depth,
however, is not direct. Deep mixed layers may not allow enough irradiance for phytoplankton to
accumulate when they are mixed over great depth. In shallow mixed layers, on the other hand,
nutrients are depleted more quickly than they can be replenished by mixing, and productivity
would subsequently decline. Conditions that provide marginal or transient stability to the water
column are optimal for phytoplankton productivity.
Very little hydrographic data for the Arabian Sea exist prior to the JGOFS effort. An offshore
section of density after the SW Monsoon shows an increase in mixed layer depth going offshore,
expected from the changing sign of the wind-stress curl (Bauer et al., 1991). Seasonally,
Sathyendranath and Platt (1994) suggest how chlorophyll biomass and mixed layer depth may
interact, using data from the CZCS and climatologies of hydrography. Given upwelling to the
north of the Findlater Jet, and downwelling to the south, however, the Arabian Sea might be
expected to resemble upwelling regimes in eastern boundary currents. The difference in the
Arabian Sea is, of course, the annual reversal of winds.
In summary, the historical data clearly indicated the importance of the monsoons to the
oceanography of the Arabian Sea. The effect of the SW Monsoon is to divide the Arabian Sea
into two zones, upwelling nearer the Arabian Peninsula, and downwelling offshore. The CZCS
data provided tantalizing glimpses of the potential for productivity in the Arabian Sea and
important ideas, but little information on the means by which environmental forcing governed
the seasonal cycle, and less still of the actual magnitudes of productivity. In reviewing the
findings of the recent programmes, I highlight the seasonal variability and the changes on- and
offshore, both in the context of a changing mixed layer depth as the indicator of water column
mixing and re-stratification. I divide consideration of the data into temporal and spatial
variability. The analysis here suggests (1) that, seasonally, productivity is less dominated by the
SW Monsoon period, and (2) that productivity is less variable going offshore from the Arabian
Peninsula.
Temporal Variability
For temporal variability in the Arabian Sea, I rely on an estimate from the bio-optical sensors
moored in the central Arabian Sea from October 1994 to October 1995 (see Dickey et al., 1998).
The mooring serves as a context for comparison with other data over the seasonal cycle. The
25
optical estimate of productivity is based on moored observations of chlorophyll (Kinkade et al.,
2000) and surface irradiance (Weller et al., 1998). The governing equation is,
P = φ(E)• a*ph•(z)• Chl-a •E(z)
where P is the rate of production (mol-C m-3 d-1), φ(E) is the quantum yield for photosynthetic
carbon fixation (mol-C/mol-photons), a*ph is the mean (over wavelength) chlorophyll-specific
absorption cross-section (m2 mgChl a-1), Chl-a is chlorophyll a (mg m-3), and E is the total daily
irradiance (mol-photons m-2 d-1) at depth z. I plot the data regardless of the year in which the
data were collected in order to see patterns in the seasonality in the productivity. The danger of
this, of course, is that there will be inter-annual variability in the magnitude and timing of the
monsoons. Also, mesoscale phenomena can cause variability among the various estimates.
The variability in chlorophyll a and primary productivity, seasonally, shows large variations, but
not what might have been expected based on prior work (Figure 1). First, there are mesoscale
phenomena present in the mooring records (Dickey et al., 1998), and also in the shipboard data
(Barber et al., 2001). The mooring recorded two major eddy-like features, one in December
1994, and a second in August 1995. The December eddy, in particular, advected high
chlorophyll a concentrations through the mooring site. Second, the productivity observed during
the NE Monsoon was much higher than anticipated (Mahdupratap et al, 1996). Convective
mixing during the NE Monsoon (Weller et al., 1998; Wiggert et al., 2000) turned out to be an
important source of nutrients, which leads to enhanced rates of production over that expected
from prior analysis of CZCS imagery. The calculation in Marra et al. (1998) shows a steady
increase in productivity with the wind-driven deepening (Weller et al., 1998) of the mixed layer
with the progression of the SW Monsoon in June and July. The maximum in chlorophyll a and
productivity occurs in August and September following the cessation of mixing. There is not a
direct relationship between mixed layer depth and the concentrations of chlorophyll a.
Chlorophyll a increases after the water column re-stratifies following the cessation of monsoon
winds, and declines during the shallow mixed layers of the Intermonsoon periods. Interestingly,
the data of Ryther et al. (1966) are consistent with the JGOFS results.
There is variability in the shipboard productivity data. However, when plotted against
chlorophyll a, nearly all the data fall within an acceptable range (Figure 2). The data from
Owens et al. (1993) are the exception. This is the closest data point to the position of the
mooring, but a nearby station shows almost twice the productivity for the same amount of
chlorophyll a, and agreement with the other data. Overall, I conclude that the magnitude of
carbon assimilation is being consistently estimated, and that the pattern that has emerged reflects
at least the seasonal variability in chlorophyll a biomass and primary productivity.
The mooring productivity estimates (Figure 1) were accomplished using a constant absorption
coefficient for phytoplankton, a*ph (the mean value over wavelength) and a constant maximum
(realizable) quantum yield for carbon assimilation, φmax. Thus, productivity in those estimates
only varied as a function of chlorophyll concentration, irradiance (and consequently, the depth of
the euphotic zone), and mixed layer depth (Marra et al., 1998). The reason for the constancy in
a*ph and φmax was that the variability of these parameters is little understood, especially at the
day-to-day time scale at which we can obtain estimates of chlorophyll and irradiance. Therefore,
we decided to keep them constant. However, other data show that these parameters change,
certainly between the monsoons (Sathyendranath et al., 1999; Johnson et al., in press), and
perhaps over shorter time periods as well. It is possible that changing bio-optical properties of
the phytoplankton are responsible for the differences between the mooring estimates and the
shipboard data, especially at low biomass and productivity (Figure 2). There is a need in future
studies to understand how phytoplankton absorption and quantum yields change in response to
environmental conditions at greater resolution than has heretofore been measured.
26
(a)
80
70
60
Chl-a
50
40
30
20
10
1995
1994
0
0
50
100
150
200
250
300
350
Day Number
(b)
C Assimilation
250
200
150
100
50
1995
0
0
1994
50
100
150
200
250
300
350
-50
-100
MLD
Day Number
Figure 1. (a) Chlorophyll a (mg m-2) as a function of day number for 1994 (after vertical line)
and 1995 (before vertical line). Data shown are from the mooring (15° 30’ N, 61° 30’ E) (dots),
the Arabian Sea Expedition (16° 12’N, 62° W) (squares), Arabesque (19° N, 62° E: circles; 19°
N, 64° E, diamonds), and from Madhupratap et al. (1998) (15° N, 64° W: triangles). (b) Carbon
assimilation (mmol-C m-2 d-1) as a function of day number [symbols as in (a)]. Mixed layer
depth (MLD) estimated from the mooring data is also shown.
Onshore-Offshore Variability
There are three features of the spatial variability in the Arabian Sea that result in large part from
the monsoons. First, sea-surface temperature (SST) always increases offshore (Gundersen et al.,
1998). During the SW Monsoon, there is a particularly large jump from the coastal stations,
followed by smaller increases further offshore. Nevertheless, despite the season, SST always
increases offshore. Second, mixed layer depths always increase offshore as well (Figure 3),
indicating a downward tilt to the pycnocline going in the offshore direction. Third, surface
nutrient concentrations almost always decrease offshore (Morrison et al., 1998; Woodward et
al., 1999), although during the Northeast Monsoon in 1995, there was a maximum in surface
nitrate at 16 °N (700 km offshore) (Dickson et al., 1999).
27
Figure 2. Carbon assimilation as a function of chlorophyll a for the data from the Arabian Sea.
All of these observations point to the large-scale upwelling regime inshore of the geographic
axis of the Findlater Jet. Coastal upwelling occurs nearshore, and Ekman driven upwelling
occurs over the broad reaches of the offshore ocean. South of the Findlater Jet, and in the region
where the curl of the wind-stress becomes negative, convergent circulation and downwelling
maintain deep mixed layers. While there is evidence for substantial eddy variability in the
Northern Arabian Sea (Flagg and Kim, 1998), the overall trends are preserved in their mean
values in the transect data on the line offshore proceeding southeast from Ras Madraka, Oman.
To be sure, there is considerable noise in the mixed layer depth data, but the downward trend is
preserved in all seasons. In addition to mesoscale phenomena, the variability can also be caused
by instrument or observational error, or by internal waves.
In contrast to what might be expected, productivity does not decrease in the offshore direction
(Savidge and Gilpin, 1999; Barber et al., 2001) despite the increasing mixed layer depths and the
decline in nutrient availability. The reason is most certainly the physiological adaptation in the
offshore communities. The suite of pigments is altered in the offshore direction toward that
indicative of greater dominance by picoplankton and other small forms (Latasa and Bidigare,
1998; Barlow et al., 1999). Marra et al. (2000) noted that the pigment suite in the offshore
populations also conferred an adaptation to higher irradiances, by having proportionately more
pigments used for photoprotection. The advection of coastal waters in upwelled filaments
(Latasa and Bidigare, 1998; Barber et al., 2001) is another factor that can complicate the
interpretation of the offshore trends. However, the larger effect is the changing photosynthetic
characteristics in the offshore populations, which permits relatively high rates of production in
these well-lit, but relatively nutrient-poor areas.
28
Figure 3. Estimates of the mixed layer depth as a function of latitude (°N) in the Arabian Sea.
The circles are the mean depth for all the cruises in the Arabian Sea Expedition in 1994-1995,
and the error bars represent the standard deviation from the mean. The individual estimates are
plotted as small diamonds.
29
2.5 Remote Sensing of Ocean Colour in the Arabian Sea
Louisa Watts and Shubha Sathyendranath
Biological Oceanography Section, Bedford Institute of Oceanography
P.O. Box 1006, Dartmouth, Nova Scotia, B2Y 4A2
Canada
Introduction
It is no exaggeration to state that the availability of remotely sensed data on surface-ocean
properties from satellites has revolutionised oceanography. The vantage point from space, which
earth-orbiting satellites provide, has allowed spatially detailed measurements to be made almost
instantaneously over wide areas and at regular frequency allowing monitoring of the ocean on a
global scale. Sea-surface colour, temperature, height and roughness measurements can be
collected by remote-sensing techniques, thus providing synoptic information on biological and
physical processes and opening up the possibility of a multidisciplinary approach to marine
ecosystem studies. This chapter considers the advances made in remote-sensing of the Arabian
Sea in a biogeochemical context, so that we are mainly focussing on advances within the remote
sensing of ocean colour and its applications (but see Capone et al., 1998; Böhm et al., 1999; Shi
et al., 1999 for applications of remotely-sensed sea-surface temperature and sea-surface height in
the Arabian Sea).
Biological oceanography has been revolutionised by the provision of ocean colour data that
provide us with a two-dimensional window into the synoptic state of the pelagic ecosystem (Platt
et al., 1995), and an opportunity to calculate ocean-basin scale primary production from biomass
fields. At the time of writing JGOFS Report 17 (1995), the only available ocean colour data had
been collected by the Coastal Zone Colour Scanner (CZCS) for the period 1978-1986. The
launch of the state-of-the-art, ocean colour satellite sensor, SeaWiFS (Sea Wide Field-of-View
Sensor), had been delayed until mid-1995 but the ocean colour data collected thereafter were to
be available for the JGOFS Arabian Sea Process Study. This Process Study was thus planned to
be “the first test of the JGOFS goal to relate remotely-sensed variability to oceanic processes of
the carbon cycle” (JGOFS Report 17, 1995). Unfortunately, the SeaWiFS sensor was not
launched until August 1997, leaving the JGOFS programme without remotely sensed ocean
colour data.
Despite the lack of SeaWiFS ocean colour data during the JGOFS cruises to the Arabian Sea,
significant advances have been made within the JGOFS framework, both in the area of algorithm
development and in the area of validation of the quality of the retrieval of phytoplankton
pigment data from SeaWiFS. Advances have also been made in protocol-development for the
computation of primary production using satellite-observed phytoplankton biomass fields and
bio-optical properties of the surface and near-surface waters. In this chapter, the progress made
in these areas is reviewed. Furthermore, some recommendations are made for building on the
JGOFS experience for remote sensing in the Arabian Sea.
Remote-sensing Algorithms: The Retrieval of Chlorophyll a Biomass and Fluorometric
Pigment Concentration
Standard protocols for processing remotely sensed ocean colour data to derive the water-leaving
radiance signal at several wavebands involve a correction for atmospheric noise. These multiwaveband data are then processed to obtain a measure of pigment concentration in the surface
waters. The accuracy of this information depends on the suitability of algorithms used to convert
the water-leaving radiance values into estimates of chlorophyll a and fluorometric pigment
30
concentration. These algorithms are based on the influence of pigments on phytoplankton
absorption and hence on ocean colour.
As part of the U.S. contribution to JGOFS, Kinkade et al. (2001) have developed an empirical
algorithm relating the ratio of downwelling irradiances at 443 nm to 550 nm (blue to green
wavelength ratio) with the integrated chlorophyll a concentration through the photic zone (r2 =
0.73). Data were determined from moored irradiance and fluorescence measurements, collected
over a year in the Arabian Sea.
Using an essentially empirical approach, Aiken et al. (1995) have developed a number of multiband algorithms for the retrieval of chlorophyll a and total fluorometric pigment concentration
from SeaWiFS data. These algorithms are based on two ratios of water-leaving radiances: the
443:555 ratio and the 490:555 ratio and were an attempt to produce global chlorophyll a and
fluorometric pigment algorithms using two band ratios. As part of the ARABESQUE
programme (the UK contribution to JGOFS Arabian Sea Process Studies), Pinkerton et al.
(1999) tested the performance of the Aiken et al. (1995) algorithms in the northwestern Indian
Ocean. Shipboard measurements were made of the depth-varying upwelling and downwelling
irradiances in parallel with phytoplankton pigment concentrations during a SW Monsoon and an
Intermonsoon period. This data set was used to test the validity of the algorithms described by
Aiken et al. (1995). The algorithms based on the 443:555 band-ratio estimated near-surface
chlorophyll a and fluorometric pigment concentrations satisfactorily, i.e., with an accuracy of ±
35% of the actual value, in accordance with the goal set within the SeaWiFS mission statement.
In contrast, the mean errors from the algorithms based on the 490:555 band-ratio were greater
than 35% over both seasons. This was attributed to the sensitivity of these algorithms to seasonal
and regional changes in the proportions and composition of photosynthetic and photoprotective
carotenoid pigments present in the phytoplankton. Such variability can arise from seasonal and
regional changes in ambient light levels (Bricaud et al., 1995; Johnsen and Sakshaug, 1996;
Culver and Perry, 1999; Stuart et al., 2000; Marra et al., 2000) and from changes in the physical
forcing, with subsequent changes in phytoplankton population composition.
Indeed, prior to the JGOFS programme, little was known about the absorption characteristics of
phytoplankton in the northwestern Indian Ocean. As a result of the Netherlands Indian Ocean
Programme, the ARABESQUE and U.S. JGOFS cruises, this database has been substantially
increased: light absorption by phytoplankton has been measured during the NE Monsoon
(Sathyendranath et al., 1996); SW Monsoon and November (Fall) Intermonsoon
(Sathyendranath et al, 1999); and Spring Intermonsoon and NE (winter) Monsoon (Marra et al.,
2000). Examination of these data sets has taught us that the intra-annual variability of the
physical forcing in the Arabian Sea, brought about by the monsoon system, causes changes in
the phytoplankton composition and cell size (Campbell et al., 1998; Garrison et al., 1998; Brown
et al., 1999; Savidge and Gilpin, 1999; Tarran et al., 1999; Banse and English, 2000;
Shalapyonok et al., 2001) and in the pigment make-up of these cells (Latasa and Bidigare, 1998;
Barlow et al., 1999; Banse and English, 2000). This, in turn, gives rise to changes in the
absorption properties of the phytoplankton and consequently in ocean-colour (Sathyendranath et
al., 1996; Sathyendranath et al., 1999; Marra et al., 2000; Stuart et al., 2000). Present universal
algorithms for the retrieval of chlorophyll a biomass and fluorometric pigment concentration, as
described above (Aiken et al., 1995), are inadequate to incorporate such time- and space-variant
bio-optical properties of the surface and near-surface waters found in the Arabian Sea. It is
therefore desirable that these algorithms be tuned to match the optical properties of the
phytoplankton present in the area and to incorporate their geographical and seasonal variability
(see Recommendations). The practical implementation of a patchwork of regional and seasonal
algorithms would, however, pose some problems that merit more attention.
31
Retrieval of Diffuse Attenuation Coefficient
Kinkade et al. (2001) developed an empirical algorithm relating the ratio of downwelling
irradiances at 443 nm to 550 nm with the average vertical attenuation coefficient for
photosynthetically active radiation (PAR; r2 = 0.845). Pinkerton et al. (1999) have shown that
the diffuse attenuation coefficient at 490 nm may be estimated robustly from remotely sensed
data using the ratio of normalised water-leaving radiance at 443 nm to that at 555 nm and the
algorithm developed by Austin and Petzold (1981), with the 555 nm band used instead of the
550 nm band.
Primary Production Estimates by Remote-sensing of Biogeochemical Properties
Another application of remotely sensed ocean colour data is in the computation of primary
production on large horizontal scales: maps of surface ocean biomass can be derived from
remotely sensed data on ocean colour. These maps can then be combined with a spectral and
angular model of submarine light coupled with a model of the spectral response of algal
photosynthesis to compute oceanic primary production (Platt and Sathyendranath, 1988;
Sathyendranath et al., 1989; Sathyendranath et al., 1995). In addition to satellite-derived
information on phytoplankton biomass, this processing of maps requires specification of the
photosynthetic characteristics of the phytoplankton: namely, the parameters of the
photosynthesis-irradiance (P-I) curve and information on the vertical distribution of biomass.
These data are accumulated by observations from ships and so tend to be sporadic in both space
and time. Prior to the JGOFS initiative, little was known about the magnitude and variation of
the P-I parameters in the northwestern Indian Ocean. As a result of the ARABESQUE and U.S.
JGOFS cruises, the database for this area has been supplemented substantially. Observations
show that there are temporal differences in the assimilation number and the slope of the P-I
curve, between the Monsoon and the Intermonsoon seasons, although these differences are not
as strong as those observed for the chlorophyll-specific absorption coefficient (Sathyendranath et
al., 1996; Sathyendranath et al., 1999). Spatial differences in the P-I parameters have also been
observed between filaments and adjacent waters during the Fall Intermonsoon (Toon et al.,
2000), which were associated with differences in the phytoplankton community structure.
Photosynthesis-Irradiance parameters measured during the Netherlands Indian Ocean
Programme, SW Monsoon and Spring Intermonsoon 1992, showed both spatial and temporal
variability (Kromcamp et al., 1997).
Watts et al. (1999) have used such shipboard data collected during the ARABESQUE cruises
(Sathyendranath et al., 1999) to successfully compute integrated primary production and new
production over a Monsoon and Intermonsoon period for the northwest Indian Ocean basin.
They combined archived ocean colour data (CZCS), the P-I and absorption data (Sathyendranath
et al., 1999), vertical biomass data (Burkill, 1999), nitrogen assimilation data (Watts and Owens,
1999) and the models as described above. The protocols developed by Watts et al. (1999) for the
computation of primary and new production incorporate the intra-annual variability of
phytoplankton growth within the Arabian Sea by partitioning the area into a system of “dynamic
biogeochemical provinces” (see below).
The Development of “Biogeochemical Provinces” in the Arabian Sea
The decomposition of an ocean basin into a suite of regions where the area enclosed by the
boundary of any region is under common physical forcing with respect to the factors that
influence the growth of the phytoplankton is called a partition into a system of “dynamic
biogeochemical provinces” (Longhurst et al., 1995; Longhurst, 1998; Platt and Sathyendranath,
1999). The underlying concept is that provinces can be used as a template for the assignment of
32
the parameters of the P-I curve, and of the function describing the vertical distribution of
phytoplankton biomass, for the modelling of oceanic primary production. According to this
concept, every pixel on an ocean colour image will be assigned parameter values according to
the province to which they are assigned. Primary production can then be computed for that pixel,
allowing horizontal extrapolation of shipboard data to ocean-basin scales. Some desirable
properties for a suite of provinces, as defined by Platt and Sathyendranath (1999), are as follows:
(1) they should be few in number; (2) the boundary of any province should be elastic, to account
for seasonal and interannual variations in the physical forcing; (3) the boundaries should be
accessible to remote-sensing; (4) within seasons the province should have predictable vertical
structure and biogeochemical rate parameters; and (5) they should provide templates for
province-specific rules. Although it is widely recognised that robust methods for the
classification of such provinces are required for global estimates of oceanic primary production,
this work is still in its infancy. The wide range of biogeochemical conditions encountered in the
Arabian Sea, however, have made it an ideal setting for the development of such protocols and
significant advances in this area have been made within the JGOFS programme.
As part of the Canadian contribution to JGOFS, Longhurst et al. (1995) and Longhurst (1998)
have made the first formal partition of the entire World Ocean into 57 biogeochemical
provinces, and used this partitioning for the computation of global primary production.
Longhurst et al. (1995) classified provinces according to available knowledge of the regional
oceanographic characteristics of the ocean basins. Combinations of the following factors were
taken into account: latitude, depth of water, proximity of coastline, seasonal irradiance cycle,
local wind regime, local precipitation regime, distant forcing of pycnocline depth, and nutrient
supply (described in Longhurst, 1998). Longhurst et al. (1995) identified two provinces for the
northwestern Indian Ocean (as compared with six by Watts et al., 1999; and four by Brock et al.,
1998; see below). It should be noted that the significant step forward made by Longhurst et al.
(1995) had a disadvantage in the sense that, in their implementation, the boundaries of the
provinces were static.
Following on from the Longhurst et al. (1995) study and also as part of the Canadian
contribution to JGOFS, Brock et al. (1998) have classified the northwestern Indian Ocean into
provinces. Their approach is different from that of Longhurst et al. (1995) in that they use a
combination of climatologies of mixed layer depths and archived ocean colour (CZCS) data to
partition the Arabian Sea into four distinct provinces, based on the modelled attenuation of light
through the photic zone and the subsequent relation of the depth of the photic zone to the depth
of the mixed layer. This approach is referred to as biohydro-optical classification.
As part of the UK and Canadian contribution to JGOFS, Watts et al. (1999) have used data
collected during the ARABESQUE cruises to partition the northwestern Indian Ocean into a
suite of six distinct biogeochemical provinces (Figure 1), characterised by a unique combination
of bathymetry, sea-surface temperature (SST) and sea-surface chlorophyll a data. The province
properties satisfy at least (1) to (4) of the criteria described by Platt and Sathyendranath (1999).
An advantage of the Watts et al. (1999) protocol is that the boundaries of the provinces are
dynamic, since their positions are determined from sea-surface properties that can be measured
synoptically by a satellite. Given that there were no satellites monitoring ocean colour at the time
of the ARABESQUE cruises, Watts et al. (1999) tested their protocol with archived ocean
colour (CZCS) and SST (AVHRR) data. The protocol was found to be statistically robust,
facilitating the computation of near real-time estimates of primary production and new
production on large spatial scales.
33
CZCS, AVHRR, Bathymetry data + Observed sea-surface properties (ARABESQUE 1994)
6 Biogeochemical Provinces (September 1994)
Shipboard data (ARABESQUE 1994) + Local
Algorithm Model (Platt and Sathyendranath)
Predicted Integrated Primary Production
(September 1994)
Computed vs. Observed Primary
Production:
R2 = 0.89 Slope = 0.72 n=10
Empirically derived algorithm
Predicted Integrated New Production (September 1994)
Computed vs. Observed F-ratio:
R2 = 0.81 Slope = 0.55 n=10
Figure 1. A summary of the work carried out by Watts et al. (1999) as part of the UK and
Canadian contributions to the JGOFS programme in the Arabian Sea (ARABESQUE cruises,
1994).
