Seasonal dynamics of zooplankton and grazing impact of microzooplankton on phytoplankton in Sanmen Bay, China

Abstract

The species composition, biomass, abundance and species diversity of zooplankton were determined for samples collected from 12 stations in Sanmen Bay, China, in four cruises from August 2002 to May 2003. Growth of phytoplankton and grazing rates of microzooplankton were measured using the dilution technique. The spatial and temporal variation of zooplankton and its relationship with environmental factors were also analyzed. The results showed that a total of 89 species of zooplankton belonging to 67 genera and 16 groups of pelagic larvae were found 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. Maximum biomass was recorded in August, followed by November and May, and the lowest biomass was recorded in February. Similarly, the highest abundance of zooplankton was observed in August, followed by May, November, and February. Grazing pressure of microzooplankton on phytoplankton in Sanmen Bay existed throughout the year, although the grazing rate of microzooplankton on phytoplankton varied with the season. Estimates for growth rate of phytoplankton ranged from 0.25 d −1 to 0.89 d −1, whereas grazing rate of microzooplankton ranged between 0.18 d −1 and 0.68 d −1 in different seasons. The growth rate of phytoplankton exceeded the grazing rate of microzooplankton in all the seasons. Grazing pressure of microzooplankton on phytoplankton ranged from 16.1% d −1 to 49.1% d −1, and the grazing pressure of microzooplankton on primary production of phytoplankton ranged from 58.3% d −1 to 83.6% d −1 in different seasons.