Predation effects of the calanoid copepod

This study investigated feeding rates and selective feeding of the common copepod Acartia tonsa on mixtures of the heterotrophic dinoflagellate Protoperidinium cf. divergens and the cooccurring red-tide dinoflagellate Gonyaulax polyedra and predation effects of A. tonsa on the population growth of P cf. divergens during simulated red-tide and non-red-tide periods. Ingestion and clearance rates of A. tonsa on P cf. d~vergens were similar to those on ciliates of similar cell volumes measured in other studies, when each heterotrophic protist was offered as the only prey and similar prey densities were used. In general, in mixed prey experiments, there was no evidence of a strong preference by A. tonsa for P cf. divergens over G. polyedra. However, there may be a weak preference at low total prey concentrations or low ratios of prey availability (G. po1yedra:P cf. divergens). A. tonsa ingested similar amounts of carbon from P cf. divergens and G. polyedra at a G. po1yedra:P cf. divergens carbon biomass ratio of about 1.8. The maximum clearance rate of A. tonsa on P cf. divergens (5.5 m1 Acartia-l h-') was higher than that on G. polyedra (3.1 m1 Acartia-' h-'). The effect of A. tonsa predation on the population growth of P cf. divergens decreased with increasing G. polyedra concentrations and was negligible at 2200 cells ml-'. At low G. polyedra concentrations, high mortality rates of P cf. divergens due to A. tonsa predation may enable A. tonsa to limit the population growth of P cf. divergens. However, a dense layer of G. polyedra may provide P cf. divergens a 'refuge' from A. tonsa predation during red tides.