34
Recommendations
1. The retrieval of chlorophyll a biomass and fluorometric pigment concentrations
The optical data collected during the ARABESQUE cruises, as described in Pinkerton et al.
(1999), clearly indicate that the global remote sensing algorithms presently in use for pigment
retrieval require some regional and seasonal adjustments. These must allow for the bio-optical
changes observed in near-surface and surface waters brought about by changes in phytoplankton
composition in response to changes in oceanographic conditions caused by the monsoonreversals in the Arabian Sea. It is therefore recommended that new algorithms be developed to
allow the retrieval of information on particular taxonomic groups from remote-sensing
measurements. This will require thorough investigation of the advanced spectral and radiometric
properties offered by the new generation of satellites (IOCCG, 1998) in relation to shipboard
measurements of the light absorption characteristics of different phytoplankton species and their
regional and seasonal variability. This can be carried out using the existing databases for the
Arabian Sea but there is no doubt that building on these databases through future Arabian Sea
cruises will facilitate this optimisation and increase the accuracy of the remote-sensing
algorithms.
We therefore recommend further ship-time in the Arabian Sea to cover all seasons, maintaining
at least the same spatial coverage used in the ARABESQUE and U.S. JGOFS cruises.
2. Primary production estimates by remote sensing of biogeochemical properties
We have available a spectrally resolved model for the computation of primary production (Platt
and Sathyendranath, 1988; Sathyendranath et al., 1989; Sathyendranath et al., 1995), given P-I
and vertical biomass information. Additional effort is required as follows.
(i) In the collection of more shipboard measurements of P-I parameters, light absorption
and pigment data over all seasons and on at least the same spatial scale as the U.S. JGOFS and
ARABESQUE cruises. This can be achieved with more sea-time in the Arabian Sea;
(ii) In the development of protocols for the assignment of the parameters that drive the
model to every pixel on the SeaWiFS image. It seems inevitable that the method applied will be
within the context of some scheme of biogeochemical provinces (Platt et al., 1995) so that the
principle question for the future is about which criteria are used to classify provinces. Is it
possible to develop algorithms that define the boundaries between provinces in real-time at the
global scale and would these be appropriate for an area such as the Arabian Sea, which exhibits
such spatial and temporal variability? The discussion on the retrieval of phytoplankton pigment
using a universal SeaWiFS algorithm (Aiken et al., 1995) would suggest not. Therefore, regional
and seasonal algorithms to take account of changing phytoplankton compositions are
recommended. Whilst the approach taken by Watts et al. (1999) for defining provinces appears
promising, the idea of dynamic biogeochemical partitions to facilitate large-scale analyses of
ocean-basins is still young and the necessary methodology is still being refined. Such approaches
require testing of shipboard data simultaneously with real-time remotely sensed ocean colour
data. Clearly, this is only possible with future sea-time in the Arabian Sea.
35
2.6 Microbial Dynamics2
Peter H Burkill
Plymouth Marine Laboratory, Natural Environment Research Council
Prospect Place, West Hoe, Plymouth, PL1 3DH
UK
Introduction
Considerable recent progress has been made in understanding the role played by microbes in the
biogeochemistry of the Arabian Sea. Almost one hundred papers have been published on this
topic in the last decade, mostly stimulated by the JGOFS Process Studies that were planned by
SCOR (1995). This chapter focuses on the microbial communities, principally bacteria and
protozoa, in pelagic and benthic biogeochemistry. Recent findings on primary production and
deep benthos are excluded, since these topics are covered elsewhere in this report.
Seasonality
A characteristic of the Arabian Sea is the strong pattern of seasonality caused by monsoonal
forcing of the basin. The response of microbial community structure to monsoonal forcing in the
Arabian Sea has been studied over an annual cycle (Campbell et al., 1998). Picoplankton
populations of heterotrophic bacteria (HBac), Prochlorococcus spp. (Pro), Synechococcus spp.
(Syn) and picoeucaryotic algae (Peuc) were shown to vary considerably along two transects
from the coast of Oman to 1500 km offshore. HBac were numerically dominant in surface
waters in all regions (<3 x 106 cells ml-1), with higher maximum abundances in coastal waters
than at offshore stations. Conversely, Pro were most abundant at the oligotrophic offshore
stations, and 100-fold lower, or absent, at coastal stations, except during the Spring
Intermonsoon when abundances were extremely high along the entire southern transect. Syn
abundances were highly variable, with no consistent trend between coastal and offshore stations.
This variability may be explained by the prevalence of mesoscale eddies, but also could be due
to overlapping distributions of multiple Syn populations distinguished by pigment type. Syn with
a low phycourobilin (PUB) to phycoerythrobilin (PEB) ratio pigment type were more abundant
at coastal stations, whereas Syn with a high PUB:PEB ratio increased in abundance offshore.
Average depth profiles for Pro, Syn, and HBac displayed uniform abundance in the surface
mixed layer, with a rapid decrease below; however, during the Spring Intermonsoon most
profiles had a peak at the base of the surface mixed layer. Distributions of Peuc typically
displayed a subsurface maximum near the base of the surface mixed layer, except during the SW
Monsoon when abundance peaked near the surface. This report is the first to describe the
seasonal and spatial variation in microbial community structure in the Arabian Sea over a
complete monsoon cycle. Overall, eukaryotes were more important at coastal stations, while
prokaryotes predominant at offshore stations. Pro abundance was restricted to warm oligotrophic
waters and was inversely related to surface nitrate concentrations; thus, an increase in the % Pro
as a fraction of total prokaryote abundance was also indicative of oligotrophic conditions. The
effects of SW Monsoonal forcing on microbial community structure resulted in an increase of
Peuc, but this response was limited to coastal stations. Pro and Syn remained dominant at
offshore stations.
2
MSS published since 1992 extracted from the ISI database using ‘Arabian Sea’, ‘microbial’,
‘bacteria’ and ‘microzooplankton’
36
The role of such forcing during the NE Monsoon has been studied by Kumar et al. (2001)
through time-series observations at a station situated at 21°N, 64°E. At this location, sea-surface
temperature cooled and the mixed layer deepened due to winter convection. Co-variation of
nitrate concentrations in the surface layers and concentrations of chlorophyll a and primary
production in the euphotic zone with mixed-layer depth (MLD) and wind suggests that carbon
fixation was controlled primarily by physical forcing. Cooler waters during winter (NE
Monsoon) 1997 relative to winter 1995 were associated with deeper MLDs, higher nitrate
concentrations, elevated primary productivity, and higher chlorophyll a concentrations, leading
to the inference that even a 1°C decrease in SST could lead to significantly higher primary
productivity. Satellite data on sea-surface temperature (advanced very high-resolution
radiometer; AVHRR) and TOPEX/POSEIDON altimeter data suggest that this interannual
variation is of basin-wide spatial scale. After the termination of winter cooling and subsequent
warming during the Spring Intermonsoon, when primary production drops, bacteria and
microzooplankton proliferate. This suggests that the organisms of the microbial loop may be an
important food source that sustains the mesozooplankton throughout this period (Madhupratap et
al., 1996).
The seasonal dynamics of the microbial activity in the euphotic zone has been modelled
(Ryabchenko et al., 1998). A nitrogen-based, seven-component ecosystem model has been
coupled to a three-dimensional quasi- geostrophic ocean general circulation model and applied
to a simulation of the seasonal variability of physical and ecosystem variables in the
northwestern Indian Ocean. Comparison with available data has highlighted many similarities
and some disagreements between the model and observations. The Ryabchenko model produces
an entrainment bloom in May-June in the central Arabian Sea for which there is no evidence in
the CZCS data and does not reproduce the observed August - September bloom in the northern
Arabian Sea. In the latter case, analysis showed that grazing control by zooplankton prevents the
development of the model bloom. A comparison of our solution with the results obtained by
McCreary et al. (1996) using a 2.5-layer physical-ecosystem model showed that, despite
numerous differences between the models, the period and location for most blooms in these
solutions coincide closely. The model reproduced the differences in primary productivity
between the NE and SW Monsoon periods and showed that the model phytoplankton in the main
upwelling areas were only seriously nutrient limited during the Spring Intermonsoon period. For
most of the year, the model predicts that phytoplankton production is closely coupled to
zooplankton grazing with blooms only occurring when there are rapid changes in phytoplankton
growth rate due either to the entrainment of nitrate into the mixed layer or decreased light
limitation (when the mixed layer is shallowing thereby allowing the phytoplankton to escape
from grazing control). The predicted particle flux from the euphotic zone was of the order of
40% of the primary production and lagged the latter quantity by 8-10 days. This temporal lag
meant that the correlation between daily values of the two quantities was low, and this may
partly explain the difficulties of finding good correlations between observations of these two
quantities.
Bacterial Biogeochemistry
Studies of bacteria in the Arabian Sea (Azam et al., 1994), suggest that temporal contiguity of
highly oligotrophic and eutrophic periods, along with high water temperatures, may result in
unique features of bacteria-organic matter coupling, nutrient cycling, and sedimentation, which
are unlike those in classical oligotrophic and eutrophic waters. Bacteria- phytoplankton
interactions may influence phytoplankton aggregation and its timing. Azam et al. (1994)
hypothesise that, within aggregates, hydrolytic ectoenzyme activity, together with condensation
reactions between the hydrolysis products, produce molecular species that are not readily
degraded by pelagic bacteria. Accumulation of a reservoir of such slow-to-degrade dissolved
37
organic carbon (DOC) is proposed to be a carbon flux and energy buffer, which moderates the
response of bacteria to the dramatic variations in primary production in the Arabian Sea. Use of
the slow-to-degrade DOC pool during the Intermonsoons could temporarily render the Arabian
Sea net-heterotrophic and a source of CO2 to the atmosphere.
DOC is an important resource for bacteria. While DOC is generally considered to be
conservative in marine waters, Kumar et al. (1990) have shown DOC to be non-conservative in
the basin. Concentrations of DOC were shown to be 3-4 times higher than those reported earlier,
and decreased northward from the equator. Total carbon dioxide (TCO2) increases in proportion
of the oxygen utilized, thereby revealing the dominant biological role in the carbon turnover.
The CO2 added through dissolution of biogenic debris is found to decrease southward.
Decomposition of organic material contributes at least 64% to the CO2 addition that increases
southward, the rest being from dissolution of skeletal material. Evidence is provided for the
utilization of oxygen and nitrate for DOC oxidative decomposition. Accumulation of DOC
without its complete oxidation to CO2 could be the main reason for the TCO2 decrease in
southern Arabian Sea. Relationships of DOC with nitrification and denitrification processes
show that the microbial population plays a major role in regulating the DOC contents in the
seawater of this region. Consumption/decomposition by denitrifying bacteria and other
microorganisms responsible for nitrogen cycling in the sea are found to be intimately related to
the DOC dynamics and are responsible for decreased DOC concentrations in the north. DOC
accumulation in the southern Arabian Sea seems to facilitate bacterio-particulate aggregate
formation and consequent nitrification, which results in excess nitrate.
Bacteria often proliferate in regions of high organic matter and may become sites for elevated
microbial activity. One source of the latter are transport exopolymer particles (TEP). Kumar et
al. (1998) have studied the behaviour of TEP in the Arabian Sea and found that this differs from
that of the Bay of Bengal. TEP concentrations were lower in the Bay of Bengal due to faster
scavenging from water column because of interaction with mineral particles. TEP concentrations
were higher and occurred even in intense suboxic layers in the Arabian Sea. These results
support the mineral ballast theory advocated for the Bay of Bengal and suggest that there is a
surplus of organic matter over and above that required to meet the higher carbon demand by
bacteria in the denitrifying waters of the Arabian Sea. The situation in the Arabian Sea during
the late SW Monsoon would seem to be different. Bacterioplankton distributions and production,
studied by Ducklow (1993) show abundances and production to be high (>1 x 109 cells l-1 and
30-92% of primary production, respectively) along the oceanic portions of the transects. These
elevated levels may indicate a response to the decline of summer phytoplankton blooms
stimulated by monsoonal deepening of the mixed layer. The bacterial production estimates of
this study, which used very conservative conversion factors, suggest that the typical carbon
sources used to support bacterial production (e.g., phytoplankton exudation, particle breakdown)
could supply only a fraction of the bacterial demand. A more recent study on the relationship
between TEP and bacteria has been carried out by Ramaiah et al. (2000) who found high
regional variability in the vertical distribution of TOC with high concentrations in surface waters
at 18-20°N along 64°E due to upwelling-induced productivity. TEP concentrations decreased
with depth and were lower between 200 and 500 m. Bacterial concentrations were up to 1.99 x
108 l-1 in the surface waters and decreased by an order of magnitude or more at depths below 500
m. A better relationship has been found between bacterial abundance and concentrations of TEP
than between bacteria and TOC, suggesting that bacterial metabolism is fuelled by TEP
availability. Assuming a carbon assimilation of 33%, bacterial carbon demand (BCD) was
estimated to be 1- 4 g C m-2 d-1 in the surface waters. The observed TEP concentrations appear
to be sufficient in meeting the surface and subsurface BCD in the northern Arabian Sea.
38
Bacteria use extracellular enzymes to degrade particulate material. Activity of enzymes such as
phosphatase (P-ase) was determined together with bacterial abundance and production rates
during two SW Monsoons (Hoppe and Ullrich 1999). Samples were collected along the cruise
tracks from the equator to the upwelling region at the shelf edge off Oman. Depth profiles of Pase activity were found to be strikingly different from those of the other enzymes. While values
of aminopeptidase and beta-glucosidase generally decreased below the euphotic zone, P-ase
increased by factors of 1 to 7. The ratio of peptidase- to P-ase activity was from 4 to 21 at the
surface and from 3 to 5 at 800 m depth. Because P-ase production (dissolved and cell-bound) in
deep waters is mainly dependent on bacteria, P-ase activities per bacterial cell were calculated:
these were, on average, 37 times higher at 800 m than at the surface. A positive correlation of Pase activity with phosphate concentrations was observed in depth profiles below the euphotic
zone, but this relationship was much more variable in the mixed surface layer. These
observations suggest that C-limited bacteria in the deep strata did not primarily focus on the
phosphate generated by their P-ase activity but on the organic carbon compounds which were
simultaneously produced and which could probably not be taken up prior to the hydrolytic
detachment of phosphate. It is hypothesised that a considerable part of the measured P-ase
activity was dissolved and may be important for the slow, but steady regeneration of phosphate
and organic C in mesopelagic waters.
Bacterioplankton activity in surface waters during and after the 1994 SW Monsoon (Pomroy and
Joint, 1999) showed that while phytoplankton production was significantly higher during the SW
Monsoon, bacterial numbers showed little difference. Bacteria were most abundant in the
euphotic zone with highest numbers (<1.6 x 109 bacterial-1) found in the shelf waters. Most
bacteria were small cocci, which were larger (diameter 0.40 µm) during the SW Monsoon period
than in the Spring Intermonsoon (diameter 0.36 µm). Bacterial production, measured by
thymidine and leucine incorporation, confirmed highest rates on the shelf, and decreased
offshore. In the central Arabian Sea, thymidine incorporation rates were similar in the SW
Monsoon and Spring Intermonsoon periods, but higher rates of leucine incorporation were
measured during the SW Monsoon season. Bacterial production was a relatively small
proportion of phytoplankton production in both periods sampled, estimated to be equivalent to
between 10 and 30% of the daily primary production.
Bacterial abundance and production in the central and eastern Arabian Sea, studied by Ramaiah
et al. (1996), have confirmed the results of Pomroy and Joint (1999) from the western side of the
basin. They found higher bacterial concentrations (~1 x 109 cells L-1) during September and
April/May compared to the SW Monsoon period of July/August and the winter period of
February/March. Although primary production was low during April/May, bacterial production
was much higher during this period, unlike July/August. Bacterial concentrations also showed an
increase from the northern to the southern Arabian Sea, suggesting the presence of high amounts
of dissolved organic carbon in this area. High picoplankton concentrations (<45 x 106 cells L-1)
were observed during February, particularly in the northern Arabian Sea. Rapid turnover of
bacteria during the Spring Intermonsoon period of April/May suggests the predominance of a
'microbial loop' in the foodweb and a prevailing source of dissolved organic carbon in the
oceanic waters.
The response of bacteria to year-round high primary productivity has shown that bacterial
productivity, both in the surface layer and integrated over the euphotic zone, was elevated less
than 2-fold during the SW Monsoon (Ducklow et al., 2001). This suggests that bacterial
populations are closer to steady state than phytoplankton that typically increase in concentration
during this period. Surprisingly, bacterial abundance and production were low during the NE
Monsoon, but then increased in March during the Spring Intermonsoon (Ducklow et al., 2001).
39
Bacterial production seems to respond differently to the seasonal pattern in different regions.
Wiebinga et al. (1997) found that during the SW Monsoon, the Somali Current showed highest
bacterial production (<849 mgC m-2 d-1) in regions with enrichment of the surface waters by
upwelling of cold, nutrient-rich, deep water. In contrast, the Gulf of Aden and the Red Sea were
most productive during the NE Monsoon (225 mgC m-2 d-1). Depth profiles of the upper 300 m
in general showed a subsurface maximum in bacterial abundance and production at 20-70–m
depth. Heterotrophic activity and primary production were closely correlated, indicating the
dependence of bacterioplankton on local phytoplankton-derived organic carbon and their ability
to adapt quickly to changes in the environment. The bacterial carbon demand in the upper 300 m
of the water column was largely supplied by phytoplankton production in the euphotic zone.
Bacterial production was 18 ± 7% (average ± S.D.) of primary production. Assuming an
assimilation efficiency of 50% for marine bacteria, they consumed up to half of the carbon
produced by the phytoplankton.
A comparison of the bacterial activity in the Arabian Sea with other ocean basins has been made
(Ducklow, 1999). While euphotic zone bacterial stocks in the Arabian Sea were remarkably
similar, averaging about 1 gC m-2, compared to subarctic and oligotrophic gyres of the north
Atlantic and north Pacific oceans and the upwelling zone of the equatorial Pacific Ocean,
bacterial production and growth rates varied more widely. This suggests that regulation of
biomass and production may occur independently.
While most of the focus in the JGOFS studies has been on carbon cycling, it has not been
entirely so. The role of bacteria in iodine cycling has been greatly facilitated by shipboard
experimental studies in the Arabian Sea. Using cultures of the ubiquitous marine bacterium
Shewanella putrefaciens, the chemical reduction of iodate in seawater has been shown to be
biologically mediated (Farrenkopf et al., 1997). Shipboard incubations of filtered seawater
collected from the oxygen minimum zone (OMZ) with this bacterium, showed evidence of
iodate reduction. Inorganic chemical reduction of iodate in these samples was ruled out since no
free sulphide was measurable and concentrations of ammonia and nitrite were found to be less
than 5 µM. To confirm that bacterial metabolism was involved, iodate reduction to iodide by
Shewanella putrefaciens strain MR-4 was studied in artificial seawater using electrochemical
methods. It is thought that MR-4 is a facultative anaerobe that may switch amongst a suite of
electron accepters to support metabolism. In all experiments, MR-4 reduced all iodate to iodide.
The rate of formation of [I-] in the culture followed pseudo-first order kinetics. This is the first
report of the marine bacterial reduction of iodate where the concentrations of iodide and iodate
were measured directly. These results may help to explain the depth distribution of iodine
speciation reported in productive waters like the Arabian Sea.
The composition of the bacterial community was studied using denaturing gradient gel
electrophoresis (DGGE) of rRNA genes (Riemann et al., 1999). During the NE Monsoon, the
total bacterial community’s genomic DNA was analysed by PCR amplification of 16S rRNA
gene fragments, followed by DGGE. In total, 20 DGGE bands reflecting unique or varying
phylotypes were excised, cloned, and sequenced. Bacterial groups commonly found in oceanic
waters (e.g., the SAR11 cluster of alpha-Proteobacteria and cyanobacteria) were present, but
surprisingly none of the sequenced amplicons were related to gamma-Proteobacteria or to
members of the Cytophaga-Flavobacter-Bacteroides phylum. Amplicons related to
magnetotactic bacteria were found for the first time in pelagic oceanic waters. In the mixed layer
the bacterial community was dominated by the same set of approximately 15 phylotypes at all
stations, but unique phylotypes were found with increasing depth. Except for cyanobacteria,
comparison of the bacterial community composition in surface waters from January and
December 1995 showed only minor differences, despite significant differences in environmental
40
parameters. These data suggest a horizontal homogeneity and some degree of seasonal
predictability of bacterial community composition in the Arabian Sea.
Microbial Role in the Vertical Flux of Particulates
Microbes play key roles in the transformation of material as it fluxes vertically through the water
column. Using NMR spectroscopy, Hedges et al. (2001) have shown evidence that there is nonselective preservation of organic matter in sinking marine particles in the Arabian Sea. Almost
all the particulate organic matter exported from ocean surface waters into the ocean interior is
recycled. The fraction of this organic matter remaining becomes buried in ocean sediments, thus
sequestering carbon and so influencing atmospheric carbon dioxide concentrations. The
processes controlling the extensive biodegradation of sinking particles remain unclear, partly
because of the difficulty in resolving the composition of the residual organic matter at depth with
existing chromatographic techniques. The chemical structure of organic carbon in both surface
plankton and sinking particulate matter has been analysed. Minimal changes occur in bulk
organic composition, despite extensive (>98%) biodegradation, and amino acid-like material
predominates throughout the water column. The compositional similarity between phytoplankton
biomass and the small remnant of organic matter reaching the ocean interior indicates that the
formation of unusual biochemicals, either by chemical recombination or microbial biosynthesis,
is not the main process controlling the preservation of particulate organic carbon within the
water column. It seems that the organic matter might be protected from degradation by the
inorganic matrix of sinking particles.
Bacterial activity has been suggested as an agent in the concentration and vertical transport of
excess manganese (Nair et al., 1999) as well as the fatty acid composition of settling particles
(Reemtsma and Ittekkot 1992).
Microbial Role in Methane Production
The Arabian Sea is a globally significant ocean basin for methanogenesis (Owens et al., 1991).
Jayakumar et al. (2001) have described the patterns of methane found in the eastern and central
basin in April - May and August - September when surface waters showed 110-2521%
saturation. The highest values were observed during the SW Monsoon in the inner shelf where
coastal upwelling combined with freshwater runoff to produce very strong near-surface
stratification. Methane inputs from coastal wetlands through seasonal runoff may have occurred
as abnormally high saturations (up to similar to 13,000%) were recorded in the estuarine surface
water. In situ production, favoured by very high biological production in conjunction with the
prevalence of suboxic conditions in the upwelled water, could be the other major source. In
comparison, sedimentary inputs seemed to be of lesser importance in spite of previously reported
occurrence of gas-charged sediments in this region. Methane profiles in the open central Arabian
Sea showed two maxima. The more pronounced deeper maximum, occurring at 150-200 m
depth, was similar to the feature seen elsewhere in the oceans, but was probably intensified here
due to an acute oxygen deficiency. It showed some correlation with the subsurface particle
maximum characteristic of the denitrifying layer. The dominant mechanism of its formation
might be in situ production within particles rather than advection from the continental shelf as
concluded by previous workers. The less pronounced and previously unreported shallower
maximum, occurring in the well-oxygenated upper 50 m of the water column, was more
dynamic probably as a result of variability of the balance between methane production due to
biological activity and its losses through microbial oxidation and air-sea exchange (see also
Chapter 2.3).
41
The importance of the upper ocean in methanogenic activity has been elucidated using isotopes
(Faber et al., 1994) at 24°N, 65°E. This investigation has shown that surface waters of the
Arabian Sea are enriched in methane, confirming that this is a source for the atmosphere. Carbon
isotope ratios point to a bacterial origin of methane that is generated in the surface waters.
