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Acta Oceanol. Sin., 2014, Vol. 33, No. 9, P. 93–99
DOI: 10.1007/s13131-014-0527-1
http://www.hyxb.org.cn
E-mail: [email protected]
Remote sensing of phytoplankton distribution in the Red Sea
and Gulf of Aden
ALKAWRI Abdulsalam1*, GAMOYO Majambo2
1
Department of Marine Biology and Fisheries, Faculty of Marine Science and Environment, Hodiedah
University, Hodiedah,Yemen
2 Coastal Oceans Research and Development in the Indian Ocean, P.O. Box 10135, Mombasa 80101, Kenya
Received 28 April 2013; accepted 13 May 2014
©The Chinese Society of Oceanography and Springer-Verlag Berlin Heidelberg 2014
Abstract
Analysis of high-resolution 4 km sea surface temperature, Chlorophyll a (Chl a), and wind datasets provides
a detailed description of the spatio-temporal seasonal succession of phytoplankton biomass in the Red Sea
and Gulf of Aden. Based on Moderate Resolution Imaging Spectroradiometer on-board aqua platform (MODIS Aqua) data andsynoptic observations in the Red Sea, Chl a varies from north to south, with the northern
part appearing to be oligotrophic. This is likely due to the absence of strong mixing and low nutrient intrusion in comparison to the southern part during winter. In the Gulf of Aden, the emergence of upwelling cell
is clearly evident along the coast of Yemen, and is only distinct from the summer-autumn seasons. Most
notable is the pulsating nature of the upwelling, with warm and cold events clearly distinguished with phytoplankton response to this physical forcing also evident. The phytoplankton biomass distribution varies
considerably between the two regions of study. In both study areas, water temperature and prevailing winds
control nutrient concentrations.
Key words: Red Sea, Gulf of Aden, Phytoplankton, chlorophyll a, SST
Citation: Alkawri Abdulsalam, Gamoyo Majambo. 2014. Remote sensing of phytoplankton distribution in the Red Sea and Gulf of
Aden. Acta Oceanologica Sinica, 33(9): 93–99, doi: 10.1007/s13131-014-0527-1
1 Introduction
The Red Sea and Gulf of Aden region is a complex and unique
tropical marine ecosystem, with extraordinary biological diversity and a remarkably high degree of endemism. It is one
of the most important repositories of marine biodiversity on a
global scale and supports a particularly high diversity of corals
(Pilcher, 2003). Unfortunately, limited scientific literature exists
for describing the biological dynamics of the Red Sea (Alkershi
and Menon, 2011). Some research has been performed on the
biological and physiological oceanography of the Gulf of Aqaba
(Eilat), the eastern extension of the northern Red Sea. This work
indicates that the Gulf of Aqaba exhibits a strong spring bloom
and a weaker fall bloom, both of which exhibit considerable interannual variability (e.g., Labiosa et al., 2003).
The Gulf of Aden is one of the least well known areas of the
Indian Ocean in terms of its biology. It holds fishery resources
of international importance due to the upwelling of cool, nutrient-rich water during the southwest and northeast monsoons
and is characterized by a prevailing high-energy climate (Sanders, 1981a, b; Sanders and Morgan, 1989; MEP, 1998). Along the
Gulf of Aden, the coast of Yemen, studies have included oceanographic research (Wilson and Klaus, 2000), fishery investigations (Sanders, 1981a, 1981b; Sanders and Morgan, 1989; MEP,
1998), and various biological assessments (Wilson and Klaus,
2000). Little is known about the coastal and marine resources
of the Gulf of Aden off Somalia. Compared with the extensive
research on phytoplankton productivity carried out in the offshore oceanic waters of the Arabian Sea during the International Joint Global Fluxes program (Smith, 2001; Marra and Barber,
2005), phytoplankton variability in the coastal waters of Yemen
*Corresponding author, E-mail: [email protected]
and Oman is poorly known. There is a lack of a dedicated program with systematic sampling along the coast at regular time
intervals: no information is available on seasonal and interannual variability of phytoplankton communities.
One important factor distinguishing the Red Sea and the
Gulf of Aden is the influence of the Arabian Ocean monsoon. In
general, the region south of 20°N latitude is subject to the influence of both the spring and fall monsoon, which affect the prevailing wind direction over the southern Red Sea, and also the
transport of dust from adjacent desert regions (Edwards, 1987).
The northern Red Sea is not notably affected by the monsoon
(Sheppard et al., 1992). Along the coast in summer, strong sea
breezes build up and strike the coast obliquely due to the influence of the prevailing winds (Sheppard et al., 1992).
