Size-fractionated mesozooplankton biomass and grazing impact on phytoplankton in northern South China Sea during four seasons

Biomass and grazing impact of three size fractions of mesozooplankton were investigated across a wide range of trophic conditions in northern South China Sea (SCS) during four seasonal cruises from July 2009 to May 2011. The grazing impact of mesozooplankton was determined by measurements of gut fluorescence and experimentally-derived gut clearance coefficients. In the northern SCS, the variation in mesozooplankton biomass was influenced by bottom-up effects. Due to riverine runoff to the shelf and winter mixing in oceanic areas, an increase in mesozooplankton biomass was observed in late spring corresponding to an increase in Chl a concentrations. However, because a significant portion of the Chl a came from pico-sized phytoplankton cannot be directly consumed by most mesozooplankton, especially in the oceanic waters during autumn and spring, the bottom-up effect was rather weak. Mesozooplankton consumed 0.7–21.5% of phytoplankton Chl a standing stocks daily, and it was higher in shallow shelf waters than in oceanic waters during spring and summer due to combined effects of mesozooplankton feeding behavior (higher herbivory in shelf waters) and phytoplankton composition (higher contribution of pico-sized phytoplankton in oceanic waters). If only the >2 μm phytoplankton were considered, the grazing impact by mesozooplankton became substantial, reaching ~31% d−1. Among the three size fractions of mesozooplankton, large-sized mesozooplankton (>1000 µm) dominated the total biomass and contributed more than 50% of the herbivory.