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 effec ts of phytoplankton seasonal succession on the selectivity and clearan ce rates of a suspension-feeding copepod in two Indiana lakes. Respons es to particle size and quality were tested by allowing adult female D iaptomus birgei feeding in natural seston to select between a small (6 x7 mu m) flagellate (Chlamydomonas reinhardii) and a large, poor quali ty food (heat-killed Carteria olivieri, 22x25 mu m). Short-term respon ses were tested in one lake by additional treatments in which copepods were acclimated for 5-6 h in filtered lake water (''starved'') or nat ural seston with added Chlamydomonas (''enriched''). Copepods from bot h 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 scarc e. These results are interpreted as an interaction between the concent ration-dependent selectivity for high quality foods predicted by optim al diet theory and a perceptual bias for large-sized particles. Select ivity 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 a gainst the poor quality par ticle in the starvation treatment and stro nger discrimination in the enriched treatment. A two-way mixed model A NOVA revealed substantial short term (37%) and seasonal (53%) componen ts to the total variance in selectivity. Clearance rates were also inf luenced by both phytoplankton succession and the short-term resource m anipulations. As expected, clearance rates on the poor quality food we re 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.