Concentration changes and variations of carbon isotope ratios also suggest that methane seeping
from sea floor sediments of the Arabian Sea is oxidized by bacterial activity and does not reach
the atmosphere.
Microzooplankton Trophodynamics
Microbial herbivory has been studied extensively in recent years, and now we have a complete
seasonal picture. During the SW Monsoon, microzooplankton grazing utilized 81% of
phytoplankton growth at offshore oligotrophic stations and 54% at upwelling coastal stations
(Brown et al., 1999). Similar results have been reported by Edwards et al. (1999) who found that
microzooplankton herbivory was greater than mesozooplankton herbivory both nearshore and
offshore. While mesozooplankton cropped ~12% of the daily primary production, the
microzooplankton grazed up to 60% of the daily algal production. At this time, the
microzooplankton community was dominated by aloricate ciliates and heterotrophic
dinoflagellates. Their concentrations reached 850 cm-3 in nearshore waters, with highest
concentrations beneath the mixed layer while offshore, concentrations peaked in surface waters
(Stelfox et al., 1999). Microzooplankton biomass varied between 1 and 12 µgC l-1, and
nanoflagellates, dinoflagellates, and ciliates reached maximum biomass at different locations and
depths. Heterotrophs comprised 18-27% of the biomass over most of the U.S. JGOFS transect
(Garrison et al., 1998).
Trophic losses of bacteria, studied during the Fall Intermonsoon period, were found to be high
(4.1% h-1) in the shallow waters of the Gulf of Oman. Lower values were found in the Arabian
Sea, with higher (1.7% h-1) rates inshore than in the central gyre. Heterotrophic nanoflagellates
were the primary bacterivores; larger microzooplankton accounted for 33% of total bacterivory
in the Gulf of Oman but only 16% in the central Arabian Sea. Time-course experiments
indicated substantial diel changes in bacterivory making extrapolation to daily rates difficult
(Weisse, 1999). In the Fall Intermonsoon, microzooplankton concentrations increased
throughout the basin reaching standing stocks in excess of 300 mgC m-2 (Stelfox et al., 1999).
Microzooplankton biomass reached a maximum of 36 µgC l-1 during the Fall Intermonsoon and
was higher than that of mesozooplankton (Gauns et al., 1996), suggesting that the microbial
communities may dominate the plankton in this season. This is in keeping with the finding that
smaller algae such as cyanobacteria predominate in the flora, and form a major food source for
microzooplankton (Burkill et al., 1993). The stocks of Synechococcus comprise up to 40% of the
POC, and up to 71% of the cyanobacteria could be grazed daily. High turnover by grazing was
mirrored by high growth rates suggesting close coupling between growth and loss processes. It is
clear that cyanobacteria are a major component of a dynamic but well-balanced microbial
foodweb present during the Fall Intermonsoon period.
During the NE Monsoon, trophic coupling seems to be much less pronounced, despite similar
stocks of microzooplankton (2.0 and 1.8 gC m-2 respectively) in both monsoon seasons (Dennett
et al., 1999). Algal growth rates average 0.8 d-1 during the NE Monsoon while microzooplankton
grazing crops ca. 0.3 d-1 (Caron and Dennett, 1999). Such values complement those of Landry et
al. (1998) who found microzooplankton grazing rates of 0.6 d-1 for surface waters. Herbivory
was estimated to account for 38% d-1 of the algal stock and 67% of the primary production
(Reckermann and Veldhuis 1997). Experimental removal of the grazers >10µm in size enhanced
algal growth and mortality suggesting a trophic cascade within the microbial community.
42
Microbial Nitrogen Cycling
Microbially driven aspects of the nitrogen cycle have been described and modelled. A study
during the NE Monsoon by McCarthy et al. (1999) revealed that only the southern most stations
(10° and 12° N) and one shallow coastal station were as nutrient-depleted as had been expected
from the few relevant prior studies in this region. Experiments were conducted to ascertain the
relative importance of different nitrogenous nutrients and the sufficiency of local regeneration
processes in supplying nitrogenous nutrients utilized in primary production. Except for the
southern oligotrophic stations, the euphotic zone concentrations of nitrate were typically 5- to
10-fold greater than those of nitrite and ammonium. There was considerable variation (20- to 40fold) in nutrient concentration both within and between the two sections on each cruise. All
nitrogenous nutrients were more abundant (2- to 4-fold) later in the NE Monsoon. Strong
vertical gradients in euphotic zone ammonium concentration, with higher concentrations at
depth, were common. This was in contrast to the nearly uniform euphotic zone concentrations
for both nitrate and nitrite. Half-saturation constants for uptake were higher for nitrate (1.7 µM
kg-1 than for ammonium (0.47 µM kg-1). Evidence for the suppressing effect of ammonium on
nitrate uptake was widespread, although not as severe as has been noted for some other regions.
Both the degree of sensitivity of nitrate uptake to ammonium concentration and the halfsaturation constant for nitrate uptake were correlated with ambient nitrate concentration. The
combined effect of high affinity for low concentrations of ammonium and the effect of
ammonium concentration on nitrate uptake resulted in similarly low f-ratios, 0.15 and 0.13, for
early and late observations in the NE Monsoon, respectively. Stations with high f-ratios had the
lowest euphotic zone ammonium concentrations, and these stations were either very near shore
or far from shore in the most oligotrophic waters. At several stations, particularly early in the NE
Monsoon, the utilization rates for nitrite were up to 50% the utilization rates for nitrate. When
converted with a Redfield C:N value of 6.7, the total N uptake rates measured in this study were
commensurate with measurements of C productivity. While nutrient concentrations at some
stations approached levels low enough to limit phytoplankton growth, light was shown to be
very important in regulating N uptake at all stations in this study. Diel periodicity was observed
for uptake of all nitrogenous nutrients at all stations. The amplitude of this periodicity was
positively correlated with nutrient concentration. The strongest of these relationships occurred
with nitrate. Ammonium concentration strongly influenced the vertical profiles for nitrate and
ammonium uptake. Both nitrite and ammonium were regenerated within the euphotic zone at
rates comparable to rates of uptake of these nutrients, and thus maintenance of mixed layer
concentrations did not require diffusive or advective fluxes from other sources. Observed
turnover times for ammonium were typically less than one day. Rapid turnover and the strong
light regulation of ammonium uptake allowed the development and maintenance of vertical
structure in ammonium concentration within the euphotic zone. In spite of the strong positive
effect of light on nitrite uptake and its strong negative effect on NO2 production, the combined
effects of much longer turnover times for this nutrient and mixed layer dynamics resulted in
nearly uniform nitrite concentrations within the euphotic zone. Responses of the NE Monsoon
planktonic community to light and nutrients, in conjunction with mixed layer dynamics, allowed
for efficient recycling of N within the mixed layer. As the NE Monsoon evolved and the mixed
layer deepened convectively, nitrite and nitrate concentrations increased correspondingly with
the entrainment of deeper water. Planktonic N productivity increased 2-fold, but without a
significant change in the new to recycled N proportionality. Consequently, nitrate turnover time
increased from about 1 month to greater than 3 months. This reflected the over-riding
importance of recycling processes in supplying nitrogenous nutrients for primary production
throughout the duration of the NE Monsoon. As a result, nitrate supplied to the euphotic zone
during the NE Monsoon is, for the most part, conserved for utilization during the subsequent
Spring Intermonsoon season.
43
The role of bacteria in denitrification has been studied by Naqvi et al. (1993) who found an
intermediate nepheloid layer (INL) associated with high microbial metabolic rates and
denitrification. A wide range of simultaneous optical, physical, chemical, and biochemical
measurements showed that the INL was invariably associated with the secondary nitrite
maximum. Maxima in particulate protein and the activity of the respiratory electron transport
system (ETS) are also found within the INL. Since the INL persisted far from the continental
margin and intensified offshore, it may not be related to the transport of material resuspended
along the continental margin. An apparent correlation of the INL with the previously reported
subsurface maximum in bacterial abundance suggests that a local increase in the abundance of
bacteria may be responsible for the increased turbidity. Positive correlations of the beam
attenuation anomaly with nitrite and nitrate deficit suggest that most of these bacteria may be
denitrifiers. The organic carbon demand within the denitrifying layer, computed from the
observed ETS activity, appears to be several fold higher than the sinking carbon fluxes to the
denitrifying layer, requiring additional modes of supply of biodegradable organic matter. It is
proposed that a bacterial maximum could be maintained with efficient utilization of the
dissolved organic matter within the denitrifying waters (see also Chapter 2.3).
Another aspect of the role played by microbes in the nitrogen cycle concerns methylamine
production. The distributions and biogeochemistry of methylamines and ammonium in the
Arabian Sea have been reported by Gibb et al. (1999). They found concentrations of NH4+ up to
two orders of magnitude greater than those of methylamines throughout the region.
Monomethylamine was generally the most abundant MA, whilst trimethylamine was found only
at concentrations < 4 nM. Low concentrations in open ocean meso- and oligo-trophic regions
contrasted with the elevated levels recorded in the highly productive coastal upwelling waters of
the NW Arabian Sea. In total, methylamines contributed < 1% dissolved organic nitrogen
(DON). Depth maxima of methylamines were generally associated with those of chlorophyll a,
and in offshore regions, also with those of ammonium (above the thermo-, oxy- and nitra-clines).
Trimethylamine maxima were recorded at the base of the thermo- and oxyclines, resolved from
the other analytes. Through correlation studies, a degree of diatom specific MMA production
was inferred and microzooplankton grazing found to influence significantly all aqueous MA
concentrations. Enhanced correlation of monomethylamine concentrations with
mesozooplankton abundance was attributed to their ability to graze diatoms. We postulate that
the nitrogen taken up in nutrient-rich, diatom-dominated regions of the Arabian Sea will be used
both biosynthetically and anabolically. This may be accompanied by introduction of
methylamines into the aqueous phase through enzymatic precursor degradation, nitrogen
detoxification, senescence or lysis and be accelerated through grazing pressure, particularly that
of mesozooplankton on diatoms. In contrast, under the more oligotrophic conditions recorded in
the remote Arabian Sea, those species of phytoplankton with a lower nitrogen demand are
favoured, e.g., prymnesiophytes and dinoflagellates. Correspondingly, lower methylamine
concentrations are recorded in these regions (see also Chapter 2.3).
Nitrogen cycling in suboxic and anoxic conditions in the Arabian Sea has been modelled
(Yakushev and Neretin, 1997). The vertical distributions of nitrogen compounds (total organic
nitrogen, ammonium, nitrate, nitrite) as well as dissolved oxygen agree reasonably well with the
observations. Bacterially mediated processes are described by first-order equations using O2
concentration. Denitrification is estimated to be 34 TgN y-1 in the Arabian Sea.
Oxygen Minimum Zone
A key oceanographic characteristic of the Arabian Sea is the presence of low oxygen
concentration in mid water. This oxygen minimum zone (OMZ) is spatially variable, showing
greatest oxygen deficiency in the northern basin. Its vertical expression is also variable showing
44
greatest deficiency over the depth horizon of ca. 200 to 2000 metres, although exact horizon of
the OMZ depends on the location and season. A summary of the seasonal interaction of the
monsoon with the OMZ has been reported by Morrison et al. (1999) for 1995. They considered
that dissolved oxygen, nitrite, nitrate and density values are all required to delineate the OMZ,
and the regions of denitrification. The suboxic conditions within the northern Arabian Sea are
documented, as well as biological and chemical consequences of this phenomenon. Overall, the
conditions found in the suboxic portion of the water column in the Arabian Sea were not greatly
different from what has been reported in the literature with respect to oxygen, nitrate and nitrite
distributions. Within the main thermocline, portions of the OMZ were found that were suboxic
(oxygen less than similar to 4.5 µM) and contained secondary nitrite maxima with
concentrations that sometimes exceeded 6.0 µM, suggesting active nitrate reduction and
denitrification. Although there may have been a reduction in the degree of suboxia during the
SW Monsoon, strong seasonality was not observed, as has been suggested by some previous
work. There was little evidence for the occurrence of secondary nitrite maxima in waters with
oxygen concentrations greater than 4.5 µM. The northern basin appeared to accumulate larger
nitrate deficits due to longer residence times even though the denitrification rates, shown by
reduced nitrite concentrations, there might be lower. Organism distributions showed strong
relationships to the oxygen profiles, especially in locations where the OMZ was pronounced, but
the biological responses to the OMZ varied with type of organism. The regional extent of
intermediate nepheloid layers in our data corresponds well with the region of the secondary
nitrite maximum. This is a region of denitrification, and the presence and activities of bacteria
are assumed to cause the increase in particles.
The importance of the OMZ for denitrification has long been known. However, measurements of
respiration associated with denitrification have been hampered by the difficulty of making
accurate measurements of O2 in the OMZ. One way around this problem is to use cellular
measurements of the electron-transport system (ETS) to quantify respiration in the OMZ (Naqvi
and Shailaja, 1993). Measurements of ETS activity at 15 stations in the Arabian Sea during the
NE Monsoon yielded high respiration rates that do not correlate with the trends in primary
productivity. The rates are unlikely to be sustained by the supply of carbon associated with the
sinking particles alone, and seem to suggest a major role for dissolved and/or suspended organic
matter in fuelling oxygen consumption and denitrification. Naqvi and Shailaja (1993) have
computed a denitrification rate of 24 - 33 TgN y-1 in the Arabian Sea. This estimate agrees with
the estimate based on the exports of nitrate deficits outside the denitrification zone. The
ventilation time of the denitrifying layer is calculated as approximately 1 year.
The effect of the OMZ on the benthos has also been studied (Schmaljohann et al., 2001) in the
northeastern Arabian Sea off Pakistan. Here the upper continental slope between 350 and 850 m
water depth, which is in the centre of the oxygen-minimum zone, is characterized by numerous
sites of small-scale seeps of methane- and sulphide-charged porewater. White bacterial mats
with diameters <1 m of filamentous sulphur-oxidizing bacteria were discovered. Seep sediments,
as well as non-seep sediments, in the vicinity were characterized by the occurrence of the
bacterium Thioploca in near-surface layers between 0 and 13 cm depth. Thioploca bundles were
up to 20 mm in length and contained up to 20 filaments of varying diameters, between 3 and 75
µm. Up to 169 bundles cm-2 were counted. Maximum numbers occurred in the top 9 cm of
sediment, which contained very low concentrations of soluble sulphide (<0.2 µM) and high
amounts of elemental sulphur (up to 10 µmol cm-3). Moderate sulphate reduction activity
(between 20 and 190 nM cm-3 d-1) was detected in the top 10 cm of these sediments, resulting in
a gradual downcore decrease of sulphate concentrations. CO2 fixation rates had distinct maxima
at the sediment surface and declined to background values below 5-cm depth.
45
Preservation and composition of organic matter in surface sediments were investigated on three
different transects on the Pakistan continental margin across the OMZ (Schulte et al., 2000). To
assess whether the OMZ has an effect on the total organic carbon (TOC), the extractable lipids
were quantified by gas chromatography, combined gas chromatography/mass spectrometry, and
compound-specific stable carbon isotope measurements. These lipids were dominated by marine
organic matter as indicated by the abundance of lipids typical of marine biota and by the bulk
and molecular isotopic composition. Sediments from within the OMZ are enriched in organic
carbon and in several extractable lipids such as phytol, n-alcohols, total sterols, n-C-35 alkanes,
relative to stations above and below this zone. Other lipid concentrations, such as those of total
n-fatty acids and total n-alkanes fail to show any relation to the OMZ. Only a weak correlation
of TOC with mineral surface area was found in sediments deposited within the OMZ. In
contrast, sediments from outside the OMZ do not show any relationship between TOC and
surface area. Among the extractable lipids, only the n-alkane concentration is highly correlated
with surface area in sediments from the Hab and Makran areas. In sediments from outside the
OMZ, the phytol and sterol concentrations are also weakly correlated with mineral surface area.
The depositional environment of the Indus Fan offers the best conditions for an enhanced
preservation of organic matter. The OMZ, together with the undisturbed sedimentation at
moderate rates, seems to be mainly responsible for the high TOC values in this area. Overall, the
type of organic matter and its lability toward oxic degradation, the mineral surface area, the
mineral composition, and possibly the secondary productivity by sedimentary bacteria also
appear to have an influence on organic matter accumulation and composition.
The biogenic origins of organic matter in sediments underlying the OMZ have been investigated
using lipids (Smallwood and Wolff, 2000) and amino acids (Suthhof et al., 2000). The high
abundance of compounds that can be ascribed to primary producers, namely diatoms,
dinoflagellates, coccolithophores, Eustigmatophycae and cyanobacteria, reflects the high
productivity of surface waters, but there is little direct molecular evidence of zooplankton
reworking of organic material, except at the deepest water site (similar to 1250 m). There is,
however, clear evidence of benthic re-working, probably by detritivorous megafauna, at one of
the sites near the base of the oxygen minimum zone (similar to 1000 m). The relative abundance
of biological markers that can be ascribed to aerobic and anaerobic bacteria in the sediments,
namely the hopanoids and branched fatty acids, respectively, mirrors the concentrations of
oxygen in overlying water. Considerable intra- and inter-site variability probably results from
the spatial and temporal variability in production and from the complexity of sedimentation
processes. There is little clear evidence that bottom water oxygenation affects quantitative
preservation of organic matter.
Benthic Microbial Biogeochemistry
Denitrification is the process by which nitrate is reduced to gaseous nitrogen species, typically
N2 or N2O, and is the dominant mechanism for removal of fixed nitrogen from the biosphere. In
the oceans, denitrification is mediated by bacteria in suboxic environments and, by controlling
the supply of fixed nitrogen, is an important limiting factor for marine productivity.
Denitrification produces substantial 15N enrichment in subsurface nitrate, which is reflected in
the isotopic composition of sinking particulate nitrogen; sediment 15N/14N ratios in regions with
suboxic water columns may therefore provide a record of past changes in denitrification
intensity. Altabet et al. (1995) report nitrogen isotope data for sediment cores from three sites in
the Arabian Sea. At all three sites, large, near-synchronous downcore variations in 15N/14N were
found, which are best explained by regional changes in the isotopic composition of subsurface
nitrate, and hence denitrification. Moreover, these variations are synchronous with Milankovitch
cycles, thereby establishing a link with climate. These large, climate-linked variations, in a
46
region that contributes significantly to global marine denitrification, are likely to have perturbed
marine biogeochemical cycles during the Late Quaternary period.
Benthic bacterial and nanoflagellate concentrations in the sediments and benthic boundary layer
have been studied during upwelling and non-upwelling seasons, in the Arabian Sea (Bak and
Nieuwland, 1997). There was a significant decrease with depth in benthic bacteria and benthic
nanoflagellates. Bacterial concentration and biomass were significantly larger in the bottom
water during the upwelling season, whereas benthic flagellates were significantly more
numerous in the sediments in the non-upwelling season.
The distribution of microbial biomass in sediments at eight sites in the Arabian Sea has been
studied using phospholipids (Boetius and Lochte, 2000). Surface sediment phospholipid
concentrations at 4 abyssal sites ranged from 7 nM cm-3 at the southern site (SAST; 10 degrees
N 65 degrees E, 4425 m) to 29 nM cm-3 at the western site (WAST; 16 degrees N 60 degrees E,
4045 m). The high values detected at the abyssal station WAST exceeded those in the literature
for other abyssal sites and were comparable to values from the upper continental slope of the NE
Atlantic and Arctic oceans. At the four continental slope sites in the Arabian Sea, phospholipid
concentrations ranged from 9 to 53 nmol cm-3 with the maximum values at stations A (2314 m)
and D (3142 m) close to the Omani coast. Records of particulate organic carbon flux to the deep
sea are available for four of the investigated locations, allowing a test of the hypothesis that the
standing stock of benthic microorganisms in the deep sea is controlled by substrate availability,
i.e. particle sedimentation. Total microbial biomass in the surface sediments of the Arabian Sea
was positively correlated with sedimentation rates, consistent with previous studies of other
oceans.
The microbiology of the benthic nepheloid layer (BNL) has been studied (Boetius et al., 2000a)
at six stations in the deep Arabian Sea during May and February. Suspended particulates and
bacterial activity were found to be more variable vertically than regionally or temporally.
Compared to the deep-water column, bacterial concentrations increased towards the seafloor
with highest concentrations of suspended particulates, bacterial leucine incorporation and the
activity of different enzymes found in the near-bottom water. Suspended particulates in the BNL
had a high chlorophyll a-to-POC ratio and were of higher organic carbon content than the
sinking particles or the particulate matter at the sediment surface. The carbon demand of the
bacterial community in the BNL, estimated by leucine incorporation experiments, exceeded the
vertical POC flux. Thus, it seems that the enhanced microbial activity and biomass close to the
seafloor is mainly supported by the resuspension of small phytodetrital particles and by the DOC
flux.
Substantial regional variation in benthic microbial biomass and activity has been found in the
Arabian Sea with highest values in the western Arabian Sea (station WAST), decreasing
approximately three-fold to the south (station SAST). Benthic microbial biomass and activity
during the NE Monsoon was as high or higher than subsequent to the SW Monsoon, indicating a
very rapid turnover of POC in the surface sediments. This variation in the biomass and activity
of the microbial assemblages in the deep Arabian Sea can largely be explained by the regional
and temporal variation in POC flux. Compared to other abyssal regions, the substantially higher
benthic microbial biomasses and activities in the Arabian Sea reflect the extremely high
productivity of this tropical basin (Boetius et al., 2000b).
Sulphate reduction in near-surface sediments of the deep Arabian Sea has been studied using
stable isotopes (Bottcher et al., 2000). Depth profiles of the stable sulphur isotopic composition
of dissolved sulphate in near-surface sediments at five stations of the deep Arabian Sea indicate
that net microbial sulphate reduction can take place in the sediments. At stations WAST and
47
NAST this occurred at depths below about 12 cm. At WAST this was more pronounced than at
station NAST, most likely due to a higher organic carbon content in turbiditic sediments.
Sulphate reduction did not take place within the upper 10 cm of the surface sediments at any
stations, and no significant isotopic indication for sulphate reduction was found down to 30 cm
below the surface (bsf) at station SAST. Results are in accordance with accumulation of reduced,
isotopically light sulphur species below about 6 cm bsf at station WAST. Bottcher et al. (2000)
concluded that the sulphur isotopic composition of remaining sulphate is more sensitive to net
sulphate reduction than the [SO4]/[Cl] ratio. The sulphur isotopic composition of a vertical
profile for dissolved sulphate through the water column at station WAST was essentially
constant (250 - 4047 m: δ34S = +20.49 ± 0.008 parts per thousand, versus V-CDT, n = 8). A
similar constancy (20 - 4565 m: δ34S = +20.57 ± 0.06 parts per thousand versus V-CDT, n = 15)
was found for the station BIOTRANS in the northeastern Atlantic Ocean (47°N 19°W),
indicating that the oxygen minimum zone in the Arabian Sea has no influence on the sulphur
isotopic composition of dissolved sulphate.
A possible microbial origin of phosphorites in the Arabian Sea has been proposed (Rao et al.,
1992). Petrographical, mineralogical and microprobe analyses of phosphorites from Error
Seamount, in the northwestern Arabian Sea, demonstrate that microbial processes played an
important role in the early diagenetic formation of phosphorites during subaerial exposure of the
seamount. The phosphorites on Error Seamount occur as laminated crusts and massive slabs.
