Diets of calanoid copepods on the West Florida continental shelf: Relationships between food concentration, food composition and feeding activity

Bottle incubations were conducted in March, July/August and October 1992, to measure the daily rations (R) and objectively characterize the diets of the calanoid copepods Eucalanus elongatus, Undinula vulgaris, Centropages velificatus and Temora stylifera from the west Florida continental shelf. Daily rations, R, were clustered around two, order-of-magnitude different means, 1.3 and 11.2% of body C d(-1), representative of quiescent and active feeding modes, respectively. The food concentration at which the transition from quiescent to active mode occurred was influenced by food particle size. In the quiescent mode: diets were dominated by nanoplankton, whereas no food type dominated the diet in the active mode. Selective feeding, defined as a statistically significant difference between the frequency distributions of foods in the diet and environment, occurred in both quiescent and active copepods. However, what appeared to be selective feeding in quiescent copepods could be explained by processes that passively modified the distribution of the diet relative to that of the food supply. Conversely, selective feeding in active copepods apparently resulted from foraging for particles >5 mu m in diameter in food environments dominated by nanoplankton (<5 mu m).