ABSTRACT: 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 ofchain-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 relativecontribution of protists <20 µm increased. The potential grazing impact of ciliates <20 µm on single celled cyanobacteria was similar to or even higher than that of nanoflagellates. Grazing experiments conducted in situ using the serial dilutiontechnique and pre-screening water on 64 µm showed that 33 to 42% of the chl a biomass and 54 to >100% of phytoplankton production were consumed daily by protozooplankton. Thus, when predation was reduced, protozooplankton were able to controlphytoplankton biomass and production. Predation by copepods, which showed high concentrations and were dominated by omnivorous Oithonidae during summer, may have played a central role in the seasonal changes in the size structure and biomass ofprotozooplankton. The observations and results from path analysis suggest the existence of 2 different food webs corresponding to bloom and summer conditions, respectively.