PREDATION OF SEVERAL DOMINANT COPEPODS ON MICROZOOPLANKTON IN CHANGJIANG RIVER ESTUARY

Predation of four dominant copepods(Calanus sinicus, Centropages dorsispinatus, Acartia pacifica, and Euchaeta concinna) on microzooplankton was estimated in Changjiang(Yangtze) River estuary and adjacent waters by in-situ incubation. E. concinna hardly fed on microzooplankton although it is a carnivorous species, and for other three species(C. sinicus, C. dorsispinatus, and A. pacifica), the ingestion rate on microzooplankton(ciliate + heterotrophic dinoflagellate) reached 0.66, 0.09, and 0.59 μg C/(ind·d), accounting for 29%, 24%, and 37% of daily carbon intake of copepod on average, respectively. Heterotrophic dinoflagellate, as an important component of microzooplankton, contributed 30% biomass and 28% food intake of microzooplankton. The correlation between the ingestion rate of C. sinicus and the initial concentration of microzooplankton was significantly positive, and the clearance rate on 20 μm ciliate was significantly higher than 20 μm individuals(P0.01), indicating its preferences to larger individuals. Although the biomass of microzooplankton was much less than that of phytoplankton, copepods preferred to feed on microzooplankton, as indicated by the Chesson's Index of selectivity. The three dominant species accounted for 77% of total abundance of copepod community on average, but they could remove just 0.8% of the microzooplankton standing stock daily. Therefore, the top-down effect by copepod on microzooplankton community is insignificant, and most microzooplankton biomass do not enter copepod community via predation.