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Distribution of phytoplankton in the tropical Eastern South Pacific at 10°S in relation to hydrographic and nutrient conditions
Jasmin Franz*, Gerd Krahmann*, Gaute Lavik** and Ulf Riebesell*
*Leibniz Institute of Marine Sciences (IFM-GEOMAR), Kiel, Germany **Max Planck Institute for Marine Microbiology, Bremen, Germany
[email protected]
1. Introduction
The tropical Eastern South Pacific is oceanographically a highly diverse region, with upwelling of nutrient-rich midwater into the surface layer above the narrow
shelf, an O2-deficient intermediate water body (so-called OMZ) and a pronounced stratification of the water column west of the shelf area. Nutrient cycling is
significantly affected by the hydrographic conditions of the water column. Nutrient availability is in turn the major factor controlling primary production and the
taxonomical composition of the phytoplankton community.
In this study we investigated linkages between hydrography, nutrient chemistry and photoautotrophic production along an east-west transect at 10°S
off the Peruvian coast.
2. Study site
RV Meteor M77/3 cruise
Ecuador - Peru
Jan. 2009
Fig. 1. Cruise track of M77/3 cruise
on RV Meteor running from
Guayaquil (Ecuador) to Callao (Peru)
from Dec. 2008 – Jan. 2009. Red dots
denote CTD cast stations. Blue frame
denotes sampled transect at 10°S.
© Kerstin Nachtigall
3. Hydrography
coastal upwelling
of NO3A
"new" production
subsurface NH4+
max. within the
thermocline
B
… calculated from measured phytoplankton marker pigments (HPLC= High Pressure Liquid
Chromatography) with the program CHEMTAX (Mackey et al., 1997; Veldhuis & Kraay, 2004)
5.1 Inner shelf
Fig. 2. For collecting hydrographic
data as well as water samples for
biogeochemical analyses, vertical
casts were conducted using a rosette
device
equipped
with
CTD
(conductivity-temperature-depth), O2
and fluorescence sensors, and 24 x
10-L Niskin bottles.
4. Primary production
Fig. 3. Hydrography
along the 10°S transect
derived from vertical
CTD casts (see fig. 1 and
A
2). (A) Temperature and
(B) O2 distribution show
upwelling of cold O2deficient intermediate
water, lifting the upper
B
boundary of the OMZ
(O2< 20 µmol/kg) to 10-20 m depth in the area next to the
coast. In contrast, the water column west of the shelf was
strongly stratified. Green arrows denote coastal upwelling.
5. Distribution of phytoplankton functional types (PFTs)
"regenerated"
production
A
B
C
D
Fig. 5. PFTs restricted in their
distribution primarily to the
eutrophic surface layer of the
narrow inner shelf. (A) Diatoms,
in terms of biomass the most
dominant PFT along the
transect, (B) cryptophytes, (C)
prasinophytes (subdivision of
the chlorophytes) and (D)
autotrophic dinoflagellates.
5.2 Inner shelf and shelf slope
Fig. 4. (A) Upwelling of NO3--rich
water into the euphotic zone above
the narrow shelf enabled "new"
production (denoted by green
dashed line). (B) Subsurface NH4+
max. within the pycnocline off the
shelf potentially produced by
zooplankton grazing and bacterial
decomposition
indicate
"regenerated" production in this
area (denoted by red dashed line).
Black dots indicate sampled depths.
6. Summary
1. Hydrographic structure of the water column along 10°S is controlling the system of primary production
- NO3--based "new" production above the narrow shelf
- NH4+-based "regenerated" production west of the shelf
2. Subsurface NH4+ maximum within the thermocline potentially formed by bacterial decomposition and zooplankton grazing of phytoplankton transported offshore
3. Partioning of PFTs along the transect in types that exclusively grow in the NO3--rich surface layer above the shelf (Fig. 5), other groups that additionally colonize the
subsurface in the realm of the NH4+ max. (Fig. 6), and the picocyanobacterium Prochlorococcus in the westerly open ocean region of the transect (Fig. 7).
B
E
A
subsurface NH4+
maximum
D
E
D
D
C
Fig. 6. PFTs occurring in the surface inner shelf
waters (N-source: NO3-; see fig. 4A), and in
addition in the subsurface water column off the
shelf (N-source: NH4+; see fig. 4B). (A)
Chlorophytes, (B) haptophytes (probably
Phaeocystis globosa), (C) chrysophytes and (D)
the picocyanobacterium Synechococcus. Deep
Chl a max. formed by these PFTs (denoted by
red dashed area) coincided with the subsurface
NH4+ max. within the offshelf pycnocline
(denoted by red lined area ) (see (E)).
5.3 Open ocean
Fig. 7. (A) A low-light adapted
Prochlorococcus strain was discovered
in the oligotrophic open ocean region
of the transect, colonizing the water
A
B
body right below the lower limit of the
oxycline or the upper boundary of the OMZ, respectively (see (3B)). Yellow dashed line mark upper limit of
OMZ. Exclusive photoautotrophic species detected in the light-limited water region below the pycnocline.
(B) A second Prochlorococcus population adapted to higher irradiances than the one below the pycnocline
(see fig. 7A).
4. Low-light adapted Prochlorococcus strain occurred along the lower boundary of the oxycline and upper boundary of the OMZ, respectively (Fig. 7A and 3B).
References
Mackey, M. D., Higgins, H. W., Mackey, D. J. and Wright, S. M. (1997) CHEMTAX - a program for estimating class abundances from chemical markers: application to HPLC measurements of phytoplankton pigments. Marine Ecology Progress Series 144, 265-283.
Veldhuis, M. J. W. and Kraay, G. W. (2004) Phytoplankton in the subtropical Atlantic Ocean: towards better assessment of biomass and composition. Deep-Sea Research 51, 507-530.
Acknowledgements
Thanks to Peter Fritsche, Kai G. Schulz, the CTD-Team and the Crew of German RV Meteor for their great support during the M77/3 cruise. This
work is a contribution of the Sonderforschungsbereich 754 "Climate-Biogeochemistry interactions in the Tropical Ocean" (www.sfb754.de) which
is supported by the Deutsche Forschungsgemeinschaft.