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.

A simple technique for cultivation of pelagic copepods through multiple generations is described. Heterotrophic dinoflagellates, cultivated together with Acartia clausi, Temora longicornis and Centropages hamatus, appeared to be very useful as food for the copepods, and controlled the fouling of water and tanks. The generation time of the copepods was about 20 d at 15 "C, mortality averaged 0.014 to 0.038 d-'. Application of the method in mass cultures and mariculture, and the role of heterotrophy in the pelagic ecosystem are discussed.

During a study on population dynamics of copepods in the southern North Sea some semiin-situ experiments were performed on predation by adult copepods on CO-occurring nauplii. In a period with a strong decline in naupliar abundance, experimental naupliar mortality by cannibalism amounted to 35% of the standing stock per day. Simultaneous observations on chlorophyll a and &atom concentrations in situ and on egg production by adult females suggested poor food conditions for adults. There appeared to be a tendency for increased predatory behaviour in the absence of other food, indicating the possible importance of cannibalism as a factor in naupliar mortality. Microscopic observations and a series of experiments on laboratory cultured Temora longicornis revealed adult female predation on stages N I through N V. Rates of capture declined with ascending developmental prey stage. Prey-specific predation rates increased continuously with prey density, but were significantly depressed m the presence of alternative algal food. Estimates of naupliar mortality in situ due only to cannibalism, based on experimentally determined predation rates, explained 5 to over 100 % of the estimated total naupliar mortality. It is suggested that cannibalism may act as a major factor in naupliar mortality and may contribute significantly to a rapid fall in naupliar densities in periods of algal food shortage.