Résumé
The effects of ultraviolet-B radiation (UVBR) on the bacterivory of a natural marine protozoan community were examined as part of a 7 d experiment designed to study the effects of different UVBR intensities on the summer planktonic assemblage of the lower St. Lawrence Estuary, Quebec, Canada. The experiment was conducted in large containers (mesocosms) subjected to 1 of the following UVBR regimes: excluded UVBR (WUVB), natural UVBR (NUVB), and natural UVBR enhanced with either 2 lamps (LUVB) or 3 lamps (HUVB). Incubations with fluorescently labeled bacteria were conducted daily as a tool to understand the interaction between the potential bacterivores (heterotrophic ciliates and nanoflagellates) and bacteria within the studied system. UVBR intensities had no significant effects on the estimated clearance and ingestion rates (CR and IR, respectively) until Day 5 of the experiment. During the following 2 d, characterized by low nutrient concentration, both CR and IR decreased with the increase of the daily UVBR (at 305 and 320 nm) doses received. The maximum difference between treatments was observed on Day 7, where both clearance and ingestion rate values in the NUVB, LUVB and HUVB treatments were significantly lower than the WUVB treatment. Our data suggest that over a 1 d period and under conditions of high nutrient concentrations, protozoan bacterivory is not affected by UVBR increases. When nutrient concentrations become low, bacterivores become more susceptible to damaging UVBR effects. We think that the deterioration of food quality, itself resulting from the synergistic action of nutrients and UVBR stresses, is responsible for the increased sensitivity of bacterivores to UVBR. UVBR-induced decreases in bacterivory would represent a considerable loss to the higher tropic levels that feed upon bacterivores.