Microzooplankton grazing and community composition in a high-productivity marine ecosystem

Abstract

Eastern Boundary Upwelling Ecosystems (EBUE's) are highly productive, and due to their extensive fisheries, of major economic importance. Compared to oligotrophic systems, however, relatively little is known about the fate of primary production in these systems. In order to investigate the role of microzooplankton in grazing primary production in the Canary Current EBUE, grazing experiments were conducted as part of the Meteor M129 expedition. Water for experiments was taken from depths with the highest chlorophyll a concentrations and prescreened to remove mesozooplankton. Microzooplankton could consume between 60% and 95% of primary production, as measured by chlorophyll growth rates. When individual prey groups were examined, however, clear grazing effects were more mixed, with strong effects on nanoautotrophs and heterotrophic bacteria, varied effects on Synechococcus, and few statistically significant grazing effects on Prochlorococcus, pico-eukaryotes or heterotrophic nanoflagellates. The microplankton grazer community was diverse, with 22-33 ciliate and 10-26 dinoflagellate morphotypes. The large size range of the microplankton grazers and their high diversity resulted in a broad collective food niche and considerable niche overlap. This suggests that with sufficient diversity in the grazer community, changes in that community over the course of experiments may have minimal functional effect.