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

Abstract. 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 (NO_x, < 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 NO_x concentrations were observed to correlate positively with temperature (τ = 0.22, p < 0.05, n = 108), whereas negative correlations were reported between surface NO_x (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 (F_v/ F_m, > 0.4).