Low-magnesium calcite and carbonate fluorapatite are the major minerals of the phosphorites,
with goethite being important only in the massive slab phosphorites. Elemental sulphur,
pyrrhotite, gypsum and chlorite are the mineral phases present in the acid-insoluble residues of
the phosphorites. Elemental sulphur, which is exclusively formed by microbial processes, occurs
as submicron-sized granules on gypsum surfaces. The laminated crust phosphorites consist of
organic-rich and organic-poor Laminae interlayered with ghost pellets, index fossils of
Oligocene to lower Miocene age, peloids and coated grains. Massive slab phosphorites are finegrained, and consist of goethitic and non-goethitic zones. Non-goethitic zones comprise mainly
microsphorite with Oligocene to lower Miocene fossils and microfossils. Goethitic zones contain
abundant Quaternary skeletons. Microprobe analyses of the Error Seamount phosphorites
indicate that the geochemistry of these phosphorites is similar to that of the Tertiary Seamount
phosphorites in the Atlantic and Pacific oceans and that they are enriched in Ca and depleted in
P compared to other seamount and island phosphorites in the Indian Ocean. Microbial activity is
evident in both types of phosphorites and occurs in the form of microborings in peloids and on
skeletal fragments, encrustation of phosphate crystals on hollow globular microstructures and
phosphate sheaths enclosing organic filaments. The presence of calcite rim cements, microsparfilled patches and recrystallised grains indicates exposure of the seamount under subaerial
conditions, during which phosphatisation took place.
Microbial Respiration
Respiration rates in the Arabian Sea have been reported for both the monsoon seasons (Dickson
et al., 2001). They have investigated the relationships among oxygen, carbon and nitrogen
production and respiration rate measurements made in the Arabian Sea. Increased biological
production characterized the SW Monsoon, with rates 12-53% higher than the NE Monsoon. In
most cases, there was remarkable similarity in production rates during the two monsoons and an
absence of strong spatial gradients in production between nearshore and offshore waters,
especially during the SW Monsoon. Daily 14C and total 15N production underestimated gross C
production, and at the majority of stations 14C and total 15N production were either the same as
net C production or between gross and net C production. Moreover, new production (15Nnitrate), scaled to carbon, was substantially less than net C production. Approximately 50% of
the POC-14C was metabolised during the photoperiod, with smaller losses (7-11%) overnight.
48
The simplest explanation for the discrepancy between gross and total 15N production and
between net C and new production was the loss of 15N-labeled particulate matter as dissolved
organic matter. Partitioning of metabolised gross C production into respiratory and dissolved
pools showed distinct onshore-offshore distributions that appeared to be related to the
composition of the phytoplankton assemblage and probably reflected the trophodynamics of the
ecosystem. The percentage of gross C production released as dissolved organic carbon (DOC)
was highest in the nearshore waters where diatoms dominated the phytoplankton assemblage,
while community respiration was a more important fate for production further offshore where
picoplankton prevailed. In general, stations that retained more gross C production as net
production (i.e., high net C/gross C ratios) had higher rates of DOC production relative to
community respiration. Locations where community respiration exceeded DOC production were
characterized by low rates of net C production and had low net C/gross C ratios. In those
ecosystems, less net C production was retained because higher metabolic losses reduced gross C
production to a greater extent than at the more productive sites.
The Electron Transport System (ETS) is a biochemical proxy for respiration and has been used
by Naqvi et al. (1996) to compare respiration in the Arabian Sea and the Bay of Bengal. They
found much lower ETS activities in subsurface waters of the Bay of Bengal than those measured
in the Arabian Sea. Lower respiration rates in the Bay of Bengal are corroborated by the much
weaker north-south gradients in oxygen and total carbon dioxide. These are, however, in conflict
with the higher sinking fluxes of organic carbon measured with sediment traps. The observations
support the view that particulate organic matter may undergo a lesser degree of oxidation in the
water column through its incorporation into rapidly sinking matter, perhaps because of the
massive inputs of terrigenous matter in the Bay of Bengal. Differential respiration rates may
cause changes in the distribution of suspended particulate matter and may also explain why the
Bay of Bengal is not a site of water-column denitrification in spite of an apparently slower
renewal of the intermediate waters as compared to the Arabian Sea.
Synthesis Studies
Few attempts have been made at synthesising the recently advanced understanding of microbial
biogeochemistry of the Arabian Sea. One of these is the synthesis of microbial dynamics from
the U.S. study made by Garrison et al. (2000). In this, the biomass in the upper 100 metres from
a number of sources are combined, the structure of the food web evaluated, and changes in food
web structure are related to carbon cycling processes. Microbial biomass ranged from
approximately 1.5 to > 5.2 gC m-2, with heterotrophic bacteria contributing 16 and 44%;
autotrophs contributing 43-64%, with the remainder made up of nano- and microheterotrophs.
Autotrophs and nano- and microheterotrophs showed a general pattern of higher values at
coastal stations, with the lowest values offshore. Heterotrophic bacteria (Hbac) showed no
significant spatial variations. The Spring Intermonsoon and early NE Monsoon were dominated
by autotrophic picoplankton, Prochlorococcus and Synechococcus. The late NE Monsoon and
late SW Monsoon periods showed an increase in the larger size fractions of the primary
producers. At several stations during the SW Monsoon, autotrophic microplankton, primarily
diatoms and Phaeocystis colonies, predominated. Increases in the size of autotrophs were also
reflected in increasing sizes of nano- and microheterotrophs. The biomass estimates based on
cytometry and microscopy are consistent with measurement of pigments, POC and PON.
Changes in community structure were assessed using the percent similarity index (PSI) in
conjunction with multidimensional scaling (MDS) or single-linkage clustering analysis to show
how assemblages differed among cruises and stations. Station clustering reflected environmental
heterogeneity, and many of the conspicuous changes could be associated with changes in
temperature, salinity and nutrient concentrations. Despite inherent problems in combining data
from a variety of sources, the present community biomass estimates were well constrained by
49
bulk measurements such as chlorophyll a, POC and PON, and by comparisons with other
quantitative and qualitative studies. The most striking correlation between food web structure
and carbon cycling was the dominance of large phytoplankton, primarily diatoms, and the
seasonal maxima of mass flux during the SW Monsoon. High nutrient conditions associated with
upwelling during the SW Monsoon would explain the predominance of diatoms during this
season. The sinking of large, ungrazed diatom cells is one possible explanation for the flux
observations, but may not be consistent with the observation of concurrent increases in larger
microzooplankton consumers (heterotrophic dinoflagellates and ciliates) and mesozooplankton
during this season. Food-web structure during the early NE Monsoon and Intermonsoons
suggests carbon cycling by the microbial community predominated in those seasons.
Recommendations
1. Almost one hundred microbial papers have been published in this field since 1992. Half of
these were in the last 3 years. The report shows how rich the literature on the Arabian Sea
has become, and how recent much of this is.
2. There have been few attempts at synthesis: a full synthesis of microbial dynamics is now
warranted.
50
2.7 Zooplankton
Sharon L. Smith
The Rosenstiel School, University of Miami
4600 Rickenbacker Causeway
Miami, FL 33149
USA
Martien Baars
Netherlands Institute for Sea Research
P.O. Box 59, 1790 AB Den Burg, Texel
The Netherlands
Introduction
The Arabian Sea is unique in the world for its upwelling areas that seem to support large pelagic
fisheries (Bakun et al., 1998). Until recently, zooplankton of the Arabian Sea region were known
primarily from three oceanographic expeditions in this century: The John Murray Expedition
(JME; September 1933 - May 1934), the International Indian Ocean Expedition (IIOE; 19591965), and the Indian Ocean Expedition (INDEX; 1979). In the 1990's, investigations by the
Netherlands (Netherlands Indian Ocean Programme; NIOP; 1992-1993), the United Kingdom
(Arabesque; 1994), Germany (German JGOFS; 1995, 1997), Pakistan (North Arabian Sea
Environmental and Ecological Research; NASEER, 1992-1994), India (Indian JGOFS, 19951997) and the United States (U.S. JGOFS, 1994-1996) expanded our knowledge of the region.
The Arabian Sea’s Paradox of the Plankton
Close scrutiny of zooplankton biomass data for the northwestern Indian Ocean, including the
Arabian Sea, has shown that seasonal oscillations thought to arise from seasonal variability in
phytoplankton abundance actually do not occur (Baars and Brummer, 1995; Madhupratap et al.,
1996; Baars and Oosterhuis, 1998; Baars, 1999). Prior to the investigations of JGOFS, there was
no explanation of the IIOE result that phytoplankton biomass varied widely while zooplankton
biomass remained reasonably constant throughout the year. With the thorough seasonal
observations of JGOFS, it is now apparent that large fluctuations in primary productivity and
surface chlorophyll a do not occur (Barber et al., 2001). Convection during the NE Monsoon
results in substantial primary production that was not apparent in the IIOE atlases. Therefore,
there is no seasonal food limitation of zooplankton productivity in the Arabian Sea. There are,
however, seasonal variations in the zooplankton community, most notably the ontogenetically
migrating Calanoides carinatus that is at the surface in upwelling areas during the SW Monsoon
season (Smith, 1995; Smith et al., 1998). Detailed seasonal comparisons of other taxa remain to
be done for the JGOFS study area. Prior seasonal investigation of the Somali segment of the NW
Indian Ocean showed shifts in abundances of other taxa in both the NE and SW Monsoon
seasons (Smith, 1988).
The Oxygen Minimum Zone (OMZ)
The sharp subsurface gradient between oxic and suboxic waters, which generally occurs between
100 and 150 metres in the JGOFS study area, is an impenetrable boundary for many zooplankton
(Morrison et al., 1999). At the station with the most intense oxygen deficiency in 1995, 85% of
the zooplankton biomass in the upper 300 meters was in the upper 100 meters day and night; diel
vertical migration was suppressed in the area of strong and persistent oxygen depletion (Smith et
51
al., 1998). Earlier studies have shown that the copepod Pleuromamma indica has its highest
abundances at low oxygen and that of seven species of this genus in the Indian Ocean only P.
indica has the ability to survive in low oxygen waters (Saraswathy and Iyer, 1986; Goswami et
al., 1992). Although Pleuromamma indica migrates daily to the surface layer (Saraswathy and
Iyer, 1986), it must have physiological adaptations to withstand long exposure to suboxic
conditions. These adaptations have not been investigated. Between 400 and 500 meters, the
abundances of many taxa fall nearly to zero (Morrison et al., 1999). Earlier studies which
compared zooplankton biomass in areas of the Arabian Sea containing an OMZ with those that
did not have shown that even in the oxic upper 100 meters biomass in an area with an OMZ was
half that in areas without an OMZ (Vinogradov and Voronina, 1962). At the lower boundary of
the OMZ (600-1000 metres), biomass in the area of the OMZ is not different from biomass in an
area that does not contain an OMZ. The copepod Lucicutia grandis is an indicator of this habitat,
and in the Arabian Sea it was found feeding and reproducing at depth year-round (Wishner et al.,
2000). The material in its guts revealed feeding on a wide range of particles (Gowing and
Wishner, 1998; Wishner et al., 2000).
Seasonal Differences in Biomass and Species Composition
Surveys of the west coast of India, extending up to 700 kilometres offshore, have shown clearly
that both zooplankton biomass (330-µm mesh) and euphausiid biomass are elevated during the
SW Monsoon season (Mathew et al., 1990). The maximum epipelagic biomass in the upper 150
meters of the eastern Arabian Sea (287 mmolC m-2; Mathew et al., 1990) is similar to that
observed in the upper 200 meters off Somalia in the SW Monsoon season (363 mmolC m-2;
Baars and Oosterhuis, 1997). The highest biomass of epipelagic zooplankton measured in the
entire Indian Ocean during the IIOE was 444 mmolC m-2 (0-200 meters), and this was located in
the coastal upwelling area off Oman (Qasim, 1977). The onshore-offshore gradient in
zooplankton biomass off Oman was strong, from 444 mmolC m-2 nearshore (20°N, 58°E) to 110
mmolC m-2 offshore at 10°N, 65°E (Qasim, 1977; his displacement volumes converted to carbon
using Wiebe, 1988, for dry weight and Ikeda, 1974, for carbon).
Although the zooplankton biomass of the Arabian Sea is largely comprised of copepods, basinscale distribution and abundance of individual species are generally poorly known. During
April-October (SW Monsoon), peak copepod abundances occur in the upwelling regions off
Somalia and Arabia, but high abundances also occur far offshore of the Arabian Peninsula.
During October-April (NE Monsoon), copepods are abundant off the Arabian Peninsula and in
the Gulf of Oman, but are somewhat less abundant (compared to the SW Monsoon) off Somalia.
Unfortunately, the IIOE plankton collections have not been exploited extensively at the species
level; consequently, the reported results from the IIOE plankton investigations leave us illprepared to specify with any confidence which copepod species are dominant regionally or
seasonally (GLOBEC, 1993).
In the northwestern Indian Ocean, large calanoid copepods of the family Eucalanidae dominate
with mean abundances of 3000 and 2250 per standard haul (0-200 m) in the SW and NE
Monsoons, respectively (Stephen et al., 1992). The genera Euchaeta (1500/1900 for SW/NE
Monsoons) and Pleuromamma (270/1100 for SW/NE Monsoons) are also abundant. In the upper
layer of the Arabian Sea, at least 231 species of copepod have been identified (Lane et al.,
1998).
One of the assets of the U.S. JGOFS investigation of the Arabian Sea was sampling of the entire
annual cycle of zooplankton. Highest biomass in the upper 300 meters was measured within 350
kilometres of the coast during the SW Monsoon (Smith et al., 1998), and highest biomass in the
0-1000–m layer also was measured during the SW Monsoon (Wishner et al., 1998). In the whole
52
study area, biomass was most variable during the SW Monsoon, ranging from 50 to 200 mmolC
m-2, and least variable during the Spring Intermonsoon and late NE Monsoon, ranging from 75
to 150 mmolC m-2 (Figure 1; Wishner et al., 1998). Different regions of the Arabian Sea
showed peaks in biomass in various seasons. For example, stations in the suboxic area had
highest biomass in the upper 300 meters in the Spring Intermonsoon, while the station farthest
offshore had a biomass peak in the late NE Monsoon (Smith et al., 1998). During the SW
Monsoon, there was an onshore to offshore decrease in biomass in the upper layer (Wishner et
al., 1998; Stelfox et al., 1999), while during the Spring Intermonsoon and late NE Monsoon
there was no onshore-offshore gradient in biomass (Wishner et al., 1998). Seasonal patterns of
size fractions of zooplankton were complex and depended on the depth interval and size under
consideration. For example, biomass in the smallest size class (200-500 µm) was ten times
greater in the SW Monsoon and early NE Monsoon than in the Spring Intermonsoon, while
biomass in the 1000-2000–µm category was greatest in the Spring Intermonsoon (Wishner et al.,
1998). The largest size class, › 2000µm, showed more seasonal variability offshore than onshore,
with the Spring Intermonsoon often having higher biomass than the other seasons. The extent to
which the seasonal patterns of biomass reflect the growth and development of some of the larger
pelagic organisms such as the crab Charybdis smithii (van Couwelaar et al., 1997) or the
myctophid fish Benthosema pterotum (Gjøsaeter, 1984), which are known to have annual life
cycles in this region, is unknown at this time. In the case of C. smithii, reproduction takes place
in May and the crabs reach their largest size in November and December, but their greatest
biomass was measured in July and August (van Couwelaar et al., 1997).
Figure 1. Total night-time zooplankton biomass (sum of all four size fractions from the dry
weight measurements) summed for all depth intervals from 0 to 1000 m for each of the four
cruises. Different symbols represent the different cruises (LNEM: Late Northeast Monsoon; SI:
Spring Intermonsoon; LSWM: Late Southwest Monsoon; ENEM: Early Northeast Monsoon).
Data are plotted versus distance from land, and the single station from the northern line (N7) is
plotted separately from the southern line transects. Figure taken from Wishner et al., 1998.
The responses of the zooplankton community structure to seasonal changes in the physical
environment have not been analysed fully yet for the investigations of the 1990s. There is one
trend, however, which has been noted in every seasonal study so far: the NE Monsoon is
characterized by increased abundance of cyclopoid copepods, such as the genera Oithona,
Oncaea and Corycella, when compared with the SW Monsoon (Smith, 1988; Madhupratap et
al., 1996; Smith et al., 1998). Small calanoid copepods predominate during the SW Monsoon
(Smith, 1982; Madhupratap et al., 1996; Smith et al., 1998). Reproductive activity of copepods
has been briefly discussed for a few selected stations of the U.S. JGOFS study area. At the
station with the strongest oxygen depletion, Pleuromamma indica was reproductive between 250
and 300 meters in both the NE and SW Monsoon seasons, with the addition of Euchaeta rimana
53
and Amallothrix indica in the SW Monsoon and Lucicutia grandis in the NE Monsoon (Smith et
al., 1998). In the upper 25 meters at this station, Undinula vulgaris and Paracalanus aculeatus
were reproductive in both the NE and SW Monsoons, with the addition of Cosmocalanus
darwini and Calocalanus plumulosus in the SW Monsoon and Acrocalanus longicornis, E.
rimana and Candacia pachydactila in the NE Monsoon (Smith et al., 1998).
At a station which is within the upwelling area during the SW Monsoon, the zooplankton at 250300 meters were dominated by P. indica, Oncaea subtilis and Clytemnestra scutellata in the NE
Monsoon and by P. indica, Eucalanus crassus, Eucalanus elongatus and Heterostylites
longicornis in the SW Monsoon. All of the SW Monsoon species have adult body lengths of
approximately 2 mm and larger; the shift to larger body size nearshore (in the upwelling area)
during the SW Monsoon is a feature not seen in other parts of the Arabian Sea. In the upper 25
meters at this station, the species most abundant during the NE Monsoon are smaller sized and
less abundant during the SW Monsoon. The most abundant species of the upper 25 meters in the
upwelling area during the SW Monsoon are Paracalanus aculeatus, Paracalanus denudatus and
Paracalanus dubius (Smith et al., 1998). Analyses of changes in the zooplankton communities at
various stations explained by seasonal shifts in the phytoplankton communities and mixed layer
dynamics is an area rich with possibilities for future study.
The Upwelling Area off Oman in the SW Monsoon
The upwelling areas of Oman and Somalia both contain at least one highly successful copepod
(35% of total copepod biomass, Smith, 1982), Calanoides carinatus, which probably evolved
during past periods of intensified and widespread upwelling (Fleminger, 1986). Its life history
evolved to exploit a seasonally predictable food supply (Smith, 1982). Another calanoid
copepod, Eucalanus crassus, has been observed co-occurring with C. carinatus off Somalia
suggesting a similar, yet undescribed, life history. Re-examination of samples of the upper 200
meters collected in the International Indian Ocean Expedition (IIOE) of 1964-1965 have shown
that C. carinatus was collected from 24°N to 29°S in the western Indian Ocean, and from the
coast of Somalia and Mozambique to Java (Stephen et al., 1992). These two species possess high
ingestion rates, and are probably capable of significantly impacting the phytoplankton
concentration and microplankton populations in the upwelling areas off Somalia and Oman.
Although the presence of C. carinatus and E. crassus at the surface is transitory and seasonal
(Figure 2; June-September), when they are present, they dominate the system, and are thus
potentially important prey items for macro-invertebrates and pelagic fishes during the upwelling
season.
The demographics of Calanoides carinatus have been described most fully in areas of upwelling
off the western coast of Africa, particularly off Ghana and the Ivory Coast. The species is in
surface layers during the upwelling season and spends the remainder of the year at mesopelagic
depths (Mensah, 1974; Binet and Suisse de St. Claire, 1975) where its respiration is fuelled by
stored lipids. The lipid that can be accumulated by copepodite stage V during feeding upon the
upwelling assemblage of the Benguela Current can be considerable, up to 71% of dry body
weight (Borchers and Hutchings, 1986). The evidence from the Somali coast is consistent with
patterns established for this species on the west coast of Africa. Secondary production, primarily
reproduction, is highest in cool, recently upwelled waters nearshore. As juveniles move offshore,
somatic growth becomes a larger fraction of total potential secondary production (Smith, 1992).
The life cycle of C. carinatus, wherever it is found, has distinctive characteristics that allow it to
exploit a predictably intermittent food supply (Smith, 1982). These characteristics are rapid
growth rate (Peterson and Painting, 1990), herbivorous feeding particularly on diatoms
(Schnack, 1982), massive lipid storage by subadult stages (Borchers and Hutchings, 1986), and
diapause at mesopelagic depths (Mensah, 1974; Binet and Suisse de St. Claire, 1975).
54
The distribution of Eucalanus crassus extends throughout seasonal upwelling regions within the
Indian Ocean (Tranter, 1977; data of S. Smith and M. Baars). Eucalanus crassus was most
abundant off the southern coast of Java during the SW Monsoon upwelling season (June-July;
Tranter, 1977). In the Arabian Sea, E. crassus co-occurred with C. carinatus off the coast of
Oman and Somalia (Smith, unpublished data). These species were present rarely at the surface
off the Omani coast during the non-upwelling season. During the non-upwelling season, C.
carinatus is found at depths of 200–1500 meters in the central Arabian Sea (Smith and Baars,
unpublished data).
What role do these large-bodied, migrating copepods play in the upwelling ecosystem? Do
ontogenetically migrating predators force ecosystem restructuring in the upwelling area?
Figure 2. Arabian Sea: annual (1995) cycle of wind speed (light blue line) measured by a
mooring positioned at 15°30’N, 61°30’E (redrawn from Weller et al., 1998); of total particulate
flux (green bars) measured by a sediment trap at 858 meters positioned at 17°13’N, 59°36’E
(redrawn from Honjo et al., 1999); of vertical distribution of the copepod Calanoides carinatus
stage V (yellow circles) collected throughout the northern Arabian Sea from Somalia to Oman
(Somali data provided by M. Baars; June data for Oman derived from samples collected by M.
Wood; remainder collected by S. Smith). Insets at the top show sea-surface temperature off
Oman obtained by the advanced very high-resolution radiometer (AVHRR) at various times
during the upwelling season in 1995. Darker blue depicts the area of cool sea-surface
temperatures (redrawn from Smith et al., 1998). Note how quickly the sea-surface warms
following cessation of upwelling-favourable winds in September.
55
Recommendations
1) Detailed comparisons of the species composition data from the 1960s and 1990s need to be
conducted to determine climate change effects on the zooplankton communities of the
Arabian Sea.
2) Detailed study of seasonal successions and life cycles of dominant zooplankton species in
this region where annual life cycles are common need to be conducted.
3) Seasonal successions in zooplankton communities need to be analysed in terms of seasonal
changes in the phytoplankton and microplankton communities and the physical environment.
4) Physiological adaptations of species adapted for life in the oxygen minimum zone need
investigation.
56
2.8 The Role of the Oxygen Minimum Zone in Biogeochemical Cycles
Sharon L. Smith
The Rosenstiel School, University of Miami
4600 Rickenbacker Causeway
Miami, FL 33149
USA
John M. Morrison
Department of Marine, Earth and Atmospheric Sciences
North Carolina State University, Raleigh, NC 27695
USA
Louis A. Codispoti
Horn Point Environmental Laboratories
University of Maryland, Cambridge, MD 21613
USA
Background
The semi-annual reversal in wind stress associated with the monsoon cycle, water mass
intrusions from marginal seas and the other oceans, and the fact that this basin has no opening to
the north and therefore no subtropical convergence or deep water formation, give the upper
waters of the Arabian Sea a unique thermohaline structure and circulation (Wyrtki, 1971;
Morrison and Olson, 1992; Morrison et al., 1998). The complex water mass structure is due in
part to advection and interleaving of water masses, and in part to formation of high-salinity
waters in the Red Sea, Persian Gulf and northern portion of the basin (Arabian Sea Water),
which sink to moderate depths in the central basin. For a summary of the physical domain of the
Arabian Sea, see the hydrography section (Chapter 2.2) of this report. Overall, the conditions
found in the suboxic portion of the water column in the Arabian Sea during JGOFS were not
greatly different from what has been reported in the literature with respect to oxygen, nitrate and
nitrite distributions (e.g., Naqvi, 1992).
