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

Abstract

Grazing by a metazooplankton community on nanoprotists <10 (JLOT was studied 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) and through predation by stickleback fry. Grazing experiments were conducted in the laboratory using 5 \im pre- filtered, 3H-labelled nanoplankton as tracer food. Grazing by mesozooplankton (Meso) and metazoan microplankton (Micro), screened through 140 and 100 |xm mesh, was studied. Owing to enrichment effects and weak predation control by fish, the biomass of Meso and Micro increased during the study period. Meso biomass consisted mostly of the copepod Eurylemora affinis and the cladoceran Bosmina longispina maritima, and Micro biomass of copepod nauplii NIII-NVI. The community clear- ance rate of Meso usually exceeded that of Micro when feeding on nanoprotists. The opposite was found for the biomass-specific clearance rate, revealing nanoprotists to be a more important food source for Micro than for Meso. Metazooplankton were not able to control nanoprotists, because Meso and MicTO were estimated to remove on average 4 and 2% of nanoprotozoan biomass daily. When inte- grated 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 production being estimated to consist of bacterial carbon trans- ferred by nanoprotists. Micro were estimated to be more closely linked to the microbial food web than Meso, suggesting that the trophic position of copepods changes slightly during their maturation from nauphi to larger copepodites.