Seasonal variation in zooplankton composition and grazing impact on phytoplankton standing stock in the Pearl River Estuary, China

The composition, abundance, distribution and grazing impact of dominant components of the meso- and macrozooplankton community were investigated in the Pearl River Estuary (PRE) wet and dry season cruises during the summer of 1999 and winter 2000, respectively. Throughout the investigation, mesozooplankton, comprised mainly of copepods, dominated numerically and by species richness, accounting for at least 73% of the total mesozooplankton in the PRE. The copepods Acartia spinicauda, Pavocalanus crassirostris, Oithona rigida, Paracalanus aculeatus and Euterpina acutifrons, numerically dominated zooplankton counts, while during the dry season the zooplankton community was dominated by the copepods Paracalanus serrulus, Pavocalanus crassirostris, Paracalanus parvus, Acartia spinicauda and Oithona spp. The average evacuation rates of the copepods were 0.032±0.006 and 0.039±0.008 min−1 for winter and summer, respectively. The grazing impact of the most abundant zooplankton taxa accounted for up to 85% of all zooplankton counted at each station. The grazing impact of zooplankton, especially copepods, changed seasonally and spatially, varying between <0.3% and 75% of the chlorophyll standing stock, or up to 104% of the daily phytoplankton production in summer and 21% in winter.