The species composition, biomass, abundance and species diversity of zooplankton were determined for samples collected from 12 stations in Sanmen Bay in four cruises from August 2002 to May 2003. Phytoplankton growth and microzooplankton grazing rates were measured by the dilution technique, in which samples were taken at the beginning and end of the experiment for chlorophyll a concentration. We have also analyzed the spatial and temporal variation of zooplankton and its relationship with environmental factors.
The results showed that the zooplankton in Sanmen Bay could be divided into 4 ecological groups, which were named coastal low saline species, estuary brackish water species, off shore warm water species and eurytopic species, respectively. A total of 89 species of zooplankton belong in 67 genera and 16 groups of pelagic larva were identified in Sanmen Bay. The coastal low saline species was the dominant ecotype in the study area, and the dominant species were Calanus sinicus, Labidocera euchaeta, Tortanus derjugini, Acartia pacifica, Pseudeuphausia sinica and Sagitta bedoti. There was considerable seasonal variation in zooplankton biomass and abundance in the surveyed area. The peak of biomass appeared in August, followed by November and May, and the lowest biomass appeared in February. Similarly, the highest abundance of zooplankton was observed in August, followed by May, November and February. There were similar horizontal distribution patterns for the biomass and abundance of zooplankton in February and May, in which they both increased from the upper of the bay to the lower of the bay in February and May. However, there were different horizontal distribution patterns for them in August. The peak of biomass appeared in the lower of the bay, while the highest abundance of zooplankton was observed in the upper of the bay in August. Biomass and abundance were distributed evenly in Sanmen Bay in November. Moreover, there was marked seasonal variation in the species diversity of zooplankton, which was conformable to the abundance of zooplankton.
Results from the dilution experiments indicated that there was grazing pressure of microzooplankton on phytoplankton in Sanmen Bay throughout the year, though the rate of microzooplankton grazing on phytoplankton varied seasonally. Estimates for phytoplankton growth rate ranged from 0.25 to 0.89d^－１, while microzooplankton grazing rate varied between 0.18 and 0.68d^－１ in different seasons. The growth rate of phytoplankton surpassed grazing rate of microzooplankton in all seasons. Microzooplankton grazed phytoplankton standing stock at a rate of 16.1 to 49.1% d^－１ and the grazing pressure of microzooplankton on phytoplankton primary production ranged from 58.3 to 83.6% d^－１ in different seasons.