Size-dependent phytoplankton growth and grazing in the northern South China Sea

ABSTRACT: Field surveys of the northern South China Sea (NSCS) were conducted during the summer of 2015 and 2016 with size-fractionated chlorophyll a (chl a) measurements and size-specific dilution experiments for 3 phytoplankton size classes, including micro- (20 to 200 μm), nano- (2 to 20 μm), and picophytoplankton (<2 μm). Our results suggest that phytoplankton size structure and size-specific rates of growth and grazing mortality could vary substantially along the coastal, transition, and oceanic zones of the NSCS. There was an elevated microphytoplankton concentration in the inner shelf, in contrast to the dominance of nanophytoplankton over the middle and outer shelves. However, the phytoplankton community was mostly dominated by pico-cells in the oligotrophic oceanic regions. We found a nonlinear relationship between nanophytoplankton and total chl a, which is different from both pico-cells (a linear decrease) and micro-cells (a linear increase). By assessing the functional responses of the size-specific growth rates to nitrate concentrations, we found a higher nitrate-saturated maximal growth rate and a larger half-saturation constant for microphytoplankton, whereas nano- and picophytoplankton showed similar lower maximal rates and smaller half-saturation constants. There was also much higher grazing mortality of microphytoplankton in response to the increase in total chl a. These findings are important for understanding plankton dynamics and the associated biogeochemical fluxes in contrasting marine ecosystems, as well as for future size-structure modeling of the NSCS.