The primary production in the entire Red Sea increases
from north to south, consistent with distribution of nutrients
(Koblentz-Mishke et al., 1970; Weikert, 1987). Productivity in the
range of 170 mg·m−2·d−1 in the central Red Sea is cited by Dowidar (1983) and Weikert (1987). Koblentz-Mishke et al. (1970)
indicated that average annual production north of 17° N ranges
from 250–500 mg·m−2·d−1. The existing literature provides little
detailed information regarding annual cycles and seasonal productivity in the northern Red Sea. The concentration of chlorophyll in general increases in the southern region of the Red Sea
relative to the northern region (Acker et al., 2008).
Using satellite data, we have made an attempt to address
some of these concerns, mainly investigating chlorophyll a
(Chl a) distribution, the fundamental element of primary production, and a proxy for phytoplankton biomass. In general, the
primary goal of the present study was to understand the sea-
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ALKAWRI Abdulsalam et al. Acta Oceanol. Sin., 2014, Vol. 33, No. 9, P. 93–99
sonal cycle of phytoplankton and the extent of coastal physical–biological coupling in the Red Sea and Gulf of Aden, which
supports a large fisheries industry (Al-Jufaili et al., 2006).
2 Data and methods
2.1 Study area
The study focused on the Red Sea and Gulf of Aden (Fig. 1).
The Red Sea, which is also a large marine ecosystem (LME), is
an important economic and environmental asset. It’s a narrow
oceanic basin which is meridionally elongated about 2 250 km,
bordered by the African and Asian continental shelves. At its
northern end, the Red Sea connects with the Mediterranean Sea
through the Suez Canal, while the southern region exchanges
its waters with the open ocean (the Gulf of Aden and the Arabian Sea via the strait of Bab-el-Mandeb (Sofianos and Johns,
2002). The southern region of the Red Sea is considered to be
among the hottest regions in the world. The air temperature in
the northern region is slightly lower than in the southern region.
Rainfall in the Red Sea region is extremely sparse and is usually localized in the form of short showers. The entire stretch of
the Red Sea lacks inputs from rivers or stream sources. The Gulf
of Aden is located in the Arabian Sea between Yemen, on the
south coast of the Arabian Peninsula and Somalia, in the Horn
of Africa.
2.2 Satellite remote sensing data
For this study, monthly L3 daytime-derived chlorophyll a
and SST data at 4 km were used, covering the 2003–2011 period. The seasonal time windows in this study were defined
as Winter: December–March; Spring: March–May; Summer:
May–September; Autumn: September–December. The dataset
came from Moderate Resolution Imaging Spectroradiometer
(MODIS) on board Aqua platform. Level 3 data are the derived
30°
30°
35°
40°
geophysical variables mapped to uniform grid scale. To display
spatial distribution and analysis of Chl a and SST, we used an
online tool known as Giovanni. Geovanni is a web application
tool developed by the Goddard Earth Sciences Data and Information Services Centre (GES DISC) that provides a simple and
intuitive way to visualize, analyse, and access vast amount of
Earth science remote sensing data without having to download
the data. Giovanni is an acronym for the Geospatial interactive Online Visualization and Analysis Infrastracture (Acker and
Leptoukh, 2007).
Besides temperatuere, wind plays a crucial role in determining the spatial and temporal variability of Chl a by advection
and Ekman pumping. To describe the air–sea interaction in
the study area, we generated sea surface wind fields covering
the period from 2003–2011 over the study area using QuikSCAT
monthly data available at 0.25 degree spatial resolution.
3 Results
3.1 Chlorophyll a distribution
Generally speaking, coastal regions tend to have higher
chlorophyll a than the open oceans/seas. The spatial distribution of chlorophyll a in the Gulf of Aden and Red sea are illustrated by the annual chlorophyll a climatology (2003–2011)
(Fig. 2). Chlorophyll a concentration was relatively high (>2.5
mg·m-3) south of the Red Sea while the northern part exhibits
oligotrophic conditions, splitting the red sea almost in half. In
the Gulf of Aden area, chlorophyll a is relatively high nearshore
along the Yemen coast, and is lower in the open sea.
The succession of chlorophyll a for the two regions (Red sea
and Gulf of Aden) can be seen from the seasonal plots. Overall,
the two study areas depict a strong seasonal pattern with maximum concentrations seen during the winter time and minimum during spring and summer.