Physical Characteristics
The horizontal extent of the oxygen minimum zone (OMZ) in the Arabian Sea may vary
seasonally, with its areal extent in the NE Monsoon exceeding that during the SW Monsoon
(Figure 1; Eremeev et al., 1994). The upper boundary of the OMZ often occurs at sigma-theta
values (~ 24.8 σθ) that are characteristic of the Arabian Sea Water (ASW) salinity maximum.
This feature has a maximum depth of ~150 m in the northern Arabian Sea. During the NE
Monsoon, this water mass can be replenished by convective processes in the northern Arabian
Sea (Morrison et al., 1998). Thus, the upper boundary of the OMZ receives some direct reoxygenation as a consequence of evaporative cooling and convection during the NE Monsoon.
Lower salinity water with σθ values in the range of ASW reach the sea surface in coastal
upwelling that occurs during the SW Monsoon along the coast of Oman (Morrison et al., 1998),
and mixing of these waters under the strong winds and intense eddy field of the SW Monsoon
also contributes to re-oxygenation of the upper waters of the OMZ.
57
Figure 1. The Arabian Sea showing the geographical position of the oxygen minimum zone
(dark blue) and the cruise tracks of the U.S. observational programme (red, yellow), U.S.
sediment traps (green triangles) and the air-sea interaction mooring (black bowtie).
The “core” of the suboxic layers with elevated nitrite values is more or less coincident with the
Persian Gulf Water (PGW) salinity maximum layer that has its “core” at a density of ~26.6 σθ.
Outflow from the Persian Gulf is only a minor component of the ambient waters in the northern
Arabian Sea, but it gives it a salinity marker that is closely associated with the suboxic portions
of the OMZ (Morrison et al., 1998).
The bottom of the suboxic portions of the OMZ are roughly coincident with the characteristic
density (~ 27.2 σθ) of Red Sea Water (RSW). This high-salinity outflow from the Red Sea sinks
to depths of ~ 750 m and forms a salinity maximum in portions of the Indian Ocean, but this
maximum is weak or absent from our stations in the northern Arabian Sea suggesting that RSW
mixes quickly with other water masses found at this density within the Arabian Sea, such as
Indian Central Water (You and Tomczak, 1993) and North Indian Intermediate Water (Kumar
and Li, 1996). The residence time of OMZ waters is relatively short, ca. 10 years (Olson et al.,
1993).
The combination of source waters impoverished in oxygen entering the Arabian Sea around the
tip of India (Swallow, 1984) or from the Southern Ocean (You and Tomczak, 1993), weak
aeration arising from a of lack of an opening to the north, and productivity year-round driven by
upwelling or convection delivering organic material to the already depleted subsurface waters
are sufficient to explain the OMZ. It is also possible that lateral transfer of organic material
58
settled on the seabed of the shelf and shelf break off India intensify decomposition processes
using oxygen (Somayajulu et al., 1996).
Kim and Flagg (2001) using TOPEX/Poseidon sea-level anomalies (SLA) and shipboard
acoustic Doppler current profiler (ADCP) data show that in the central Arabian Sea there is a
large increase in eddy kinetic energy in the west and southwest and to a lesser extent to the south
and a large area of relatively reduced eddy activity over much of the eastern and northern
Arabian Sea. The area of reduced eddy activity coincides with the location of the most intense
portions of the oxygen minimum zone found in the northern Arabian Sea. They show that not
only is the level of kinetic energy lower in this area but the area also shows reduced vertical
penetration of the eddy field. This suggests that at least part of the reason for the existence of the
OMZ in the area, or at least its proximity to the surface, may be the lack of mixing and vertical
motion associated with a vigorous eddy field.
During the SW Monsoon, the only organized circulation off the Arabian Peninsula is a
northeastward coastal jet that develops along the south coast of Oman in early May and persists
throughout the SW (summer) Monsoon. This Ras al Hadd Jet (Böhm et al., 1999) flows past Ras
al Hadd at the southwest corner of the Gulf of Oman and meanders into the interior, transporting
cooler, coastally upwelled waters with high biogenic content into the warmer environment of the
Gulf of Oman and northern Arabian Sea. This is the most plausible way to transport biological
material from the upwelling region into the eastern portion of the OMZ. The distribution of the
OMZ seems to match this hypothesis in that it is closest to the surface with the largest vertical
extent in the northeast and gets progressively deeper and thinner to the south and to the west
(Kim and Flagg, 2001).
Chemical Characteristics
The standard U.S. JGOFS Arabian Sea station sampling grid in relation to the oxygen minimum
zone (Figure 1; shaded area) where the work of Naqvi (e.g., Naqvi, 1991a; Naqvi, 1991b;
Naqvi, et al., 1992; Naqvi, 1994) suggests that average secondary nitrite maximum values are =
1 µM. These features are called secondary nitrite maxima because they normally occur deeper in
the water column than the primary nitrite maxima that are a typical feature near the base of the
euphotic zone and which arise from phytoplankton reduction of nitrate, nitrification or both
(Codispoti and Christensen, 1985). Such nitrite concentrations in suboxic water (dissolved
oxygen < ~0.1 ml l-1 or <~4.5 µM or <~4.5 µmol kg-1) indicate that denitrification is a prominent
respiratory process. Naqvi et al. (1992) point out that, unlike the denitrification zone off Peru
(Codispoti and Packard, 1980), the zones of lowest oxygen and highest primary production are
geographically separated in the Arabian Sea. The shaded region in Figure 1 indicates the
horizontal extent of the main denitrification zone, and it is obvious that it lies offshore of the
upwelling that is most intense adjacent to the Arabian Peninsula. This situation has led to
speculation about how organic matter is supplied to the denitrification zone, and the assumption
is that horizontal processes are important in supplying such material (Naqvi et al., 1992). It
should also be noted that transient secondary nitrite maxima occur outside of the shaded zone
(Morrison et al., 1998).
The U.S. JGOFS programme included measurements of trace metals and their alteration by the
oxygen minimum zone (Lewis and Luther, 2000; Witter et al., 2000). In the OMZ, two distinct
dissolved manganese (d-Mn) maxima were observed (Lewis and Luther, 2000) at depths of 200300 m and 600 m. The latter peak was largely confined to stations north of 19°N. This mid-depth
maximum was found in the low oxygen core of the OMZ in the region where denitrification was
observed and appears to be maintained by a southward, horizontal, advective-diffusive flux of
59
dissolved manganese from highly reducing Pakistan margin sediments. Nepheloid layers,
presumably bacterial, were also found at depths of 150 - 350 m (Gardner et al., 1999; Gundersen
et al., 1998; Morrison et al., 1998). Particulate Mn profiles displayed corresponding minima for
this same depth interval, suggesting reductive dissolution of Mn-oxyhydroxides. These
observations imply active, in situ, microbially mediated redox cycling of manganese in the upper
OMZ, as observed in other suboxic environments. Witter et al. (2000) measured organic Fe (III)
speciation at the surface and in the OMZ during the Spring Intermonsoon season, and found that
total ligand concentrations increased by a factor of 2-3 from the surface to the oxygen minimum
zone. Ligand concentrations were always higher than total iron concentrations. This suggests
that in the OMZ, ligands are produced during organic matter degradation. In addition, "excess"
ligand concentrations in the OMZ were 2 to 20 times higher than surface waters (upper 100 m).
Although stability constant values are comparable in magnitude to those reported in the Pacific
and northwestern Atlantic oceans, measured total ligand concentrations in the Arabian Sea are
higher. This suggests that in areas that receive high Fe inputs through upwelling and/or
atmospheric deposition, marine organisms produce "excess" ligands to keep Fe soluble in
seawater for extended intervals.
Biological Characteristics
The oxygen minimum zone, where it occurs, occupies a large segment of the water column
extending from ~100 meters to depths of 1000-1200 meters. Earlier work has noted that for
some species assemblages in the northwestern Indian Ocean, the numbers of species decline
progressing northward from the equator: copepods of the genus Pleuromamma (Saraswathy and
Iyer, 1986) and myctophid fish (Kinzer et al., 1993). The abundances of those few species that
have adaptations for the OMZ increase in the northern Arabian Sea. When the OMZ impinges on
the shelf break or shelf regions, catastrophic mortality occurs in the benthic communities
(Vinogradov and Voronina, 1962) and demersal fish populations (Banse, 1959). Large expanses
of dead fish at the surface in the open Arabian Sea have also been attributed to upward
movement of the oxygen-depleted waters (Vinogradov and Voronina, 1962; Foxton, 1965).
Vertical profiles of zooplankton biomass often show a large decrease of biomass within the
OMZ layer (Vinogradov, 1970).
The Pelagic Organisms. In the Arabian Sea, several distributional patterns show the impact of
the OMZ (Morrison et al., 1998; Herring et al., 1998): (1) exclusion from the suboxic (< 4.5
µM) core of the OMZ of most zooplankton biomass and many zooplankton and nekton species
and groups (Figure 2), but, paradoxically, (2) the occurrence of extremely high abundances of a
few species of diel vertical migrators at depth during the daytime, well within the suboxic zone
(Figure 2), (3) organism-specific (and probably species-specific) distribution boundaries at the
upper and lower edges of the OMZ, probably associated with particular oxygen concentrations,
and (4) the appearance of a subsurface abundance peak for many organisms and particles in the
lower OMZ associated with the oxygen gradient between 2.3 - 4.5 µM (0.05 - 0.1 ml l-1).
Oxygen values below about 4.5 µM that extend over vertical ranges of hundreds of meters are
associated with major distributional responses of the mesozooplankton and nekton, as seen in
other regions of the world’s ocean such as the Eastern Tropical Pacific Ocean. In these
situations, most zooplankton are absent from the suboxic part of the water column, and
biomasses at depth (except for the daytime vertical migrators) are extremely low (Wishner et al.,
1998). Presumably, most organisms were unable to tolerate the extremely low oxygen conditions
and were excluded from the OMZ by physiological constraints. The total water column biomass
(0 - 1000 m) in areas where the OMZ existed showed consistent vertical pattern. Biomass was
high in the surface layer, markedly reduced in the range of the OMZ, and increased again
60
beneath the OMZ, but not to levels observed in the surface layer (Wishner et al., 1998). At
stations away from the OMZ, the vertical profile lacked the marked reduction of biomass within
the OMZ. Investigation of the distribution of the swimming crab Charybdis smithii showed that
in its one year life cycle it remained above the OMZ at all times of the year, and moved inshore
over the continental shelves to breed during the NE Monsoon (van Couwelaar et al., 1997).
Occasionally high concentrations of dead C. smithii (1 m-2) are observed on the seafloor;
whether this is caused by fluctuations in the OMZ is unknown but when this occurs, the carbon
flux via C. smithii can be equal to 30% of the annual POC flux (Christiansen and Boetius, 2000).
The exclusion of larger organisms from OMZ depths suggests that sinking material transits this
zone relatively rapidly, unaltered by consumers, and arrives at depth as a relatively fresh food
source for deeper-living organisms.
The presence of a very high biomass of diel vertical migrators that moved between the surface
waters at night and the suboxic waters during the day was a surprising finding from the JGOFS
sampling (Morrison et al., 1998; Herring et al., 1998; Luo et al., 2000; Ashjian et al., 2002).
Many of these animals spent the day at depths where the oxygen was less than 0.1 ml l-1 (or 4.5
µM). It is notable that the depth to which vertical migrators descend (150-400 m) is similar to
the depths at which the intermediate nepheloid layers occur. Material transported and excreted
by migrators may form a substrate for denitrifying processes. Prominent diel vertical migrators
61
in samples analysed so far were euphausiids (especially Euphausia diomedaea) and several
species of myctophid fish (L. Madin, pers. comm.).
The ADCP showed the diel migration and the movement into and out of the OMZ of large
species, such as myctophid fish and euphausiids (Morrison et al., 1998; Luo et al., 2000), but a
similar migration of smaller species, such as copepods was not observed (Smith et al., 1998;
Wishner et al., 1998). Daytime acoustic returns from depth were strong, and the dawn sinking
and dusk rise of the fauna were obvious. However, at night the biomass remaining in the suboxic
zone was so low that no ADCP signal was detectable at these depths. ADCP records also
showed that there were at least two groups of plankton (used loosely), one that stayed in the
upper mixed layer and another that made daily excursions. As the gaps between the mixed layer
and the deep scattering layer indicate, the near-surface and deep plankton populations are
completely separate vertically, with few plankton found in between. This was also seen in the
MOCNESS profiles (Smith et al., 1998; Wishner et al., 1998). The acoustic records show that
the vertical migration was composed of separate groups that began their migrations at slightly
different times. These could be different size classes of myctophids as has been observed in the
Gulf of Oman (Valinassab, 2001).
An increase in zooplankton biomass and abundance typically occurred in the lower OMZ within
the depth range of the oxygen gradient between 0.05 - 0.1 ml l-1 (2.3 - 4.5 µM) (where oxygen
started to increase again) at locations with a strong and extensive OMZ (Wishner et al., 1998).
This subsurface biomass feature is also known to be a location of relatively high zooplankton
feeding rates and potential food resources (POC) (Wishner et al., 1998). In the Arabian Sea at
the depth of the subsurface biomass peak, a peak in aggregates was sometimes observed with a
camera profiler (Walsh, pers. comm.). The occurrence of high plankton and microbial biomass
and a potentially short food chain suggest the possibility of intense utilization and biological
alteration of suspended and sinking particles within a narrow midwater depth zone, defined
physically by the oxygen gradients of the OMZ. The copepod Lucicutia grandis is an indicator
of the lower OMZ interface; L. grandis occurs where the oxygen gradient is 0.07-0.15 ml l-1
(Wishner et al., 2000). In this habitat at 600-1000 metres and within the OMZ, L. grandis is
reproductive year-round and feeds on a wide range of material (surface–derived, deep-sea
detritus, zooplankton and aggregates; Wishner et al., 2000). Results from the JGOFS sediment
trap programme also suggest that substantial changes in the quantity and quality of the sinking
flux, possibly due to zooplankton processes, occurred within this depth zone (Lee et al., 1998).
Various taxa in the subsurface biomass peak showed different distributional responses to the
oxygen gradients, probably because of physiological differences in their ability to cope with low
oxygen. For example, the abundance peak of gelatinous taxa such as salps and doliolids was
relatively shallow, where oxygen was still very low in the OMZ. Crustaceans (copepods, shrimp)
and fish (Cyclothone) apparently required more oxygen and increased deeper in the water
column where oxygen was somewhat higher.
Another group of organisms, the seasonal migrants such as the copepod Calanoides carinatus,
spent a portion of their life cycle in the suboxic zone and in the more oxygenated water beneath
it, mostly in a non-feeding, diapausing stage. They rose into the surface waters in large numbers,
where they fed and reproduced (Smith et al., 1998) during the SW Monsoon. Thus, although the
resident zooplankton biomass in the core of the OMZ was extremely low, there were several
assemblages of organisms that inhabited parts of the suboxic zone, especially its upper and lower
edges, at different times of the day or year.
Zooplankton biomass profiles showed substantial geographic variability across the Arabian Sea,
but comparatively little change between seasons at particular stations, except in the surface water
62
and nearshore (Wishner et al., 1998). The consistency of the biomass profiles through time
implies long-term stability in the structure and function of the mid-water ecosystem over a broad
geographic region and shows the strong influence of the OMZ on the biology of the deep
Arabian Sea.
A deep chlorophyll a maximum showing high primary productivity was identified in the Spring
Intermonsoon of 1987 (Pollehne et al., 1993) but the cells composing the layer could not be
identified. The position of this layer at the bottom of the euphotic zone but in high nutrients
suggested it was a major component of new production and sub-euphotic zone flux. Its
relationship to the OMZ was not identified. In the Fall Intermonsoon of 1986, again subsurface
maxima of cyanobacteria were identified but the role of the OMZ was not discussed (Burkill et
al., 1993). During the U.S. JGOFS cruises of 1995, details of the oxic-suboxic interfaces of the
OMZ were investigated. Johnson et al. (1999) found secondary fluorescence maxima (SFM)
near the oxic-suboxic interface at the base of the euphotic zone. “These secondary peaks were
most prominent during the early NE Monsoons in the central oligotrophic portion of the Arabian
Sea…Based on high performance liquid chromatography…and flow cytometry analyses, SFM
were determined to be populated almost exclusively by the marine cyanobacterium
Prochlorococcus spp…. SFM were about half the magnitude of primary fluorescence
peaks…Photosynthesis versus irradiance response curves revealed an efficient population
adapted to extremely low light (~0.02-0.05% surface irradiance) largely through increased light
absorption capabilities…Deck-incubated C-14 uptake as well as dilution growth experiments
revealed instantaneous growth rates on the order of µ = 0.01/d…” In another study (Goericke et
al., 2000), the OMZ is implicated in the distribution and physiology of Prochlorococcus in the
Arabian Sea but not conclusively. They state, “…we report discovery of populations of
Prochlorococcus in layers below the oxyclines of the oxygen minimum zones of the Arabian Sea
and the Eastern Tropical Pacific off Mexico. The unusual aspects of these populations are that
these were at times virtual monoalgal cultures found at a depth of 80 to 140 m…As only
Prochlorococcus thrives in these layers but Prochlorococcus and eukaryotic picoplankton
coexist in and below subsurface chlorophyll maxima, we conclude that the low oxygen
concentrations at the deep Prochlorococcus maxima are the determining factor, but we are not
able to identify any specific physiological functions that are affected by low oxygen
concentrations in eukaryotes but not Prochlorococcus.” The last fifteen years have demonstrated
the huge impact of new instrumentation that permits routine identification and sorting of
planktonic individuals and new methods for describing plant pigments.
The Benthic Organisms. Coincident investigations addressed the effect of the OMZ on the
benthic biota. Where the OMZ impinges on the continental slope off Oman, spionid worms
showed significantly increased surface area of their gas-exchange organs (size and number of
Branchiae), which is presumably an adaptive response to the low oxygen (Lamont and Gage,
2000). The diversity and evenness of the benthic community impacted by the OMZ was reduced
compared to similar communities unaffected by suboxic conditions, but abundance and biomass
of the community in the OMZ were greater than outside the OMZ (Levin et al., 2000). The
community living in contact with the OMZ was composed primarily of worms and an ampeliscid
amphipod (Levin et al., 2000). Food availability (surface pigment concentration) and oxygen
concentration explained most of the variance in species richness, diversity and eveness.
Bioturbation is another aspect of the benthic habitat and geochemistry that is altered by an OMZ
impinging on a shelf or slope. Bioturbation in sediments of the Oman margin was reduced in
comparison with sediments impacted by oxygenated waters in the Atlantic and Pacific oceans
(Smith et al., 2000). It is noteworthy that in both the pelagic and benthic ecosystems, the OMZ
of the Arabian Sea occurs in association with reduced diversity, reduced evenness but increased
biomass of the few species which can withstand suboxic conditions.
63
Recommendations
1) Since the OMZ is a source of nitrogen to the atmosphere, and rates of denitrification here
may be much higher than previously estimated, the oceanic nitrogen cycle and its impact on
climate change need to be studied.
2) Daily migration of large biomass into and out of the OMZ, probably mostly myctophids but
also Euphausiids and copepods, suggests physiological adaptations that have not been
investigated yet.
3) The potential role of physical forcing in determining the position and strength of the Arabian
Sea’s OMZ needs explicit investigation.
4) The OMZ may be an indicator of climate change exhibiting rapid response to perturbation
and therefore merits monitoring in this context.
64
2.9 Sedimentation
T. Rixen*, M.V.S. Guptha+ and V. Ittekkot*
* Zentrum für Marine Tropenökologie, Universität Bremen
Fahrenheitstraße 6, D-28359 Bremen
Germany
+ National Institute of Oceanography
Dona Paula, Goa 403 004
India
Introduction
One of the major JGOFS tasks is to quantify the export of carbon from the ocean’s surface into
the ocean's interior, and to shed light on processes coupling carbon flux and climate. Sediment
traps are at present the only tools that can intercept directly and continuously particles settling
through the water column but their trapping efficiency, and thus, their accuracy can be biased by
hydrodynamic and biological effects (Lee et al., 1988; Gust et al., 1992; 1994). Scholten et al.
(2001) showed that the 230Thorium trapping efficiency of sediment traps moored at different
sites and water depths in eastern North Atlantic Ocean varies between 9 and 143%. In general,
these trapping efficiencies increase with increasing water-depth (Scholten et al., 2001). By
comparing data from various ocean basins, Yu et al., (2001), found that sediment trap data
obtained at water depths above 1200 m are problematic whereas traps deployed below that depth
exhibit acceptable trapping efficiencies. The latter is supported by the Indo-German sediment
trap experiment that showed co-variations between surface ocean processes and deep ocean
fluxes measured between 1986 and 1997 in the western, central and eastern Arabian Sea (e.g.,
Nair et al., 1989; Haake et al., 1993; Rixen et al., 1996; 2000).
During the Dutch, German, Indian and the U.S. JGOFS field experiments between 1993 and
1995 approximately 40 moored sediment traps were deployed at different water depths in the
Arabian Sea (Figure 1). The U.S. sediment trap moorings were arranged along a transect from
the highly productive upwelling area off Oman past the long-term Indo-German sediment trap
sites in the western and central Arabian Sea into the oligotrophic southern Arabian Sea. The
Dutch activities included sediment trap experiments in the upwelling area off Somalia and in
addition, one trap was deployed in the northern Arabian Sea in collaboration with Pakistan and
Germany.
The Monsoonal Impact on Deep Ocean Fluxes
The seasonal reversing monsoon winds over the Arabian Sea are stronger during summer (SW
Monsoon) and winter (NE Monsoon) than during the Intermonsoon periods. This is reflected by
increased particle fluxes in the deep western, central and eastern Arabian Sea pointing to a close
coupling between the monsoon and the deep sea (Nair et al., 1989). During the JGOFS field
experiments this bimodal, monsoon-driven flux pattern, was also observed in the northern
Arabian Sea off Pakistan (Suthhof et al., 1999) and along the U.S. JGOFS sediment trap transect
in the western Arabian Sea (Lee et al. 1998; Honjo et al., 1999) and off Somalia (Broerse et al.,
2000). In the southern open Arabian Sea this pattern is only weakly evident and the peak organic
carbon fluxes are even lower than those during the Intermonsoon periods at the other sites
(Honjo et al., 1999; Rixen et al., submitted). This implies that the southern boundary of the
monsoon-driven high flux region is located somewhere between 10 and 15°N in the open
Arabian Sea.
65
Within the monsoon-driven, high flux region, the organic carbon transport into the deep sea is
generally lower during the NE than during the SW Monsoon. This difference is most
pronounced off Somalia (Broerse et al., 2000), in the western Arabian Sea and disappears almost
entirely in the northern Arabian Sea off Pakistan where the NE and SW Monsoon fluxes are
almost equal (Suthhof et al., 1999). However, these results demand interdisciplinary
investigations such as those carried out during the international JGOFS to elucidate the
processes that transfer monsoonal signals into to the deep sea.
Figure 1. Sediment trap sites in the Arabian Sea.
Black dots show the long-term Indo-Germans
sediment trap locations (deployed since 1986),
black squares indicate the Dutch sediment trap sites
off Somalia (deployed: 1992/1993; e.g., Koning et
al., 1997), light grey circles reveal the U.S.
locations (deployed: 1994/1995; Lee et al., 1998,
Honjo et al., 1999) and the open circle shows the
German/ Pakistan site (deployed: 1994/1996;
Suthhof et al., 1999). The lower panel shows the
mean SW Monsoon fluxes obtained by traps
deployed at a water-depth of approximately 3000 m
versus the distance between the trap site and the
coast. Data are from Lee et al. (1998; grey circles)
and from Rixen et al. (submitted, black circles).
