Contribution of nanoprotists to metazooplankton diet in a mesocosm experiment in the coastal northern Baltic

Grazing by a metazooplankton community on nanoprotists <10µm was studied four times during a 21 day enclosure experimentcarried out off the SW coast of Finland in late summer. Duringthe study, the pelagic community was manipulated through nutrientenrichment (N + P) and through predation by stickleback fry.Grazing experiments were conducted in the laboratory using 5µm prefiltered, ³H-labelled nanoplankton as tracer food. Grazing by mesozooplankton (Meso) and metazaan microplankton(Micro), screened through 140 and 100 µm mesh, was studied.Owing to enrichment effects and weak predation control by fish,the biomass of Meso and Micro increased during the study peridMeso biomass consisted mostly of the copepod Eurytemora affinisand the cladoceran Bosmina longispims maritima, and Micro biomassof copepod nauplii NIII–NVI. The community clearance rateof Meso usually exceeded that of Micro when feeding on nanoprotistsThe opposite was found for the biomass-smc clearance rate, revealingnanoprotists to be a more important f d source for Micro thanfor Meso. Metazmplankton were not able to control nanoprotists,because Meso and Micro were estimated to remove on average 4and 2% of nanoprotowan biomass daily. When integrated throughthe study period, grazing on nanoprotists could meet 5 and 17%of the carbon need for Meso and Micro, 3 and 12% of their productionbeing estimated to consist of bacterial carbon transferred bynanoprotists Micro were estimated to be more closely Linkedto the microbial food web than Meso, suggesting that the trophicpusition of copepods changes slightly during their maturationfrom nauphi to larger copepodites. ¹ Present address: Department of Ecology and Systematics, Divkionof Hydrobiology, University of Helsinki, FIN-00014, Helsinki,Finland