CONTRIBUTION OF NANOPROTISTS TO METAZOOPLANKTON DIET IN A MESOCOSM EXPERIMENT IN THE COASTAL NORTHERN BALTIC

Grazing by a metazooplankton community on nanoprotists <10 mu m was st udied four times during a 21 day enclosure experiment carried out off the SW coast of Finland in late summer. During the study, the pelagic community was manipulated through nutrient enrichment (N + P) sind thr ough predation by stickleback fry. Grazing experiments were conducted in the laboratory using 5 mu m prefiltered, H-3-labelled nanoplankton as tracer food. Grazing by mesozooplankton (Meso) and metazoan micropl ankton (Micro), screened through 140 and 100 mu m mesh, was studied. O wing to enrichment effects and weak predation control by fish, the bio mass of Meso and Micro increased during the study period. Meso biomass consisted mostly of the copepod Eurytemora affinis and the cladoceran Bosmina longispina maritima, and Micro biomass of copepod nauplii NII I-NVI. The community clearance rate of Meso usually exceeded that of M icro when feeding on nanoprotists. The opposite was found for the biom ass-specific clearance rate, revealing nanoprotists io be a more impor tant food source for Micro than for Meso. Metazooplankton were not abl e to control nanoprotists, because Meso and Micro were estimated to re move on average 4 and 2% of nanoprotozoan biomass daily. When integrat ed through the study period, grazing on nanoprotists could meet 5 and 17% of the carbon need for Meso and Micro, 3 and 12% of their producti on bring estimated to consist of bacterial carbon transferred by nanop rotists. Micro were estimated to be more closely linked to the microbi al food web than Meso, suggesting that the trophic position of copepod s changes slightly during their maturation from nauplii to larger cope podites.