Photophysiological state of natural phytoplankton communities in the South China Sea and Sulu Sea

Cheah, W., Taylor, B. B., Wiegmann, S., Raimund, S., Krahmann, Gerd, Quack, Birgit and Bracher, A. (2013) Photophysiological state of natural phytoplankton communities in the South China Sea and Sulu Sea Biogeosciences Discussions, 10 (7). pp. 12115-12153. DOI 10.5194/bgd-10-12115-2013.

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In recent years, an increasing number of studies on phytoplankton in the tropical South China Sea (SCS) and Sulu Sea (SS) have been conducted. However, still little is known about the photophysiological state of natural phytoplankton communities under varying environmental conditions. This study investigates the photophysiological state of natural phytoplankton communities in the southern SCS and SS based on high horizontal and vertical resolution field observations collected during the SHIVA (Stratosphere ozone: Halogens in a Varying Atmosphere) cruise (SO 218) in November 2011 on board RV Sonne. At the surface, pigment results revealed that total chlorophyll a (TChl a ) concentrations at all offshore stations were low at the surface and were generally dominated by cyanobacteria. Enhanced concentrations of TChl a were only observed below the upper mixed layer and above the euphotic depth with haptophytes, prochlorophytes and prasinophytes contributing most of the biomass. At stations close to the coast and river outflows, surface phytoplankton blooms (between 1 to 2.2 mg m−3) dominated by diatoms were observed. Overall, the study region exhibited strong nitrate + nitrite (NOx, < 1 μmol L−1), and phosphate (PO4, < 0.2 μmol L−1) depletion from surface down to about 50–60 m. Silicate (Si) exhibited similar trends with the exception of some near shore stations in which high Si concentrations (> 2 μmol L−1) were observed in conjunction with increased TChl a and diatoms concentrations. Surface NOx concentrations were observed to correlate positively with temperature (τ = 0.22, p < 0.05, n = 108), whereas negative correlations were reported between surface NOx (τ = −0.27, p < 0.05, n = 108), Si (τ = −0.68, p < 0.05, n = 108) and salinity indicating that the enhancement in nutrients at the surface was probably supplied through fresher and warmer river waters near the coast. In contrast, the opposite was observed between temperature, salinity and all nutrients in the water column suggesting that nutrients were supplied from the bottom through upwelling. Pigment gradients show that phytoplankton were optimising their pigment composition to acclimate to changes in the light climate and cells were in a competent state as suggested by high maximum photochemical efficiency values (Fv/ Fm, > 0.4)

Document Type: Article
Additional Information: This discussion paper has been under review for the journal Biogeosciences (BG). The revised manuscript was not accepted.
Research affiliation: OceanRep > GEOMAR > FB2 Marine Biogeochemistry > FB2-CH Chemical Oceanography
OceanRep > GEOMAR > FB1 Ocean Circulation and Climate Dynamics > FB1-PO Physical Oceanography
Refereed: No
DOI etc.: 10.5194/bgd-10-12115-2013
ISSN: 1810-6285
Projects: SHIVA
Date Deposited: 29 Oct 2013 12:14
Last Modified: 04 Oct 2016 11:20

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