Phaeocystis blooming enhanced by copepod predation on protozoa: evidence from incubation experiments

Abstract

Laboratory experiments were carried out to investigate the effect of protozoan, copepod and comb~ned grazing on Phaeocystis biomass.Phaeocystis cf.globosa single cells were offered to 3 different protozoan species, to the calanoid copepod Temora longicornis, as well as to mixtures of both grazer types.The heterotrophic dinoflagellate Oxyrrhis marina and the oligotrich clhate Strombidinopsjs acuminatum ingested Phaeocystis at much higher rates than did the copepod Nevertheless, protozoan growth and ingestion rates were submaximal, indicating Phaeocyst~s to be subopt~mal food.The ohgotrich ciliate Strombidium elegans did not feed on Phaeocystis.In grazing experiments w ~t h mixtures of both predator types, the decline of Phaeocystis single cells could be explained by protozoan grazing alone, implying no grazing by the copepods on Phaeocystis.Instead, copepods ingested the protozoans at high rates.Predation on 0. marina and S. acuminatum by T. longicornis resulted in a reduct~on of the total grazing pressure on Phaeocystis of 21 and 67 % respectively.We conclude that mesozooplankton predation on herbivorous ciliates and heterotrophic dinoflagellates, which consumed Phaeocystis cells, can considerably reduce the overall grazing pressure and may enhance Phaeocystis bloomng.