The Southwest (SW) Monsoon
The SW Monsoon is characterised by a low level Jet (Findlater Jet) blowing almost parallel to
the Arabian coast over the Arabian Sea (Findlater, 1966, 1969; Rixen et al., 1996). In the
western Arabian Sea north of the axis of the Findlater Jet, the monsoon winds induce upwelling,
which is strongest along the Oman coast (e.g., Luther et al., 1990; Brock et al., 1991; Rixen et
al., 2000). South of the Findlater Jet axis, downwelling occurs due to the wind stress curl.
During the U.S. JGOFS field experiments, the ONR Arabian Sea Central Mooring (ASCM) was
deployed almost below the axis of the Findlater Jet between the western and central Arabian Sea
trap sites (Dickey et al., 1998). It revealed a deepening of the mixed layer during the initial onset
of the SW Monsoon in 1995, while in the course of the SW Monsoon the mixed layer became
shallower (Figure 2). The initial mixed layer deepening was due to downwelling or wind
stirring, whereas the decreasing mixed layer depth can result from upwelled water propagating
offshore. This agrees with the mean SW Monsoon organic carbon fluxes that generally decrease
with increasing distance from the coast (Figure 1). The deviation from this trend close to the
coast is probably caused by marine organisms which are assumed to be exported into the deepsea during their later resting stage and not during their earlier growing stage (Smetacek, 1985).
On the other hand, spatial and temporal variations during the advection of upwelled water
(Manghnani et al., 1998) can also contribute to the observed deviation.
However, during the early onset of the SW Monsoon, increasing organic carbon fluxes are
associated with enhanced carbonate to biogenic opal ratios at all long-term trap sites (Figure 3;
Rixen et al., submitted). This indicates an increasing share of carbonate-bearing organisms in the
organic matter exported into the deep sea. During the SW Monsoon, the carbonate-to-biogenic
66
opal ratios decrease due to a higher contribution of silicate-bearing organisms such as diatoms to
the exported matter. This feature is well known in the western Arabian Sea and has been related
to the nutrient distribution in the water column showing an increase in silicate concentrations
that is steeper than that of nitrate and phosphate (Haake et al., 1993). The initial upwelling or
mixed layer deepening seems, thus, to be too weak to carry sufficient silicate into the euphotic
zone to favour diatom blooms. This situation changes during the later phase of the SW Monsoon
when an increasing influence of coastal upwelling leads to diatom blooms at the trap site in the
western Arabian Sea (Rixen et al., 2000). Detailed plankton studies during this period revealed
that flagellates succeed diatoms between the western and central Arabian Sea (Garrison et al.,
2000) probably due to the lack of dissolved silicate. This might be responsible for the relatively
high carbonate/biogenic opal ratios during the later phase of the SW Monsoon in the central
Arabian Sea compared to those measured at the other sites (Figure 3).
Figure 2. Depth of the mixed layer (MLD black line) and euphotic zone (1% PAR light level;
green line) measured at the ONR central Arabian Sea mooring between October 1994 and 1995.
The data are redrawn from Dickey et al. (1998). MLD represents the water-depth at which the
temperature is 1°C lower than at the surface. Additionally, the organic carbon fluxes (blue line)
and the carbonate biogenic opal ratio (red line) determined at the sediment trap site (WAST)
during the year 1995 are shown.
Tracer experiments carried out with a 3-D coupled ocean-atmosphere model (Maier-Reimer,
submitted) indicate that nutrients upwelled off Oman can propagate along with the coastal
current into the northern Arabian Sea, and further along the Indian coast into the eastern Arabian
Sea. Thus, the enhanced fluxes at these sites might also be influenced by the upwelling off
Oman. This implies that the western Arabian Sea upwelling influences the peak organic carbon
export throughout the open Arabian Sea north of at least 15°N during the SW Monsoon and also
causes the low carbonate/biogenic opal ratios in eastern Arabian Sea. Off Somalia, the high SW
Monsoon fluxes are linked to upwelling occurring at the western exteriors of the Great Whirl
(e.g., Conan and Brummer, 2000).
To gain more quantitative information on the link between deep ocean fluxes and upwelling,
biweekly averaged wind fields derived from satellite measurements for the period between 1987
and 1995 were converted into up- and downwelling velocities. Subsequently, upwelling
velocities off Oman were correlated with deep ocean fluxes obtained at the western Arabian Sea
trap site (Rixen et al., 2000). This analysis revealed that changes in the upwelling velocities
caused by variations in the strength of the Findlater Jet exert the main control of organic carbon
67
fluxes into the deep western, (Rixen et al., 2000b), and most likely also into the central, eastern
and northern Arabian Sea, during the SW Monsoon.
Figure 3. The mean annual organic carbon fluxes and carbonate to biogenic opal ratios at the
trap sites in the western (WAST), central (CAST) and eastern Arabian Sea (EAST).
The Northeast (NE) Monsoon
During the NE Monsoon, mixed layer deepening caused by winter cooling and wind mixing
entrains nutrient-enriched subsurface waters into the euphotic zone (e.g., Banse and McClain,
1986; Madhupratap et al., 1996). The resulting phytoplankton bloom has been assumed to cause
the enhanced organic carbon fluxes measured in the deep western, central and eastern Arabian
Sea (Nair et al., 1989; Haake et al., 1993). During the NE Monsoon of 1994/95, the ONR
ASCM buoy recorded the deepest mixed layer approximately between December and February
(Figure 2). The respective euphotic zone was almost half as deep as the mixed layer and the
corresponding organic carbon fluxes in the western Arabian Sea were even lower than during the
following Spring Intermonsoon period (May/June; Figure 2; Rixen et al., submitted). The peak
organic carbon flux occurred at the end of the NE Monsoon as the mixed layer became shallower
68
than the euphotic zone. High organic carbon fluxes at the end and low fluxes during the peak of
the NE Monsoon are a regular, repeating feature that have been observed not only in the western
but also in the central and eastern Arabian Sea. A second organic carbon flux peak was generally
measured at the beginning of the NE Monsoon in December during the initial deepening of the
mixed layer (Rixen et al., submitted). A similar bimodal flux-pattern during the NE Monsoon
was also observed at the U.S. and the Dutch sediment trap sites, which shows that it is a
characteristic feature in the NE Monsoon season in the Arabian Sea (Prahl et al., 2000; Broerse,
et al., 2000). The interrelation between mixed layer depth and depth of the euphotic zone leading
to the early and late NE Monsoon blooms is similar to those causing autumn and spring blooms
in the temperate open oceans (Kiefer and Kremer, 1981).
In the Arabian Sea, the bloom of the early NE Monsoon is associated with enhanced carbonate/
biogenic opal ratios, whereas the sinking particles during the bloom of the late NE Monsoon
reveal low carbonate/biogenic opal ratios (Figure 3). This indicates that in contrast to the early
NE Monsoon bloom, the late NE Monsoon organic carbon flux peak is mainly driven by diatom
blooms. Since the early NE Monsoon, bloom is caused by the initial deepening of the mixed
layer and the late NE Monsoon bloom succeeds the maximum mixed layer depth, the shift in the
phytoplankton composition can be related to the depth from which the water in the euphotic
zone originates.
Organic Carbon Export into the Ocean’s Interior
The organic carbon flux from the euphotic zone into the deep-sea and its remineralisation in the
water column is assumed to be associated with primary production (Deuser and Ross, 1980).
Primary production and sediment trap data are used to develop models describing this link by
including an additional term accounting for the influence of water-depth on organic carbon
fluxes (e.g., Suess, 1980; Betzer, 1984; Pace et al., 1987; Berger; 1988). These models have
been applied for global analysis (e.g., Jahnke, 1996) and are often integrated into large
circulation models utilised for climate predictions (e.g., Maier-Reimer, 1993). The organic
carbon export derived from the 234Th-method reveal, in contrast to the assumption within the
carbon flux models, a decoupling between primary and export production (Buesseler, 1998;
Buesseler et al., 1998). However, the annual averaged primary production rates (Buesseler et al.,
1998) converted into deep ocean organic carbon fluxes by using the models, correlate with the
organic carbon fluxes measured by sediment traps at water-depths > 1000 m (Table 1). In
addition to this, the model introduced by Pace et al. (1987), shows the best quantitative
agreement between measured and modelled data (Figure 4). The correlation can be improved by
running the models with primary production rates derived from the CZCS between 1978 and
1986 (e.g., Antoine et al., 1996; Behrenfeld and Falkowski, 1997; Table 1). Here, the primary
production rates were averaged for an area of one degree around the trap sites and the U.S. flux
data were reduced by 11% since the 1994 and 1995 data are 11% above the mean annual flux as
indicated by the long-term record in the western Arabian Sea (Rixen et al., 2000b). Despite the
improved correlations, the data derived from the Pace et al. (1987) model are slightly lower than
the measured ones (Figure 4). This might be attributed to an underestimation of the satellitederived mean annual primary production rates due to insufficient data coverage during the
highly productive SW Monsoon (Banse and English, 2000). Nevertheless, the agreement
between modelled and measured data can be improved by modifying the coefficients or by using
the coefficients originally introduced by Betzer et al. (1984) (Table 1; Figure 5).
The ratios between primary and export production implied by Betzer et al. (1984) are higher
than those reported by Buesseler (1998; generally < 0.1) and Buesseler et al. (1998; Figure 6).
69
Figure 4. Annual mean organic carbon fluxes measured at the U.S. and the Indo-German trap
sites at water-depths > 1000 m versus fluxes calculated with the equation introduced by Pace et
al. (1987) driven by primary production rates obtained from Lee et al. (1998, black circles),
from Antoine et al. (1996; grey circles) and Behrenfeld and Falkowski (1997; open circles).
Ref.
Berger et al., 1988
Berger et al. 1988
Suess, 1980
Betzer et al., 1984
Pace et al., 1987
This work
Equations
J=17/z + 0.01 pp
J=9pp/z + 0.7 pp/z0.5
J=pp/(0.0238z + 0.212)
J=0.409pp1.41/z0.628
J=3.523pp/z 0.734
J=0.01pp2/z0.628
1
(n=12)
2
(n=19)
3
(n=19)
r
0.717
0.721
0.683
0.699
0.712
0.653
r
0.892
0.842
0.706
0.898
0.798
0.929
r
0.933
0.929
0.880
0.932
0.918
0.923
Table 1: Correlation coefficients derived from the correlation between modelled and measured
fluxes. 1,2,3 indicate that different primary production rates (PP) are used to calculate fluxes. “z”
represents the water (trap) depth and “n” reveals the number of data, which equals the number of
traps. Correlation coefficients marked bold are shown in Figures 4 and 5.
1 PP from: Lee et al. (1998)
2 PP from: Antoine et al. (1996)
3 PP from: Behrenfeld and Falkowski (1998)
Accordingly, the Betzer et al. (1984) model possibly overestimates the export production by
applying it for a global analysis. The primary and export production ratios obtained from Pace et
al. (1987) are within the range of those measured in the Arabian Sea (Buesseler et al., 1998) but
are above 0.1. Consequently, export production rates, especially in low productive regions,
might also be overestimated by using this model. Our model produces primary and export
production ratios higher than those in the Arabian Sea but it meets the results presented by
Buesseler (1998) when the primary production is < 180 g C m-2 a-1.
70
Figure 5. Annual mean organic carbon fluxes measured at the U.S. and the Indo-German trap
sites at water-depths > 1000 m versus modelled fluxes. “PP”=Primary production; “TP”=
sediment trap depth.
Figure 6. Primary versus export production as derived from various equations and from Arabian
Sea data (circles; data are from Buesseler et al., 1998).
To quantify carbon export into the ocean interior we used the model introduced by Pace et al.
(1987) and our equation. The primary production rates (Antoine et al., 1996) were interpolated
on a one-degree grid. The resulting export production into the Arabian Sea north of 6°N (3.52 X
1012 m2 excluding shelf and slope regions; water-depths > 2000 m), ranges between 84 to 91 X
1012 g C a-1. This is approximately 50% lower than the export production rates derived from
nitrate assimilation measurements (Watts et al., 1999; Bange et al., 2000). However, the global
organic carbon flux at a water-depths of 2000 m resulting from our equation and from Pace et al.
(1987) agrees quite well with the mean annual fluxes derived from a global summary of
71
sediment trap data (340 X 1012 g C a-1; Lampitt and Antia, 1997; Table 2). Overall, these data
show that the Arabian Sea accounts for approximately 3% of the global organic carbon transport
into the ocean’s interior.
Table 2. Annual mean carbon fluxes derived from primary production rates (Antoine et al.,
1996; Behrenfeld and Falkowski, 1997). PP = primary production; Exp100 = export production;
Exp2000 = organic carbon flux at 2000 m water-depth; Surf. Sed. = the organic carbon flux which
reaches
the
sediment
surface
(water-depths
were
obtained
from:
http://ingrid.ldgo.columbia.edu/SOURCES/WORLDBATH).
J=0.01pp2/z0.628; PP=1
J=3.523pp/z 0.734; PP=1
J=3.523pp/z 0.734; PP=2
2
0.628
J=0.01pp /z
; PP=1
0.734
J=3.523pp/z
; PP=1
J=3.523pp/z 0.734; PP=2
Exp.100
Exp.2000
PP
[1012 C g]
[1012 C g]
[1012 C g]
Arabian Sea (area: 3.5 1012 m2)
700.1
86.3
13.2
700.1
84.0
9.3
761.7
91.4
10.1
9.8
6.4
7.1
global (area: 302.1 012 m2)
29924.9
1890.7
29924.9
3588.7
34484.6
4135.6
188.1
238.0
274.0
288.1
398.1
458.8
Surf. Sed.
[1012 C g]
1 Antoine et al. (1996); 2 Behrenfeld and Falkowski (1997)
Deep-sea Fluxes and Sedimentary Organic Carbon Burial
The difference between the deep-sea organic carbon fluxes and the organic carbon burial rates in
the underlying sediments allows us to estimate the remineralisation rates of organic carbon at the
sediment-water interface. Nevertheless, it should be considered that the top 1 cm of the surface
sediment should have an age of several hundred years according to the sedimentation rates
published by Paropkari et al. (1992) and the longest record of deep-sea fluxes is approximately 9
years (Rixen et al., submitted). However, Jahnke (1996) used calcium carbonate-corrected
organic carbon burial rates and data on oxygen consumption measured in the Atlantic and
Pacific oceans to study the relation between organic carbon fluxes and burial rates. He
introduced a regression equation that describes the relationship between oxygen consumption
and calcium carbonate-corrected organic carbon burial. After converting oxygen consumption
into organic carbon remineralisation, this equation could be transformed so that it allows the
calculation of the organic carbon fluxes at the sediment surface. This transformed equation fits
the data derived from the Arabian Sea (Rixen et al., 2000b), which supports the global validity
of the equation introduced by Jahnke (1996). However, Jahnke (1996) suggested that globally
396 X 1012 gC y-1 reaches the surface sediments, whereas our analysis results in an organic
carbon flux of 188 to 274 X 1012 gC y-1 (Table 2). Although this, in addition to the results
discussed previously, indicates an agreement between the different global approaches for
estimating the deep ocean carbon budget, within the order of a factor of two, there are significant
local mismatches. For example, our analysis shows higher organic carbon fluxes along sea floor
spreading zones compared to Jahnke (1996) (Figure 7).
72
Figure 7. Annual mean organic carbon fluxes reaching the sea floor (water-depths > 2000 m)
derived from primary production rates (Antoine et al., 1996), water-depth
(http://ingrid.ldgo.columbia.edu/SOURCES/WORLDBATH) and the equation: POC flux =
0.01pp2/z0.628 (pp=primary production, and z= water depth).
Conclusion
Similar to temperate open ocean environments, variations of the depth of the mixed layer and
the euphotic zone control the organic carbon export in the Arabian Sea except in regions
influenced by upwelling during the SW Monsoon. The strength of the upwelling and the height
of the subsequent export of organic matter are mainly determined by the intensity of the
Findlater Jet.
Moreover, the strength of the physical forcing mechanism, also influences the plankton
composition whereby stronger forcing favours diatom blooms and lowers the carbonate/biogenic
opal ratios. Such changes, so far attributed to an enhanced dust supply, are assumed to have
lowered the atmospheric CO2 concentration significantly during glacial times (Harrison, 2000;
Tréguer and Pondaven, 2000). The marine silica cycle, however, is still poorly understood and
new investigations are needed to adequately integrate the silica cycle into global models (Dittert
et al., 2001).
Satellite-derived primary production rates can be used to estimate deep ocean fluxes on
annual time-scales by using organic carbon models such as those introduced in this study and by
Pace et al. (1987). In the Arabian Sea, the differences between modelled and measured deep
fluxes are lower than by a factor two. Extending this approach globally results in a mean deepsea flux, which meets those derived from sediment trap data within the same error range. The
implied primary/export production ratios are close to those reported by Buesseler (1998).
The export production rates for the Arabian Sea range between 83 and 91 gC y-1 and account
for approximately 3% of the global open ocean export production (excluding shelf and slope
regions). This suggests that the export production in the Arabian Sea is close to the estimated
annual emissions of CO2 from the Arabian Sea into the atmosphere (74-94 X 1012 gC;
Somasundar et al., 1990; George et al., 1994; Körtzinger et al., 1997; Ewald, 1998) and more
than twice as high as those suggested by Goyet et al. (1998). Accordingly, small changes in the
marine biosphere could have the potential to influence the CO2 emissions significantly.
In the Arabian Sea, organic carbon burial rates can be related to primary production and
carbonate accumulation rates as suggested by Jahnke (1996). Jahnke’s and our budgets reveal an
acceptable agreement on a global scale. Problems, however, occur on regional scales in, for
example, sea floor spreading zones and continental margins.
73
2.10 Arabian Sea Benthic Dynamics
Greg Cowie
Marine and Environmental Geoscience Group, Department of Geology and Geophysics
University of Edinburgh, West Mains Road, Edinburgh EH9 3JW
UK
Chemical and biological processes occurring in the benthic boundary layer (BBL) and surficial
sediments are of central importance to the biogeochemical cycling and fluxes of numerous key
elements, and thus were an important focus of the international JGOFS research programme.
The Arabian Sea was recognised as an exceptional site for studies of these processes (cf. JGOFS
Report 17). Strongly seasonal and spatially variable productivity results in a correspondingly
variable supply of organic material to the BBL. The different margins (e.g., Africa/Arabia versus
India/Pakistan) experience widely different oceanographic conditions (circulation, upwelling,
lithogenic input, seasonality, and so forth). These differences, combined with an intense, basinwide oxygen minimum zone (OMZ) that impinges on the continental margins, result in an
exceptional range of BBL and sedimentary environments, with respect to critical factors
including food supply, redox conditions, and the nature and activity of benthic communities.
Therefore, different regions of the Arabian Sea, and locations across its margins, offer striking
contrasts likely to provide important and generally applicable insights on the nature of benthic
processes and on their significance to the global cycles of key elements and to the makeup of
sedimentary records.
Because of the unusual conditions that prevail, the sediments and benthic processes of the
Arabian Sea are also of major significance in their own right. For example, high accumulation
rates, particularly on the margins, produce detailed records of past changes in climatic and
oceanographic conditions (e.g., in the intensity of the monsoons and associated upwelling and
productivity, and in the extent and intensity of the OMZ). In some locations (e.g., the Indus
margin), surficial sediments from within the OMZ are undisturbed by macrofauna, resulting in
laminated (varved) sediments offering particularly detailed Holocene records (ca. 1 mm y-1).
Furthermore, margin sediments in the Arabian Sea are, in general, unusually rich in organic
matter. This may represent a globally significant long-term carbon sink. Indeed, such margin
settings, with high productivity and severe oxygen depletion, are often cited as the type of
depositional environment for organic-rich facies found in the geological record. Benthic studies
under the strongly contrasting depositional conditions across the Arabian Sea are likely to
provide new, comparatively unambiguous, information on the relative importance of
contributing factors (e.g., productivity versus oxygen depletion), which remains the subject of
considerable debate.
Benthic processes within the OMZ of the Arabian Sea are also likely to be important to the
global cycles of N and P and to the generation of the mid-water OMZ. The relative importance
of sedimentary denitrification in the Arabian Sea remains unclear, but, given the areal extent of
the OMZ and the amount of organic matter remineralised across the BBL, it may be significant
relative to rates occurring in the water column, which account for ca. 30% of the global pelagic
total. Consequently, the sediments may also make a significant contribution to the net flux of the
greenhouse gas N2O that occurs from the Arabian Sea. Similarly, burial of authigenic P in the
reducing margin sediments of the Arabian Sea has potential global significance for oceanic P
inventories and productivity. The contrasting redox conditions across the margins of the Arabian
Sea also potentially offer valuable information on the cycling of redox-sensitive metals and on
the nature and importance of suboxic microbial processes that use Fe and Mn oxyhydroxides as
74
electron acceptors. Studies of these and other biogeochemical processes should clarify the role of
the sediments in determining the oxygen depletion and other unusual features of Arabian Sea
water-column chemistry.
In recognition of the broad significance of Arabian Sea benthic processes, benthic studies
became an important objective of the international JGOFS research programme (see JGOFS
Report 17). The proposed studies built on various investigations of the physical, biological and
geochemical characteristics of surficial deep-sea and marginal Arabian Sea sediments carried out
over the last century during several past expeditions to the Arabian Sea. The U.S. JGOFS
programme (1994-1996), involving some 17 cruise legs, was largely centred on water-column
processes and was carried out mainly in the western basin of the Arabian Sea and on the Oman
margin. However, in addition to some sediment coring, the programme included sediment trap
deployments in various locations and with differing sampling strategies (depths, durations,
sampling intervals), which are providing important information on the magnitude, composition
and seasonality of particle fluxes to the underlying BBL, and the degree of alteration that occurs
in the water column.
Benthic processes were a more central focus of the Dutch, UK, German, and Indian
programmes. The Dutch programme involved 11 RV Tyro cruises in 1992-1993 and included an
important benthic component (geological and biogeochemical) focused on both deep-sea and
marginal sediments (both Oman and Pakistan margins). Eight German JGOFS cruises were
carried out in 1995-1997 and included sediment trap deployments as well as a scientific
programme that included a major sedimentary component, focused on benthic geochemistry and
biology (microbial and macrofaunal). The major study sites were at five abyssal stations where
long-term sediment trap deployments were made, in the northern, southern, eastern, central and
western basins, but ancillary FS Sonne cruises (PAKOMIN, 1993) also involved trap
deployments and detailed sediment studies on transects across the Pakistan margin. The UK
JGOFS programme (ARABESQUE) involved four RRS Discovery cruises in 1994, one of which
was dedicated to linked studies of the biology and biogeochemical processes of sediments on the
southern Oman margin (biological surveys, sediment accumulation and bioturbation rates,
benthic respiration and fluxes, organic geochemistry). Finally, the Indian JGOFS programme
involved 5 cruises on ORV Sagar Kanya (1992-1995) in the eastern Arabian Sea, and included
studies of active sedimentary processes (sediment trap deployments and sediment coring), with
the objectives being to ascertain burial rates of biogenic elements in deep-sea and marginal
regions and the role of the continental margins in the removal/supply of material to the Arabian
Sea.
The Arabian Sea JGOFS studies outlined above are at various stages of completion and in many
cases, the results have yet to be published. Nonetheless, some important new findings that
concern benthic processes in the Arabian Sea have appeared recently, as a result of JGOFS and
ancillary research projects. The subject areas of both geochemical and biological studies are
diverse.