45°
50°
55°
60°
65° E
N
25°
Egypt
Red
Sea
Saudi Arabia
20°
Oman
Sudan
Yemen
15°
Gulf of Aden
10°
4 000
3 500
3 000
2 500
2 000
Depth/m
1 500
1 000
500
0
Fig.1. Schematic of the study area bound by latitude 30°–10°N and longitude 60°–30°E and the associated bathymetry.
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ALKAWRI Abdulsalam et al. Acta Oceanol. Sin., 2014, Vol. 33, No. 9, P. 93–99
33°
36°
39°
42°
45°
48°
52°
55°
57°
60° E
a
30
10
26°
2.5
24°
1.0
0.7
22°
0.6
20°
0.5
18°
0.4
0.3
16°
0.2
14°
0.1
12°
36°
39°
42°
45°
48°
52°
55°
57° 60° E
b
30°
N
28°
33°
36°
39°
42°
45°
48°
52°
55°
57° 60° E
c
30
10
26°
26°
24°
24°
1.0
22°
22°
0.7
20°
20°
18°
18°
16°
16°
14°
14°
12°
12°
30°
N
28°
33°
36°
39°
42°
45°
48°
52°
55°
57° 60° E
d
30°
N
28°
2.5
0.6
0.5
0.4
0.3
0.2
0.1
Chlorophyll a concentration/mg∙m−3
33°
0
0
33°
36°
39°
42°
45°
48°
52°
55°
57° 60° E
e
30
10
26°
26°
24°
24°
1.0
22°
22°
0.7
20°
20°
18°
18°
16°
16°
0.3
14°
14°
0.2
12°
12°
2.5
0.6
0.5
0.4
0.1
Chlorophyll a concentration/mg∙m−3
30°
N
28°
Chlorophyll a concentration/mg∙m−3
30°
N
28°
0
Fig.2. Modis aqua Chl a concentration of the study area for the period 2003–2011. The upper single plot (a) represents annual
mean/climatology while the double plots below represents the seasons (b. winter, c. spring, d. summer and e. autumn).
3.2 Sea surface temperature (SST)
SST climatology (2003–2011) clearly shows the distribution
of water temperature in the study area (Fig. 3). It is evident from
the climatology plot that the southern part of the Red Sea and
the Gulf of Aden exhibit high SSTs of 27–30°C, with the upwelling cell along the coast of Oman also being evident. SST is highest (>28°C) during summer, and is distributed evenly across the
whole gulf of Aden. During the winter monsoon, SSTs are low
(<26°C) in the gulf of Aden and along the strait.
3.3 Wind pattern
The prevailing winds over the northern region of the Red Sea
do not exhibit significant seasonal variations. However, there
is seasonal variations both in wind direction and speed over
the southern half of the Red Sea. Winds over the southern Red
Sea are in a northward direction during the winter monsoon
at about 18°N, where a strong convergence zone exists (Fig. 4).
This convergence zone marks the boundary between the monsoon-dominated atmosphere in the south, and continental at-
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ALKAWRI Abdulsalam et al. Acta Oceanol. Sin., 2014, Vol. 33, No. 9, P. 93–99
33°
36°
39°
42°
45°
48°
52°
55°
57°
30°
N
28°
60° E
a
30
29
26°
27
22°
26
20°
25
24
18°
23
16°
22
21
14°
20
12°
36°
39°
42°
45°
48°
52°
55°
57° 60° E
b
30°
33°
36°
39°
42°
45°
48°
52°
55°
30°
N
28°
57° 60° E
c
30
29
26°
26°
24°
24°
22°
22°
20°
26
20°
18°
25
18°
24
16°
16°
23
14°
14°
22
12°
12°
21
10°
10°
20
30°
30°
N
28°
33°
36°
39°
42°
45°
48°
52°
55°
57° 60° E
d
28
27
30°
30°
N
28°
33°
36°
39°
42°
45°
48°
52°
55°
Temperatuere/°C
33°
30°
N
28°
57° 60° E
e
30
29
26°
26°
24°
24°
22°
22°
26
20°
20°
25
18°
18°
24
16°
16°
23
14°
14°
22
12°
12°
21
10°
10°
28
27
Temperatuere/°C
30°
Temperatuere/°C
28
24°
20
Fig.3. Modis aqua SST (0°C) of the study are for the period 2003–2011. The upper single plot (a) represents annual mean/climatology while the double plots below represents the seasons (b. winter, c. spring, d. summer; and e. autumn).
mosphere in the north. During summer (May–September), the
convergent wind pattern is replaced by weaker northweaterly
winds over the entire Red Sea basin. During summer, there are
strong southwesterly winds that appear in the Gulf of Aden and
over the Arabian Sea. These southwest monsoon winds results
induce the Ekman transport, which causes upwelling along the
coasts of Yemen and Oman.
