We studied the development and decline of the 1990 phytoplankton spring bloom in the Marsdiep area of the North Sea (The Netherlands) with emphasis on the cause of the decline of the Phaeocystis bloom, the role of microbial organisms and the utilization of organic material produced by the algae. At the top of the bloom Phaeocystis was nitrogen limited. The bloom declined through cell lysis. Sinking of colonies and grazing were found to be relatively unimportant as loss factors. Biomass in the microbial foodweb (bacteria and protozooplankton) remained low during the bloom but increased sharply as the bloom started to decline indicating that organic carbon released by the phytoplankton was rapidly utilized in the lnlcrobial foodweb Results suggest that dissolved organic carbon produced by phytoplankton through excretion and lysis was the main source of carbon for the microbial foodweb includ~ng copepods.

No abstract

Between 30 March and 1 1 May 1990, total copepod abundance and the abundance. b~o m a s s and gut fluorescence of Temora longlcornls were determined and related to the abundance and succession of phytoplankton development in a Dutch tidal inlet Gut plgment values were h~g h e s t In females and lowest In young copepodites, but weight-spec~f~c pigment concentrations were about slrnilar Pigment levels measured In the guts were relatively high at the beginning and end of the penod of investigation when diatoms d o m~n a t e d the phytoplankton community, dnd low d u n n g the Phaeocystls d o m~n a t e d period, when ambient chlorophyll concentrations were h~g h e s t For the latter period, calculated ingestion rates In T longlcornls were low and estimated dally consumption amounted to less than 1 % of the phytoplankton standing stock, s u g g e s w g a negl~gible grazlng Impact on the development of the Phaeoc)/stls bloom In splte of the low grazrng on phytoplankton. T long]-corms biomass increased by one older of magnitude The discrepancy between low giazlng pressuie and copepod development is explained by assumlng that T long~cornls scvltched to heterotrophic food a bloom of ciliates present during the Phaeocystls dominated period

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.

Mesozooplankton grazing on Phaeocystis cf. globosa was investigated by laboratory and field studies. Tests on 18 different species by means of laboratory incubation experiments, carried out at the Biologische Anstalt Helgotand, revealed that Phaeocystis was ingested by 5 meroplanktonic and 6 holoplanktonic species; filtering and ingestion rates of the latter were determined. Among copepods, the highest feeding rates were found for Calanus helgolandicus and Temora longicornis. ton grazing by the dominant copepod -femora longicorms to be negligible during the Phaeocysti, spring bloom. T longicornis gut fluorescence was inversely related to Phaeocystis dominance. Th~ hypothesis has been put forward that 7". 1ongicornis preferentially feeds on microzooplankton and b) this may enhance rather than depress Phaeocystis blooms. Results from laboratory incubatioc experiments, including three trophic levels -Phaeocystis cf globosa (algael. Strombidinopsis sp Iciliatel and Temora longicornis (copepod) -support this hypothesis.