First, as outlined above, a number of programmes (German, U.S. and Indian), in both deep-sea
and margin settings, have included sediment trap deployments at various depths and durations
(e.g., Lee et al., 1998). These are yielding valuable information on the magnitude, seasonality
and composition of the pulsed particle inputs to the BBL, how these vary with region, and how
they impact benthic communities, fluxes and C cycling (e.g., Andruleit et al., 2000; Boetius and
Lochte, 2000; Boetius et al., 2000a,b; Broerse et al., 2000; Conan and Brummer, 2000; Grandel
et al., 2000; Honjo et al., 1999; Koppelmann et al., 2000; Pfannkuche et al., 2000; Rixen et al.,
2000a,b; Sommer and Pfannkuche, 2000; Turnewitsch et al., 2000; Witte and Pfannkuche, 2000;
Witte, 2000). Further information is provided on the nature and extent of particle alteration
75
within the water-column or across the BBL (e.g., Hedges et al., 2001), and implications for
interpreting sedimentary records (e.g., Prahl et al., 2000), and on the relative importance of
benthic processes in generating the OMZ and other features of water-column chemistry. The
recent trap deployments build on near-continuous deployments made by German and Indian
groups at open-ocean sites in the eastern, central and western Arabian Sea since the 1980s (e.g.,
Ramaswamy and Nair, 1994; Rixen et al., 1996).
Studies of benthic biogeochemical processes during the Dutch, UK and German expeditions
included various down-core investigations of the solid-phase and pore-water chemistry of
surficial sediments from both marginal and deep-sea sites. Detailed profiling of oxygen,
nutrients and trace metals in the porewaters was carried out, coupled with determinations of
solute fluxes across the benthic interface (via benthic chambers) and uptake rates of different
electron acceptors. Along with assessments of microbial community structure and activity, these
results provide key information on the role of benthic processes in C cycling in the Arabian Sea,
and how C burial and the contributions of aerobic and anaerobic processes vary with location.
Comparatively little of the work on process rates is published yet. Somayajulu et al. (1996) used
Ra isotopes to estimate the contribution of margin sediments to total denitrification in the
Arabian Sea, and Schenau et al., (2000) recently provided evidence for active phosphorite
formation in surficial OMZ sediments. Schmaljohann et al. (2001) also have reported on the
occurrence of sulphur oxidizing bacteria in OMZ sediments, which, along with biomarker and
stable isotopic data (e.g., Cowie et al., 1999; Schulte et al., 2000), may indicate an important
role for chemoautotrophic C sources within the OMZ. Other published studies on benthic
processes have mainly been limited to abyssal sites from contrasting regions of the Arabian Sea.
These have included studies on near-bottom and sedimentary bacterial activity (Boetius and
Lochte, 2000; Boetius et al., 2000a, b), benthic respiration rates (Witte and Pfannkuche, 2000)
and nutrient fluxes (Grandel et al., 2000), bioturbation rates (Turnewitsch et al., 2000), sulphur
isotopes as indices of sulphate reduction rates (Bottcher et al., 2000) and distributions of
biogenic compounds as indices of benthic community activity (Pfannkuche et al., 2000), and
how these respond to temporal and spatial variability in particle fluxes. Luff et al. (2000) used a
biogeochemical modelling exercise to compare C cycling and benthic process rates at the same
sites, based on observed porewater and solid-phase compositions. Other published compositional
information on surficial Arabian Sea sediments includes assessments of the extent of sulphate
reduction (e.g., Littke et al., 1997; Luckge et al., 1999), clay mineral distributions (e.g., Rao and
Rao, 1995; Chauhan and Gujar, 1996) and the biochemical, inorganic and stable-isotopic
compositions of Pakistan, Indian and Oman margin sediments (e.g., Satyanarana and Ramana,
1994; Calvert et al., 1995; Crusius et al., 1996; Nath et al., 1997; Passier et al., 1997; van der
Weijden et al., 1998; Babu et al., 1999; Cowie et al., 1999; Keil and Cowie, 1999; Smallwood
and Wolff, 2000; Schulte et al., 2000; Suthhof et al., 2000). Organic geochemical studies have
also included determination of radiocarbon ages of individual organic compounds (Eglinton et
al., 1997), and calibration of unsaturation indices in long-chain ketones (Uk’37) in surficial
sediments against measured sea-surface temperatures (SST; Sonzogni et al., 1997a, b) for
subsequent use in paleo-SST interpretations (e.g., Rostek et al., 1997).
Many of these geochemical characterisations and process rate determinations also will contribute
to ongoing debates on the processes that control organic matter distributions in marine sediments
and, specifically, on the factors that lead to the organic-rich sediments found on the continental
margins of the Arabian Sea. The central issue is whether oxygen depletion is a primary,
independent cause of organic matter enrichment. As examples, Pedersen et al. (1992) and
Calvert et al. (1995) found no clear physical relationship between the depths of the sedimentary
organic matter maximum and the mid-water oxygen minimum, and no consistent difference in
bulk organic matter composition (C/N ratios, hydrocarbon content, δ13Corg) for surficial
76
sediments within and outside the OMZ, on the Oman and Indian margins, respectively. They
concluded that factors including winnowing and offshore changes in productivity, depth and
sedimentation rate are responsible for mid-slope maxima in organic matter concentrations, along
with sediment texture and dilution effects. Similar conclusions were drawn by Thamban et al.
(1997), Rao and Veerayya (2000) and Babu et al. (1999), but also invoking an effect of bottom
topography (enhanced accumulation on marginal highs). In contrast (also as examples),
Paropkari et al. (1992) concluded that the overall distribution of organic matter in Arabian Sea
sediments could only be explained by preferential preservation in the absence of oxygen as the
dominant influence. More recent comparisons of the organic matter contents of surficial
sediments (van der Weijden et al., 1998) and longer sediment records (e.g., Reichart, 1998) from
the Pakistan margin and Murray Ridge also conclude that oxygen depletion is of primary
importance, possibly because the OMZ is more intense in this area (undisturbed, varved
sediments within the OMZ, in contrast to bioturbation across the entire Oman margin).
However, more detailed organic geochemical studies suggest that the controls are complex.
Comparisons with faunal abundances and bioturbation indicate that benthic macrofauna may
form a major control on the distributions of organic matter content and quality across the Oman
margin (Smallwood et al., 1999; Smallwood and Wolff, 2000), not clearly linked with bottomwater oxygen concentrations. Other biochemical analyses also show compositional differences
in surface sediments across the Pakistan margin (e.g., Cowie et al., 1999; Suthhof et al., 2000;
Schulte et al., 2000), but, in general, these are subtle and organic-rich sediments occurring under
more oxidizing conditions below the strict OMZ indicate that factors other than oxygen
depletion strongly influence mid-slope organic matter enrichments. If cross-margin differences
in organic matter content are due to differences in preservation, one of the more enigmatic
results of recent studies of surficial Arabian Sea sediments is that there is little indication of
down-core sedimentary organic matter decay. Moreover, although net sulphate reduction is
evident even in abyssal sediments, particularly at depth (e.g., Luckge et al., 1999; Bottcher et al.,
2000), it is at most moderate and insufficient to significantly deplete porewater sulphate
concentrations within the surficial 50 cm, even in the core of the OMZ (Crusius et al., 1996;
Passier et al., 1997; van der Weijden et al., 1998; Cowie et al., 1999) even in the core of the
OMZ. This contrasts strongly with organic-rich littoral sediments and even with much less
organic-rich sediments on other ocean margins with oxygenated bottom waters. It suggests only
moderately reducing redox conditions, even on the Pakistan margin where the OMZ is most
pronounced, and that the organic matter present is unusually refractory and/or that post-oxic
organic matter decay is inhibited.
Biological investigations of surficial Arabian Sea sediments on several JGOFS (and other)
programmes have included studies of the distributions of foraminifera (e.g., Naidu and
Malmgren, 1995; Gooday et al., 1998; Jannink et al., 1998) and dinoflagellate cysts (Zonneveld,
1997) as a function of depth, redox conditions, productivity and other oceanographic parameters
(with comparisons to distributions in other oceans). These provide important tools for paleoenvironmental interpretations of nanofossil assemblages in Arabian Sea sediment records (e.g.,
den Dulk et al., 1998). The UK JGOFS expedition and the recent German JGOFS programme
have included the most comprehensive investigation of benthic microbial and faunal
abundances, diversity, community structure and activity carried out across the Oman margin and
at abyssal sites across the Arabian Sea. Some of the results of these studies have recently
appeared in dedicated volumes of Deep-Sea Research (Gage et al., 2000; Pfannkuche and
Lochte, 2000).
The UK JGOFS studies included further work on foraminiferan communities and their
relationships to organic matter enrichments, oxygen availability and metazoan abundances
(Gooday et al., 2000). In another paper, Gooday et al. (2000) reported on a new species of giant
77
protozoan from the Oman margin below the OMZ, which complements a previous report by
Levin and Edesa (1997) of a first finding of mudball-building cirratulid polychaetes within the
OMZ on the Oman margin. Also, Lamont and Gage (2000) examined morphological adaptations
in polychaetes to hypoxia, while Cook et al. (2000) test its influence on nematode abundance,
and Creasey et al. (2000) examine the population biology, parasite pressure and population
genetics of galatheid crabs inhabiting the lower boundary of the OMZ. Meadows et al. (2000)
and Murray et al. (2000) investigate relationships between bioturbation, sediment geochemistry
and geotechnical parameters. Levin et al. (2000) reported on relationships between
environmental conditions and macrofaunal assemblages and lifestyles across the OMZ at the
Oman margin, with a notable observation that abundance maxima occur well within the OMZ.
Smith et al. (2000) examined the nature and extent of bioturbation across the OMZ, and
Smallwood and Wolff (2000) reported on spatial and depth distributions of organic matter
quality, and related these to distributions in biological abundances and activity. Biological
studies from the German JGOFS programme include reports on sedimentary and BBL microbial
populations and activity (Boetius and Lochte, 2000; Boetius et al., 2000a, b), bioturbation rates
(Turnewitsch et al; 2000), and distributions, abundances and diversity of foraminifera
(Kurbjeweit et al., 2000), metazoan meio- (Sommer and Pfannkuche, 2000) and macrofauna
(Witte, 2000), and benthopelagic nekton (Christiansen and Martin, 2000) in relation to
regionally and seasonally differing particle fluxes.
Recommendations
The published articles outlined above provide a sampling of the wealth of information that will
come from the variety of ongoing studies on Arabian Sea benthic dynamics and processes,
which are exceptional in nature and of great biogeochemical importance. Suggestions for future
research would include:
• Systematic studies of the structure, function and trophic interactions of Arabian Sea benthic
communities (microbes to megafauna), and how these vary with region, with depth, and in
relation to the OMZ, and in response to seasonal variation in organic matter supply.
• Linked to these should be a comprehensive assessment of the nature and rates of sedimentary
biogeochemical processes, and how these vary seasonally and in relation to the OMZ. This
would combine:
detailed geochemical characterisation of the sediments (both solid-phase and
porewaters) and their redox status
measurements of microbial process rates (both aerobic and anaerobic), characterisation
and quantification of biological mixing and irrigation processes, and an assessment of
the importance of chemoautotrophic processes within the OMZ
in situ determinations of the directions and magnitudes of benthic fluxes of oxygen,
nutrients, trace metals and dissolved organic matter.
Together with a concerted modelling effort, these studies would combine to provide an
important and broadly relevant view of benthic biogeochemistry in the Arabian Sea. Specific
advances would include new information on:
• the global significance of benthic bio-element fluxes to/from one of the planet’s largest
expanses of suboxic, organic-rich sediments
• the mechanisms of C cycling, and differences in C burial efficiency, in contrasting redox
settings and, perhaps, a conclusive answer as to the primary controls on organic C
preservation and distribution
• the environmental factors that control benthic community structure and function, and how
the benthic community contributes to C cycling and burial.
78
2.11 Modelling Physical-Biogeochemical Interactions
Jerry Wiggert
Earth System Science Interdisciplinary Center
University of Maryland
College Park, MD 20742-2465
USA
Introduction
The various Arabian Sea JGOFS field campaigns that took place from 1990 to 1997 have
provided a significant expansion of the observational database for this region (van Weering et
al., 1997; Smith et al., 1998b; Burkill, 1999). The ongoing analysis of these data has led to a
seminal advancement in our understanding of the physical and biogeochemical processes at
work in this complex region (Smith, 2001 and references within). The in situ biogeochemical
observations collected over this time period provide a comprehensive sampling of the
constituents of the primary and secondary ecosystem (Landry et al., 1998; Smith et al., 1998a;
Ducklow et al., 2001), along with distributions of chemical species and fluxes of particulate
matter (Lee et al., 1998; Morrison et al., 1998; Honjo et al., 1999). Measurements of the region's
physical characteristics have provided new insights into how the ocean's dynamical response to
the monsoon's annual cycle of meteorological forcing impacts biogeochemical processes
(Dickey et al., 1998; Weller et al., 1998; Lee et al., 2000). These physical and biogeochemical
data sets have prompted several modelling studies that have already appeared in the recent
literature (Wiggert et al., 2000; McCreary et al., 2001). These data, as both solution validation
and as a basis for ecosystem model parameter optimisation (Ryabchenko et al., 1998; Fasham
and Evans, 2000), are also an integral component of ongoing research employing coupled
physical-biological models that cover the full range of spatial domains (i.e., 1-D to Arabian Sea
to Indian Ocean) as part of the U.S. JGOFS Synthesis and Modelling Project (SMP). The SW
(summer) Monsoon has received significant attention in modelling studies in the past, due to the
elevated primary production that occurs during this period (Bauer et al., 1991; Brock and
McClain, 1992; Keen et al., 1997). More recently, it has become apparent that primary
productivity during the NE (winter) Monsoon is also relatively high and plays a prominent role
in defining the ecosystem’s annual cycle (Smith et al., 1998; Barber et al., 2001). A short
summary of the pre-JGOFS paradigm, followed by a synopsis of the fresh insights garnered
from modelling investigations stimulated by the recent observations, is presented for each
monsoon period. This is followed by some closing remarks, which include some suggested focus
areas for future analyses that would benefit from the JGOFS observations collected over the last
decade.
The Southwest Monsoon
Oceanic response to large-magnitude wind forcing dominates the circulation pattern of the
Arabian Sea during the SW Monsoon (Schott and McCreary, 2001). The so-called Findlater Jet
hugs the coast of Somalia before heading offshore as strong southwesterlies that parallel the
Arabian Peninsula (Shetye et al., 1994). Notable current features resulting from the annual onset
of the SW Monsoon winds include the Great Whirl off of the Somali coast and the offshore
advection of waters upwelled along the southern coast of Oman (Schott, 1993; Flagg and Kim,
1998). The synoptic distributions of surface chlorophyll a first revealed by the Coastal Zone
Color Scanner (CZCS) stimulated a number of modelling studies that looked to identify the
specific monsoon-driven physical processes that caused these prominent SW Monsoon
79
phytoplankton blooms. Clearly, upwelling along the Omani coast plays a major role during this
period (Brock and McClain, 1992; Kindle, this volume). Though hampered by excessive
cloudiness, the CZCS observations also indicated that elevated chlorophyll a concentrations
extended offshore at least an order of magnitude beyond the internal Rossby radius of O(50 km)
(Brock et al., 1991). A region of persistent, large-magnitude Ekman pumping, resulting from the
horizontal distribution of the monsoon winds, extends ~ 450 km offshore of the Arabian
Peninsula during the SW Monsoon (Brock and McClain, 1992). This led to a debate in the
literature as to whether Ekman pumping (Brock et al., 1991; Bauer et al., 1992) or a combination
of horizontal advection and wind-forced vertical entrainment (Young and Kindle, 1994;
McCreary et al., 1996; Keen et al., 1997) leads to the nutrient enrichment that supports the
elevated, offshore phytoplankton biomass.
High levels of eddy kinetic energy, and current filaments that transported distinct physical and
bio-optical properties more than 700 km offshore, were observed during the JGOFS cruises
(Flagg and Kim, 1998; Lee et al., 2000). In addition, HPLC (high-performance liquid
chromatography) observations and primary productivity measurements obtained by JGOFS
researchers during the 1995 SW Monsoon extended the CZCS-suggested offshore distribution
pattern by revealing elevated chlorophyll a concentrations and phytoplankton growth rates at
least 1000 km offshore (Latasa and Bidigare, 1998; Barber et al., 2001). These offshore
phytoplankton distributions may be linked to the eastward advection of upwelled waters within
the Somali Current (Young and Kindle, 1994; McCreary et al., 1996; Murtugudde et al., 1999).
Furthermore, new results from a U.S. JGOFS SMP-funded modelling study demonstrate that a
series of detrainment/entrainment phytoplankton blooms result from a shoaling/deepening mixed
layer responding to high-frequency wind forcing observed by a meteorological buoy located
along the climatological axis of the Findlater Jet (R. Hood, pers. comm.). The JGOFS
observations and the noted model study belie the impact of Ekman pumping as the mechanism
that defines the distribution of the SW Monsoon phytoplankton bloom, since these results are
from areas well offshore of the region of positive wind curl (Manghnani et al., 1998).
The Northeast Monsoon
The cool, dry northeasterly winds that characterize the NE Monsoon result in a typical
wintertime convection/nutrient enrichment scenario in the northern Arabian Sea (Banse and
McClain, 1986; Madhupratap et al., 1996). Evaporative cooling increases the salinity of the
surface waters, which further promotes the convective mixing. However, this convection is
inhibited during the day by solar heating of the surface waters. These competing surface heat
fluxes result in a persistent, large-magnitude diurnal oscillation in mixed layer depth (e.g.,
Wiggert et al., 2000). In comparison to previously reported values, surprisingly high rates of
primary productivity were measured by JGOFS researchers in concert with this convective
mixing (Barber et al., 2001; Smith, 2001) and an accumulation of phytoplankton biomass was
apparent over the course of the NE Monsoon (Wiggert et al., 2000). Values of the f-ratio
determined by nutrient assimilation experiments during the NE Monsoon showed that
phytoplankton growth was primarily supported by ammonium uptake (McCarthy et al., 1999;
Watts and Owens, 1999). Indeed, McCarthy et al. (1999) note that rates of primary productivity
increased 2-fold while nitrate slowly accumulated in the surface layer. These observations
indicate that some physical and/or biological control limits the full utilization of available
nutrients, which runs counter to the previously tendered supposition that variations in wintertime
nitrate entrainment lead to the interannual variability noted in distributions of surface
chlorophyll a concentrations (Banse and McClain, 1986; Banse and English, 1993).
Results from the JGOFS observations suggest that this incomplete utilization of nutrients is a
result of the combined impact of zooplankton grazing and the mixed layer's diurnal cycling.
80
Microzooplankton grazing accounted for 86% of phytoplankton growth (Landry et al., 1998).
Furthermore, estimated rates of fecal pellet production by mesozooplankton were significantly
higher than the observed rates of export production (Roman et al., 2000). These studies suggest
significant grazing control and recycling of biogenic material within the upper layer, which is
consistent with the f-ratios determined in the nutrient cycling studies already noted. The diurnal
mixing homogenizes the surface layer's physical and biogeochemical properties down to the
seasonal thermocline, nominally on a daily basis (Gardner et al., 1999). This restrains the
accumulation of phytoplankton biomass in the euphotic zone and provides a continual supply of
plant material subject to remineralisation in the sub-euphotic zone (Wiggert et al., 2000;
McCreary et al., 2001). By incorporating the diurnal irradiance cycle within their coupled biophysical model, McCreary et al., (2001) introduced a diurnal mixed layer which generated
phytoplankton blooms that exhibited more realistic magnitude, timing, and temporal extent than
observed in a previous solution (McCreary et al., 1996). Through the further inclusion of daily
forcing, the best correspondence to a phytoplankton production time-series (Marra et al., 1998)
was achieved. In another modelling study focused on diurnal mixing, Wiggert et al., (2000)
suggested interannual variations in the depth of the permanent mixing barrier (i.e., the seasonal
thermocline), in conjunction with a diurnally cycling mixed layer, could account for the
interannual variability in surface chlorophyll a distribution noted previously.
This hypothesis has been tested with notable success (Wiggert et al., 2001). The analysis made
use of the first three years of NE Monsoon-period ocean colour distributions observed by
SeaWiFS (Sea-viewing Wide Field of view Sensor) and the corresponding thermocline depths
extracted from an interannually forced ocean general circulation model (OGCM) solution
(Murtugudde and Busalacchi, 1999). SeaWiFS ocean colour distributions (8-day composites)
from the same period for three successive NE Monsoons are shown (Figure 1). These reveal
significantly lower pigment concentrations during the 97/98 NE Monsoon. The boxed region
depicted here defines the area over which spatial averages were made for the subsequent
analysis.
The time-series (of the spatial mean within the boxed region) of thermocline depth for the three
NE Monsoon periods is shown (Figure 2a), revealing that for the 97/98 NE Monsoon, the
OGCM-predicted thermocline was consistently at least 2σ below the 35-year mean. The
distribution of chlorophyll a concentration as a function of thermocline depth for the full
sequence of SeaWiFS 8-day composite imagery for the three NE Monsoon periods is shown
(Figure 2b). The two solid lines represent the surface chlorophyll a concentration (at noon and
dusk) as predicted by a simple, 1-D growth/mixing scheme (complete details are provided in
Wiggert et al., 2001). This result suggests that the convolution of a diurnally cycling mixed layer
and an interannually varying thermocline depth results in the observed interannual variation in
pigment concentration.
Closing Remarks and Suggestions for Future Modelling Efforts
The extensive in situ observational database that is now available as a result of the JGOFS
activities in the Arabian Sea has already altered our understanding of the spatial extent of the
SW Monsoon’s phytoplankton bloom and the magnitude of primary production during the NE
Monsoon. These data are certain to provide additional new insights for years to come. The
modelling studies that have so far resulted from the JGOFS expeditions in the Arabian Sea
consistently demonstrate that including the full spectrum of physical variability, especially high
frequency processes, is vital to developing an accurate simulation of the biogeochemical system
(Wiggert et al., 2000; McCreary et al., 2001). Modelling efforts aimed at achieving insight into
the impact of mesoscale current structures on the region's biogeochemical processes would also
81
be timely, given the eddy and filament structures that manifest during the SW Monsoon (Kindle,
this volume).
The synthesis of the JGOFS observations with an eye toward providing the modelling
community with conceptualisations that lead to more robust representations of ecosystem
behaviour without overwhelming computational resources will be an ongoing challenge that
should be vigorously pursued. For example, the food web structures developed in Garrison et al.
(2000), which evolved from the analysis of Campbell et al. (1998), provide genuine insight into
the seasonal/spatial variations/transitions among, and between, various ecosystem components.
The conceptual food webs developed in these analyses will be invaluable for assessing how well
ecosystem models can reproduce the observed spatial and temporal structures. Furthermore, such
conceptualisations will help illuminate the present shortcomings of the various ecosystem
models as well as aid the continuing exploration for model forms that best capture the full
dynamic range of the natural variability (e.g., Yajnik and Sharada, 1992; Sharada and Yajnik,
1997; Fasham and Evans, 2000).
Figure 1. Surface chlorophyll a distribution for 11-18 December is shown for three NE
Monsoon periods observed by SeaWiFS (97/98, 98/99, 99/00). We have defined a focus region
(15-18°N, 60-65°E) away from coastal influences. This is superimposed on each image, and aids
in making consistent comparisons between images. During 97/98, pigment concentrations range
from 0.3 to 0.6 mg Chl a m-3. During 98/99 and 99/00, the typical range is 0.7 to 1.0 mg Chl a
m-3, with values ranging up to 2.0 mg Chl a m-3.
