Effects of food quality on the functional ingestion response of the copepod Acartia tonsa

Effects of food quality on the functional ingestion response of the copepod Acartia tonsa were determined using 3 algal species 6 to 12 pm in diameter (dinoflagellate Amphidnium carten, coccol~thophorid Hymenomonas carterae, senescent and actively growing diatom Thalass~osira weissflogii), over a wide range of concentrations. The algal cells were characterized by their contents of carbon, nitrogen, prote~n, carbohydrate, lipid and chlorophyll a, and by dry weight. A. tonsa Ingestion rates Increased to an asymptote and were fit to the Ivlev model without a feeding threshold at low algal concentratlons. Maximum ingestion rates of algal cells were inversely proportional to their protein, nitrogen, and carbon content. Maximum lngeshon rates of protein and nitrogen increased w t h increasing cell content of these variables. In contrast, maximum ingestion rates of algal carbon were generally constant w t h increasing cellular carbon content. The effects of cellular carbohydrate, hpid, and chlorophyll a content on maximum ingeshon rates were variable. The rate at which the ingestion of phytoplankton cells approached the maximum ingestion rate increased w t h greater protein and nitrogen content. Our results suggest that cellular protein and nitrogen content (or factors related to them) are important in regulating A. tonsa ingestion rates.