4 Discussion
The southern part of Red Sea and the Gulf of Aden have
characteristics of mesotrophic waters receiving relatively high
energy from solar radiation, which creates favorable conditions
for the photosynthesis of phytoplankton, and also experience a
seasonal reversal monsoon.
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30°
34°
38°
42°
46°
50°
54°
58° E
30°
30°
N
30°
N
26°
26°
22°
22°
18°
18°
14°
14°
10°
34°
38°
42°
46°
50°
54°
58° E
10°
January
12.2 m/s
6°
June
17.5 m/s
6°
Fig.4. Red Sea and Gulf of Aden climatological QuikSCAT wind speed (m/s) winds for January (winter) and June (summer) at the
height of the NE and SW monsoon seasons. The black arrows highlight the different wind direction between the two seasons, whereas the red arrow indicates the intrusion of water masses through the facilitation of northwards winds.
4.1 Seasonality of Chl a in the Red Sea
The annual Chl a climatology (2003–2011) reveal distinct
distribution patterns with the northern part exhibiting oligotrophic conditions (Chl a concentrations <1 mg/m3), and the
southern part being mesotrophic (Chl a concentrations > 1 mg/
m3), splitting the area into almost two (Fig. 2a). The seasonality
of phytoplankton biomass can be seen from the monthly means
of Chl a for the 2003–2011 period (Figs 2b, c and d). The Red
Sea Chl a depicts a strong seasonal pattern with high concentrations seen during winter time, reaching their lowest during
summer, with autumn being the initiation period, and spring
being the termination period. The high Chl a concentrations
in the southern region of the Red Sea during winter time might
be attributed to vertical mixing and intrusion of nutrient-rich
waters from the Gulf of Aden (Salem et al., 2001). The seasonal
changes of the Red Sea exchange through the strait is driven
primarily by the seasonal change in winds (monsoon) over the
southern Red Sea and Gulf of Aden (Fig. 4).
The highest Chl a values occuring in the winter season indicates that Chl a in the Red Sea exhibits a significant negative
correlation with Sea surface temperature (SST). Fahmy (2003)
indicates that productivity in the Red Sea is controlled by phosphate concentrations and is also related to temperature and salinity variability.
In summer, there appears to be a seasonal pattern in which
slightly increased Chl a occurs near the southern boundary of
the Red Sea. Acker et al., 2008 linked this phenomenon to the
strength and timing of the monsoon in the Arabian Sea. It is
therefore possible that effects of the monsoon could extend
into the Red Sea. On the other hand, during the summer in the
Red Sea region, sea breezes caused by marked temperature differences between the land and ocean waters strengthen during
the day. These sea breezes strike the coast obliquely due to the
influence of prevailing winds (Sheppard et al., 1992). Elevated
wind events have been shown to cause sediment resuspension
and transport on carbonate banks (Acker et al., 2002), so it is
likely that elevated wind speeds would also cause sediment re-
suspension in the Red Sea reef complexes. Sediment resuspension increases bottom reflectance (Acker et al., 2002) and resuspended sediments can also be a significant source of nutrients
to the water column (Fanning et al., 1982). Red Sea coral reef
waters have significantly higher nutrient concentrations than
the open Red Sea waters, due primarily to bacterial respiration
of organic matter in the sediments (Rasheed et al., 2002).
SST in the Red Sea increases from north to south, and decreases in the far southern Red Sea to the straits of Bab al Mandeb. Winter to summer SST variability is greater in the northern
Red Sea than in the southern Red Sea, and SST distribution in
the southern Red Sea is markedly influenced by the wind direction induced by the monsoon. The water column throughout
the Red Sea is highly stratified, with a thermocline extending
from 50 mto 250–300 m (Edwards, 1987). The stratification
is generally weaker in the winter than in the summer. Below
250–300 m, the entire Red Sea basin is comprised of water with
extremely uniform salinity of 40.6‰ (Edwards, 1987). The homogeneity of the deep water of the Red Sea, forming a persistent thermocline and halocline, is the primary reason nutrient
renewal from deeper waters is suppressed, particularly in the
northern Red Sea. The biological productivity of surface waters
is therefore dependent on the nutrient concentrations found in
the upper 200–300 m of the Red Sea water column (Acker et al.,
2008).
