Grazing impact of natural populations of ciliates and dinoflagellates in a river-influenced continental shelf

The abundance and biomass of ciliates and dinoflagellates in a nearshore area affected by freshwater runoff and coastal upwelling events were assessed during 7 campaigns distributed over winter and spring months between 2006 and 2008 in the Itata river plume and off Coliumo Bay in Central Chile (similar to 36 degrees S). Grazing rates were also established for both aloricate ciliate and naked dinoflagellate populations using data on instantaneous food vacuole content via the uptake of fluorescently labelled algae (FLA). Both dinoflagellates and ciliates were more abundant in the less saline waters of the river plume. The dominant micrograzers on FLA were the dinoflagellates Gyrodinium spp. and Gymnodinium spp. and the ciliates Strombidium spp. Ingestion by naked dinoflagellates ranged from 0.06 to 0.14 prey d(-1), whereas aloricate ciliates ingested between 20 and 35 prey d-1. Maximum carbon ingestion of naked dinoflagellates occurred during spring, whereas it peaked both in winter and in spring in aloricate ciliates. The highest grazing impact of ciliates on nanoplankton (similar to 99% of primary production d(-1)) occurred during winter within the river plume. The higher abundance of small potential prey, as well as the resulting higher grazing impact of microzooplankton during winter in the river plume area, suggest an adequate environment for protozoan growth.