82
Figure 2(a). The individual time-series of 21°C-isotherm depth for the three years of SeaWiFS
coverage are shown. The 35-year mean is also shown, with (± s) depicted by the grey shaded
region. This figure indicates that the thermocline during the 97/98 NE Monsoon is consistently
at least 2σ below the mean. (b). Chlorophyll a versus thermocline depth. The blue and brown
curves are chlorophyll a at noon and sundown, respectively, from the 1-D model. The SeaWiFS
and CZCS chlorophyll a observations, matched up with their corresponding OGCM-predicted
isotherm depths, have been superimposed. The dashed black line is a linear fit to the SeaWiFS
data. The SeaWiFS data and the model result show an excellent correspondence.
A similar distillation of the observations of biogeochemical processes associated with the
Arabian Sea’s extensive oxygen minimum zone (OMZ) would facilitate the incorporation of this
feature within future ecosystem modelling efforts. In addition to a full chapter devoted to
describing the OMZ's impact on biogeochemical cycling (Smith et al., this volume), no fewer
than four of the other chapters presented herein call for further analysis of, and insight into, its
significance in this region. The hydrographic surveys performed during the U.S. JGOFS cruises
have resulted in a new appreciation of the extent of the OMZ (Morrison et al., 1999).
Furthermore, it has become apparent that the OMZ significantly reduces the diel vertical
migrations of the zooplankton community (Smith et al., 1998; Wishner et al., 1998). Since
zooplankton significantly modify carbon fluxes through grazing, excretion, repackaging and
vertical motility, their absence from the mid-depths of the water column could have profound
effects on regional carbon cycling (Wishner et al., 1998). These processes need to be better
understood so enhancements to carbon cycle modelling efforts (e.g., Swathi et al., 2000) can be
83
implemented, which should, in turn, further our understanding of whether the Arabian Sea is a
net source or sink of atmospheric CO2 (Lal, 1994; Smith, 2001). It is also worth noting that
denitrification within the OMZ produces the nitrous oxide (N2O) that subsequently effluxes to
the atmosphere at an estimated rate of 0.4 Tg N2O y-1 (Bange et al., 2000). This rate does not
include the significant contribution made by waters from the West Indian continental shelf
(Naqvi, this volume). Nitrous oxide is a potent greenhouse gas that merits attention in future
climate studies since denitrification rates appear to be increasing, which may be indicative of a
global warming-monsoon intensification feedback scenario (Naqvi, et al., 2000).
Finally, the anoxic conditions of the OMZ result in greater iron solubility and higher
concentrations of bio-available iron below the ocean's surface layer (Measures and Vink, 1999;
Witter et al., 2000). Iron is a requirement for phytoplankton photosynthesis (Martin et al., 1991)
and the surface waters of the Arabian Sea are characterized by non-limiting iron concentrations
throughout the year (Measures and Vink, 1999). While significant aeolian iron deposition is
documented (Tindale and Pease, 1999), questions remain as to whether this deposition is
sufficient to maintain the observed surface iron concentrations or if entrainment/upwelling of
iron-rich OMZ waters must also be taken into account. Applying models that include explicit
iron biogeochemistry (e.g., Christian et al., 2001) to the Arabian Sea would help characterize the
OMZ's possible role as a source of iron to the euphotic zone.
Acknowledgements
The author gratefully acknowledges the support of the NASA Oceanography Program (NAG
58595). This chapter has benefited significantly from the comments of (in alphabetical order)
Raleigh Hood, John Kindle and Ragu Murtugudde. I would also like to thank Sharon Smith for
inviting this contribution and Louisa Watts for her tireless assistance, which was very much
appreciated.
84
3. MAJOR ACHIEVEMENTS, RECOMMENDATIONS AND FUTURE TIME-LINE
3.1 Major Achievements
At the time that the JGOFS Arabian Sea studies were organized, first-order issues such as
whether or not the Arabian Sea was a sink or a source for atmospheric carbon dioxide were still
unclear. Did high rates of primary productivity and large concentrations of sedimentary carbon
make it a sink or were these processes overwhelmed by outgassing of carbon dioxide arising
from the warming of deeper waters reaching the sea surface via upwelling and mixing? The wide
range of climatic variability revealed in the Arabian Sea’s sedimentary record, the inter-annual
variability in the strength of monsoons that has been documented for the recent past, and
suggestions that the mean strength of the monsoon would change under plausible scenarios of
climate change, all suggested that the Arabian Sea is an excellent environment in which to study
biogeochemical cycling and climate change. Since the goals of JGOFS are (1) to determine and
understand on a global scale the processes controlling the time-varying fluxes of carbon and
associated biogenic elements in the ocean and to evaluate the related exchanges with the
atmosphere, sea floor and continental boundaries, and (2) to develop a capability to predict on a
global scale the response of oceanic biogeochemical processes to anthropogenic perturbations, in
particular those relating to climate change (SCOR, 1990), it was possible to make a compelling
case that JGOFS should support a major international research effort in the Arabian Sea. The
JGOFS studies were designed to provide a seasonally and spatially resolved carbon budget for a
basin exhibiting some of the highest and lowest concentrations of plant biomass observed
anywhere in the world’s ocean. To date, disciplinary papers have given us extraordinary new
understanding of the components of the carbon cycle in the Arabian Sea, and these have been
constrained by emerging interdisciplinary studies and modelling. Some of our new knowledge is
summarized below.
Understanding the Physical Forcing
•
•
•
•
•
•
•
•
•
•
•
Unlike currents depicted from ship-drift data, currents in the upper 400 meters did not follow
the winds
The velocity field was dominated by eddies
The only organized flow was the coastal current and Ras al Hadd Jet
Heat was lost in December/January and gained in August
Highest rates of heating occurred in the Intermonsoon seasons
Upwelled water in the SW Monsoon season is geostrophically advected offshore in filaments
due to
wind stress curl and sea level height
Deepest mixed layers occur in the NE Monsoon season
Upwelling comes from as deep as 200 meters
In the NE Monsoon, surface cooling intensified and mixed layer deepens dramatically with
distance
offshore due to convective overturning
In the SW Monsoon, cooling offshore is due to filaments and wind driven entrainment and
not open
ocean upwelling (Ekman pumping)
Eddy kinetic energy is high in the west and southwest of the Arabian Sea, but much reduced
in the
eastern and northern Arabian Sea
85
Understanding the Biogeochemical Response
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
Flux of organic material to the seabed is dominated by an event at the end of the SW
Monsoon
Areally and annually the Arabian Sea is a slight sink for CO2 from the atmosphere
Primary production in the SW Monsoon is equal to that during the NE Monsoon
Iron is delivered from the atmosphere to the sea surface during SW Monsoon and exceeds
phytoplankton needs
Microbial loop is balanced except during the SW Monsoon
Diatom phytoplankton blooms develop late during the SW Monsoon
Substantial biomass inhabits the OMZ and vertically migrates
Zooplankton biomass is high all year round
A deep, active Prochlorococcus spp. layer exists during the NE Monsoon
Complex seasonal variability: TOC is lowest in the fall and highest in July, but bacteria
exhibit little
seasonality
Mesozooplankton seem to exert top-down control of the microbial loop during the SW
Monsoon
The spatial area of surface chlorophyll, as observed by remotely-sensed ocean colour
(CZCS), is
larger than that suggested by observations (possible effects of dust)
Both benthos and pelagos respond similarly to the OMZ with decreased diversity and
increased
biomass of those few taxa able to exploit suboxic conditions
The oceanic nitrogen cycle is far from steady-state; denitrification is increasing
Nutrient ratios suggest nitrogen is limiting and denitrification dominates nitrogen fixation
Mixed layer is deeper in the NE Monsoon than in the SW Monsoon
3.2 Recommendations
1) The JGOFS emphasis on basin-scale carbon budgets forced observational schemes that did
not allow the study of mesoscale spatial and temporal variability in the Arabian Sea. It is
therefore recommended that investigations be made in the following areas:
•
•
•
•
•
•
•
•
•
•
•
Coastal upwelling: physiology of phytoplankton (spin up) and washout
Scales of advection and ecological processes
Source and physiology of diapause zooplankton
Top-down control of the microbial loop in upwelling areas
Taxonomy and the microbial loop
The role of filaments in biogeochemistry
The role of eddies in biogeochemistry
Mechanisms generating filaments
Sediments as a source of Fe for phytoplankton
Biogeochemical responses
Role of larger predators such as crabs, myctophids, squid
86
2) The JGOFS emphasis on carbon cycling forced observational strategies that minimised the
study of the nitrogen cycle and OMZ of the Arabian Sea. Therefore, investigations into the
following areas are recommended:
•
•
•
•
Nitrogen cycle of the OMZ
Sediment biogeochemistry; denitrification
Chemical transformations
Physiological adaptation of all organisms in the OMZ
3) Understanding processes in the Arabian Sea would benefit from a comprehensive approach
(models + data assimilation + remote sensing) to integrated problems. In this respect the
following integrations should be highlighted for future research:
•
•
•
•
•
•
Sedimentation flux, surface forcing, and carbon burial rates
Diapause zooplankton, phytoplankton blooms and physical forcing
Potential linkage of Oman and Somalia upwellings
Aerosols, delivery of micronutrients (Fe), phytoplankton blooms, surface forcing
Onset of SW Monsoon
Food web structure adjustments during upwelling and convection
4) Monitoring of the OMZ should be made over the long term, as it is a sensitive barometer of
climate change
5) Rigorous floristic and faunistic comparisons between the 1960’s and 1990’s should be made
with respect to species shifts, as these provide evidence for ecosystem adjustment to climate
change
3.3 Future Time-line and Research
Synthesis and modelling of the JGOFS data sets generated from the Arabian Sea campaign will
continue until 2003. Major milestones envisaged are as follows:
2002
In February, Indian Ocean Synthesis Group (IOSG) committee members (Smith, Naqvi and
Burkill) will host a Special Session on Arabian Sea Synthesis at the ASLO meeting in Hawaii.
Those presenting papers at this open meeting will be encouraged to focus on international
disciplinary syntheses. If there is sufficient interest, we may consider a further Special Issue of
Deep-Sea Research with an intended publication date of spring 2003.
2003
We intend one or more multidisciplinary syntheses to be organised by the Indian Ocean
Synthesis Group (IOSG) for presentation at the Final JGOFS Open Science Conference. This is
being organised for Washington, USA in April 2003. In addition, there may be a variable
number of other syntheses, probably with a disciplinary focus. Publication would depend on the
Symposium Organising Committee.
87
4. APPENDICES
APPENDIX 4.1
Membership of JGOFS Indian Ocean Synthesis Group (July 2001)
Professor Sharon Smith (Chair), University of Miami, The Rosenstiel School (RSMAS),
Division of Marine Biology and Fisheries, 4600 Rickenbacker Causeway, Miami, Florida
33149-1098, USA. [email protected]
Dr. Shahid Amjad, National Institute of Oceanography, ST-47, Block 1 Clifton, 75600 Karachi,
PAKISTAN. [email protected]
Dr. Martien Baars, Netherlands Institute of Sea Research (NIOZ), P.O. Box 59, 1790 AB Den
Burg, Texel, THE NETHERLANDS. [email protected]
Professor Karl Banse, University of Washington, Dept. of Oceanography,
Seattle, Washington 98195-5351, USA. [email protected]
Box 355351,
Professor Peter Burkill, Plymouth Marine Laboratory, Natural Environment Research Council,
Prospect Place, West Hoe, Plymouth, PL1 3DH, UNITED KINGDOM. [email protected]
Dr. John Kindle, Oceanography Division, Naval Research Laboratory, Stennis Space Center,
MS 39529-5004, USA. [email protected]
Dr. Wajih Naqvi, National Institute of Oceanography, Dona Paula, Goa 403 004, INDIA.
[email protected]
Dr. Tim Rixen, Zentrum für Marine Tropenökologie, Universität Bremen, Fahrenheitstraße 6,
D-28359 Bremen, GERMANY. [email protected]
Dr. Shubha Sathyendranath (Vice Chair), Bedford Institute of Oceanography, Biological
Oceanography Section, P.O. Box 1006, Dartmouth, Nova Scotia, B2Y 4A2, CANADA.
[email protected]
Dr. Kirit S. Yajnik, Centre for Mathematical Modelling and Computer Simulation
(CMMACS), Council of Scientific and Industrial Research (CSIR), F2, Regal Manor, 2/1 Bride
St., Belur Campus, Longford Town, Bangalore 560037, INDIA. [email protected]
88
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111
APPENDIX 4.3
Seminal Literature:
1971-Present
1971. Wyrtki K. Oceanographic Atlas of the International Indian Ocean Expedition. National
Science Foundation, Washington, D.C., 531pp.
1973. Zeitzschel B. (Ed.) The Biology of the Indian Ocean, Ecological Studies 3. Proceedings
from symposium held at Kiel University, Germany, March 31 – April 6, 1971. Published by
Springer-Verlag, Berlin, Heidelberg, New York, 549pp. ISBN 3-540-06004-9.
1976. Krey J. and Babenerd B. Phytoplankton Production Atlas of the International Indian
Ocean Expedition. Intergovernmental Oceanographic Commission, Paris, 70pp.
1977. Proceedings of the Symposium on Warm Water Zooplankton. Published by the National
Institute of Oceanography and Casa J.D. Fernandes, Goa, India, 722pp.
1984. Angel M. (Ed.) Marine Science of the North-west Indian Ocean and Adjacent Waters.
Deep-Sea Research Volume 31, No.6-8. Published by Pergamon Press, London. ISBN 0 08
030285 8.
1988. Thompson M.F. and Tirmizi N. Marine Science of the Arabian Sea: Proceedings of an
International Conference. Published by the American Institute of Biological Sciences,
Washington, D.C., 658pp.
1991. Smith S.L., K. Banse, J.K. Cochran, L.A. Codispoti, H.W. Ducklow, M.E. Luther, D.B.
Olson, W.T. Peterson, W.L. Prell, N. Surgi, J.C. Swallow and K. Wishner. U.S. JGOFS: Arabian
Sea Process Study. U.S. JGOFS Planning Report No.13, Woods Hole Oceanographic Institution,
Woods Hole, Massachusetts, 164pp.
1992. Desai B.N. (Ed.) Oceanography of the Indian Ocean. A collection of peer-reviewed
papers presented at the International Symposium on the Oceanography of The Indian Ocean,
National Institute of Oceanography, Goa, India, January 1991. Published by Oxford and IBH
Publishing Co. PVT. Ltd., New Delhi, India, 772pp. ISBN 81-204-0735-0.
1993. Ittekkot V. and Nair R.R. Monsoon Biogeochemistry. SCOPE/UNEP Sonderband Heft 76,
192pp. ISSN 0072-1115.
1993. Topical Studies in Oceanography. Biogeochemical Cycling in the Northwestern Indian
Ocean. Deep Sea Research II Volume 40, No. 3, 1993. Ed. J.D. Milliman, Guest Eds. P.H.
Burkill, R.F.C. Mantoura and N.J.P. Owens. Published by Pergamon Press, London. ISSN 09670645.
1994. Baars M. (Ed.) Monsoons and Pelagic Systems. Cruise Reports from the Netherlands
Indian Ocean Programme Volume 1. Published by the National Museum of Natural History,
Leiden. ISBN 90-73239-24-9.
1994. Lal D. (Ed.) Biogeochemistry of the Arabian Sea. Published by the Indian Academy of
Science, Bangalore, 560 080, India, 253pp.
112
1994. Biogeochemical Processes in the Arabian Sea. Proceedings from the US-CIS Arabian Sea
Workshop, Sevastopol, Crimea, Ukraine, September 20-25, 1993. Published by Woods Hole
Oceanographic Institution, Marine Hydrophysical Institute, U.S. National Science Foundation.
Edited by The Marine Hydrophysical Institute of National Academy of Sciences of the Ukraine.
1995. Thompson M.F. and Tirmizi N. (Eds.). The Arabian Sea: Living Marine Resources and
the Environment. Published by A.A. Balkema, Rotterdam, Netherlands, 730pp.
1997. Topical Studies in Oceanography. Netherlands Indian Ocean Program. (1992-1993) First
Results. Deep Sea Research II Volume 44, No. 6-7. Ed. J.D. Milliman, Guest Eds. T.C.E. Van
Weering, W. Helder and P. Schalk. Published by Pergamon Press, London. ISSN 0967-0645.
1998. Sherman K., Okemwa E.N., and Ntiba M.J. (Eds.). Large Marine Ecosystems of the Indian
Ocean, Assessment, Sustainability and Management. Published by Blackwell Science Inc.,
Malden, MA, 394pp. ISBN 0 632 04318 0.
1998. Topical Studies in Oceanography. The 1994-1996 Arabian Sea Expedition: Oceanic
Response to Monsoonal Forcing, Part 1. Deep Sea Research II Volume 45, No. 10-11. Ed. J.D.
Milliman, Guest Ed. S.L. Smith. Published by Pergamon Press, London. ISSN 0967-0645.
1999. Topical Studies in Oceanography. ARABESQUE: UK JGOFS Process Studies in the
Arabian Sea. Deep Sea Research II Volume 46, No. 3-4, 1999. Ed. J.D. Milliman, Guest Ed.
P.H. Burkill. Published by Pergamon Press, London. ISSN 0967-0645.
1999. Topical Studies in Oceanography. The 1994-1996 Arabian Sea Expedition: Oceanic
Response to Monsoonal Forcing, Part 2. Deep Sea Research II Volume 46, No. 8-9. Ed. J.D.
Milliman, Guest Ed. S.L. Smith. Published by Pergamon Press, London. ISSN 0967-0645.
2000. Gaur V.K. (Ed.). Proceedings of the Indian Academy of Sciences. Earth and Planetary
Sciences. Special Issue on the International JGOFS Symposium on Biogeochemistry of the
Arabian Sea. Vol. 109, No.4. Published by Brilliant Printers Pvt. Ltd., Bangalore, India. ISSN
0253-4126.
2000. Topical Studies in Oceanography. Benthic Processes in the Deep Arabian Sea. Deep Sea
Research II Volume 47, No. 1-2. Ed. J.D. Milliman, Guest Eds. J.D. Gage, L.A. Levin and G.A.
Wolff. Published by Pergamon Press, London. ISSN 0967-0645.
2000. Topical Studies in Oceanography. The 1994-1996 Arabian Sea Expedition: Oceanic
Response to Monsoonal Forcing, Part 3. Deep Sea Research II Volume 47, No. 7-8. Ed. J.D.
Milliman, Guest Ed. S.L. Smith. Published by Pergamon Press, London. ISSN 0967-0645.
2000. Topical Studies in Oceanography. The Biogeochemistry of the Deep Arabian Sea. Deep
Sea Research II Volume 47, No. 14. Ed. J.D. Milliman, Guest Eds. O. Pfannkuche and K.
Lochte. Published by Pergamon Press, London. ISSN 0967-0645.
2001. Topical Studies in Oceanography. The 1994-1996 Arabian Sea Expedition: Oceanic
Response to Monsoonal Forcing, Part 4. Deep Sea Research II Volume 48, No. 6-7. Ed. J.D.
Milliman, Guest Ed. S.L. Smith. Published by Pergamon Press, London. ISSN 0967-0645.
113
The JGOFS Report Series includes the following:
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
Report of the Second Session of the SCOR Committee for JGOFS. The Hague, September 1988
Report of the Third Session of the SCOR Committee for JGOFS. Honolulu, September 1989
Report of the JGOFS Pacific Planning Workshop. Honolulu, September 1989
JGOFS North Atlantic Bloom Experiment: Report of the First Data Workshop. Kiel, March 1990
Science Plan. August 1990
JGOFS Core Measurement Protocols: Reports of the Core Measurement Working Groups
JGOFS North Atlantic Bloom Experiment, International Scientific Symposium Abstracts. Washington, November 1990
Report of the International Workshop on Equatorial Pacific Process Studies. Tokyo, April 1990
JGOFS Implementation Plan. (also published as IGBP Report No. 23) September 1992
The JGOFS Southern Ocean Study
The Reports of JGOFS meetings held in Taipei, October 1992: Seventh Meeting of the JGOFS Scientific Steering
Committee; Global Synthesis in JGOFS - A Round Table Discussion; JGOFS Scientific and Organizational Issues in the
Asian Region - Report of a Workshop; JGOFS/LOICZ Continental Margins Task Team - Report of the First Meeting.
March 1993
Report of the Second Meeting of the JGOFS North Atlantic Planning Group
The Reports of JGOFS meetings held in Carqueiranne, France, September 1993: Eighth Meeting of the JGOFS
Scientific Steering Committee; JGOFS Southern Ocean Planning Group - Report for 1992/93; Measurement of the
Parameters of Photosynthesis - A Report from the JGOFS Photosynthesis Measurement Task Team. March 1994
Biogeochemical Ocean-Atmosphere Transfers. A paper for JGOFS and IGAC by Ronald Prinn, Peter Liss and Patrick
Buat-Ménard. March 1994
Report of the JGOFS/LOICZ Task Team on Continental Margin Studies. April 1994
Report of the Ninth Meeting of the JGOFS Scientific Steering Committee, Victoria, B.C. Canada, October 1994 and The
Report of the JGOFS Southern Ocean Planning Group for 1993/94
JGOFS Arabian Sea Process Study. March 1995
Joint Global Ocean Flux Study: Publications, 1988-1995. April 1995
Protocols for the Joint Global Ocean Flux studies (JGOFS) core measurements (reprint). June, 1996
Remote Sensing in the JGOFS programme. September 1996
First report of the JGOFS/LOICZ Continental Margins Task Team. October 1996
Report on the International Workshop on Continental Shelf Fluxes of Carbon, Nitrogen and Phosphorus. 1996
One-Dimensional models of water column biogeochemistry. Report of a workshop held in Toulouse, France, NovemberDecember 1995. February 1997
Joint Global Ocean Flux Study: Publications, 1988-1996. October 1997
JGOFS/LOICZ Workshop on Non-Conservative Fluxes in the Continental Margins. October 1997.
Report of the JGOFS/LOICZ Continental Margins Task Team Meeting, No 2. October 1997
Parameters of photosynthesis: definitions, theory and interpretation of results. August 1998
Eleventh meeting of the JGOFS SSC; Twelfth meeting of the JGOFS SSC; and the Second meeting of the North Pacific
Task Team. November 1998
JGOFS Data Management and Synthesis Workshop, 25-27 Sept. 1998, Bergen, Norway. Meeting Minutes. January 1999
Publications 1988-1999. January 2000
Thirteenth meeting of the JGOFS Scientific Steering Committee. Fourteenth meeting of the JGOFS Scientific Steering
Committee. Fifteenth meeting of the JGOFS Scientific Steering Committee. October 2001
Meeting of the Southern Ocean Synthesis Group, Year 1998. October 2001.
Joint IGBP EU-US Meeting on the Ocean Component of an integrated Carbon Cycle Science Framework. October 2001
First Meeting of the North Atlantic Synthesis Group, 1998; Second Meeting of the North Atlantic Synthesis Group,
1999; Third Meeting of the North Atlantic Synthesis Group, 2001. October 2001
The following reports were published by SCOR in 1987 - 1989 prior to the establishment of the JGOFS Report Series:
• The Joint Global Ocean Flux Study: Background, Goals, Organizations, and Next Steps. Report of the International
Scientific Planning and Coordination Meeting for Global Ocean Flux Studies. Sponsored by SCOR. Held at ICSU
Headquarters, Paris, 17-19 February 1987
• North Atlantic Planning Workshop. Paris, 7-11 September 1987
• SCOR Committee for the Joint Global Ocean Flux Study. Report of the First Session. Miami, January 1988
• Report of the First Meeting of the JGOFS Pilot Study Cruise Coordinating Committee. Plymouth, UK, April 1988
• Report of the JGOFS Working Group on Data Management. Bedford Institute of Oceanography, September 1988