Optimal foraging by a suspension-feeding copepod: responses to short-term and seasonal variation in food resources

Laboratory radioisotope experiments were used to investigate the effects of phytoplankton seasonal succession on the selectivity and clearance rates of a suspension-feeding copepod in two Indiana lakes. Responses to particle size and quality were tested by allowing adult female Diaptomus birgei feeding in natural seston to select between a small (6×7 μm) flagellate (Chlamydomonas reinhardii) and a large, poor quality food (heat-killed Carteria olivieri, 22×25 μm). Short-term responses were tested in one lake by additional treatments in which copepods were acclimated for 5-6 h in filtered lake water ("starved") or natural seston with added Chlamydomonas ("enriched"). Copepods from both lakes fed selectively on the small live flagellate during the spring bloom of edible phytoplankton but fed selectively on the larger, poor quality particle during the "clear water phase" when food was scarce. These results are interpreted as an interaction between the concentration-dependent selectivity for high quality foods predicted by optimal diet theory and a perceptual bias for large-sized particles. Selectivity for high-quality food was intermediate and clearance rates were depressed when total phytoplankton abundance was high but dominated by filamentous cyanobacteria. In each experiment copepods also responded to the short-term manipulations by exhibiting weaker discrimination against the poor quality particle in the starvation treatment and stronger discrimination in the enriched treatment. A two-way mixed model ANOVA revealed substantial short term (37%) and seasonal (53%) components to the total variance in selectivity. Clearance rates were also influenced by both phytoplankton succession and the short-term resource manipulations. As expected, clearance rates on the poor quality food were more sensitive to the abundance of alternative foods. These results show how the feeding behavior of a freshwater copepod is modulated by both seasonal and short-term variation in natural food.