4.2 Seasonality of Chl a in the Gulf of Aden
The seasonal cycle of Chl a, in the Gulf of Aden and coast
of Oman, is dominated by two periods of high concentrations:
January–March and August–October. These months correspond
to the end of the southwest monsoon (SWM) and northeast
monsoon (NEM) (Banse and English, 2000; Marra and Barber,
2005). It is unclear whether the distribution of this bloom observed by us is strictly regulated by seawater temperatures, or if
factors such as water column stratification, nutrient availability
and grazing play a part in this horizontal gradient.
In summer, Chl a content is relative high and uniform
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in the whole Gulf (Fig. 2); very high Chl a concentrations are
limited only to a narrow band along the coast. Because of the
strong SWM that blows parallel to the coast, the Ekman transport produces a net movement of surface water 90 degrees to
the right. Stratification vanishes and vertical mixing develops
during this period. Upwelling brings nutrients from deep waters to surface waters, and these nutrients then support phytoplankton blooms. Also, the most distinct variations in SST are
the sharp drops observed during this period. Coles and Seapy
(1998), Claereboudt et al. (2001), and Al-Azri et al. (2010) attributed this to the upwelling and cyclonic eddies as cause; deeper
nutrient-rich waters to shoal may be responsible for the abrupt
decreases in SST during this period. From March to May, with
weak winds and large sea surface heating, water stratification is
well developed, which results in relative uniform SSTs in the entire Gulf. This water stratification limits upwelling of nutrients
and phytoplankton growth. High Chl a concentration along the
coastal waters may also be due to nutrients from runoff from
coastal cities and some small rivers that discharge into the gulf.
During SWM, water-masses are enriched with nutrients
brought up by monsoon upwelling during June-September.
At the end of September, diatom blooms are triggered. During
December, nutrients (specially Nitrate) is depleted and dinoflagellate blooms appear (Gomes et al.,2008). The satellite Chla
data indicate that the largest winter blooms in the southern
Red Sea, the Gulf of Aden, which extend to the Gulf of Oman,
were formed in January and persisted until March, where winter
convective mixing is the strongest. Many studies reported that
the Arabian Sea is a prominent site for the appearance of the
blooms of Noctilucamiliaris (Devassy, 1987; Gomes et al., 2008;
Matondkar et al., 2012). However, N. miliaris appears in coastal
waters (Devassy 1987) as well as in the open ocean (Gomes et
al., 2008). In the coastal waters along the West coast of India,
bloom is recorded during SWM, and in the open ocean and
north east Arabian Sea, during NEM (Feb-Mar period). More
recent studies report the appearance of N. miliarisin the SWM
(Sahayak et al., 2005) when colder, oxygen-poor, deep water is
brought to the surface by coastal upwelling. Although we don’t
have any in situ data, perhaps a similar event is taking place in
the Gulf of Aden, which may extend to the southern region of
the Red Sea.
Gomes et al. (2008) suggested from their satellite altimetry
data a strong coupling between N.miliaris blooms and mesoscale eddy activity in the north Arabian Sea. They hypothesized
that cyclonic cold eddies, through their ability to bring up nutrients and oxygen-poor subsurface waters from the depths
(100–300 m), are facilitating the genesis and evolution of N.
miliaris blooms in the north Arabian Sea. In a recent review,
Wiggert et al. (2005) have emphasized the need to ascertain the
importance and role of mesoscale eddies in the biogeochemistry of the Arabian Sea. A few observations from the Arabian
Sea JGOFS program cite the enhancement of Chla during the
passage of a cold eddy (Dickey et al., 1998), although this phytoplankton bloom was not characterized taxonomically. A study
similar to that of Crawford et al. (2005), which details the influence of mesoscale eddies in the eastern Gulf of Alaska on primary productivity and their role in advecting phytoplanktonrich coastal waters offshore into the deep-sea region, would be
of immense benefit.
Remote-sensing data, such as that provided by MODISAqua, provide a new capability for examination of the Red Sea
and Gulf of Aden, despite the lack of in situ data to support the
satellite observations, and only a few references which discuss
processes here. For that reason, inferences regarding the causation of the patterns observed in the remote-sensing data are
necessarily provisional. The observations in this paper can provide an introductory framework for further investigation of this
region, perhaps utilizing other types of remote-sensing data
and data with increased spatial and temporal resolution.
Acknowledgements
Chlorophyll a and sea surface temperatures analysis and
visualizations used in this paper are produced using the Geovanni online data system, and developed and maintained by
the NASA GES DISC.
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