Grazing by protists and seasonal changes in the size structure of protozooplankton and phytoplankton in a temperate nearshore environment (western Gulf of St. Lawrence, Canada)

Seasonal changes in the biomass and size structure of phytoplankton and protozooplankton and the grazing effect of protozooplankton on phytoplankton were assessed in a temperate nearshore environment (Baie des Chaleurs, western Gulf of St. Lawrence, Canada). The size structure of protozooplankton and phytoplankton assemblages showed relatively similar seasonal changes. During spring, dominance by large (>20 μm) heterotrophic or mixotrophic ciliates and flagellates coincided with high biomass of chain-forming diatoms, suggesting that protists were important grazers of the spring diatom bloom. During summer, concentrations and biomass of ciliates and flagellates >20 μm were lower than they were during spring and, consequently, the relative contribution of protists