Seasonal variations in the effect of microzooplankton grazing on phytoplankton in the East China Sea

Abstract

The effect of microzooplankton grazing on phytoplankton in the East China Sea (ECS) was investigated in summer 2009 and winter 2009/2010 using the dilution technique. There were no significant differences in phytoplankton growth rates and microzooplankton grazing rates between coastal (influenced by the Changjiang River plume) and offshore (influenced by the Kuroshio) waters in either season. The mean rates of phytoplankton growth (0.77±0.53d−1) and microzooplankton grazing (0.69±0.42d−1) in summer (n=26) were significantly higher than those in winter (0.39±0.18d−1 and 0.21±0.08d−1 for mean rates of growth and grazing, respectively) (n=24). In both seasons, phytoplankton growth and microzooplankton grazing rates were significantly higher at the surface than at the depth of 5% surface irradiance. Aloricate ciliates were abundant and dominated microzooplankton in the ECS. There were no significant differences in microzooplankton abundance and biomass between seasons or depths. The grazing per microzooplankton was higher in summer than in winter. Phytoplankton growth rates were positively correlated with temperature in the surface waters in summer. Microzooplankton grazing balanced the primary production in summer, while the grazing was low in winter. In winter, the low picophytoplankton and temperature may have been responsible for the low microzooplankton grazing in the ECS.