Seasonal variation of phytoplankton growth and microzooplankton grazing in a tropical coastal water (off Kochi), Southwest coast of India

Phytoplankton growth and microzooplankton grazing in the coastal waters off Kochi, Southwest coast of India, were assessed during three seasons (Pre-Monsoon; Southwest Monsoon and Northeast Monsoon) using the dilution technique. Nutrients concentration in the study area was low during the Pre-Monsoon (PRM) compared to the Southwest Monsoon (SWM) and Northeast Monsoon (NEM) periods. Phytoplankton biomass (Chl. a) and growth rate were the highest during the SWM followed by the NEM. They were influenced mainly by the coastal upwelling and estuarine influx. Microzooplankton grazing rate varied from 0.11–0.27 day−1 and their grazing pressure on phytoplankton production varied from 26–80% day−1. The highest grazing pressure of microzooplankton on phytoplankton was found during the PRM, when nutrient concentration was low. It was found that ~80% of the daily phytoplankton production and 31% of their biomass were grazed by microzooplankton during the PRM. On the other hand, microzooplankton grazing on phytoplankton biomass and production were low (11% and 26%, respectively) during the nutrient-repleted SWM. Grazing experiments using size-fractionated phytoplankton showed that microzooplankton prefers nano size fraction of the phytoplankton community. This study presented a baseline information on microzooplankton grazing in the nearshore waters along the southwest coast of India during different seasonal hydrographical setting.