PHAEOCYSTIS BLOOMING ENHANCED BY COPEPOD PREDATION ON PROTOZOA - EVIDENCE FROM INCUBATION EXPERIMENTS

Laboratory experiments were carried out to investigate the effect of p rotozoan, copepod and combined grazing on Phaeocystis biomass. Phaeocy stis cf. globosa single cells were offered to 3 different protozoan sp ecies, to the calanoid copepod Temora longicornis, as well as to mixtu res of both grazer types. The heterotrophic dinoflagellate Oxyrrhis ma rina and the oligotrich ciliate Strombidinopsis acuminatum ingested Ph aeocystis at much higher rates than did the copepod. Nevertheless, pro tozoan growth and ingestion rates were submaximal, indicating Phaeocys tis to be suboptimal food. The oligotrich ciliate Strombidium elegans did not feed on Phaeocystis. In grazing experiments with mixtures of b oth predator types, the decline of Phaeocystis single cells could be e xplained by protozoan grazing alone, implying no grazing by the copepo ds on Phaeocystis. Instead, copepods ingested the protozoans at high r ates. Predation on 0. marina and S. acuminatum by T longicornis result ed in a reduction of the total grazing pressure on Phaeocystis of 21 a nd 67 % respectively. We conclude that mesozooplankton predation on he rbivorous ciliates and heterotrophic dinoflagellates, which consumed P haeocystis cells, can considerably reduce the overall grazing pressure and may enhance Phaeocystis blooming.