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 an d protozooplankton and the grazing effect of protozooplankton on phyto plankton 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 simi lar seasonal changes. During spring, dominance by large (> 20 mu m) he terotrophic or mixotrophic ciliates and flagellates coincided with hig h biomass of chain-forming diatoms, suggesting that protists were impo rtant grazers of the spring diatom bloom. During summer, concentration s and biomass of ciliates and flagellates > 20 mu m were lower than th ey were during spring and, consequently, the relative contribution of protists < 20 mu m increased. The potential grazing impact of ciliates < 20 mu m on single celled cyanobacteria was similar to or even highe r than that of nanoflagellates. Grazing experiments conducted in situ using the serial dilution technique and pre-screening water on 64 mu m showed that 33 to 42% of the chl a biomass and 54 to > 100% of phytop lankton production were consumed daily by protozooplankton. Thus, when predation was reduced, protozooplankton were able to control phytopla nkton biomass and production. Predation by copepods, which showed high concentrations and were dominated by omnivorous Oithonidae during sum mer, may have played a central role in the seasonal changes in the siz e structure and biomass of protozooplankton. The observations and resu lts from path analysis suggest the existence of 2 different food webs corresponding to bloom and summer conditions, respectively.