An overview of the abundance and role of protozooplankton in Antarctic waters

Abstract

The classic view of the Antarctic pelagic system has suggested that food web dynamics are dominated by the diatom-krill food web link. Recent observations, however, have indicated that this is an oversimplification and that the antarctic food web has a complexity similar to that found in lower latitude systems. More specifically, small particulate feeding protozoans appear to have a much greater importance than was previously assumed. Only a few studies have been sufficiently extensive to characterize the Antarctic pelagic protozoan assemblage. These indicate that heterotrophic flagellates (dinoflagellates and other heterotrophic nanoplankton) and ciliates (mostly non-loricate oligotrichs) dominate the protozooplankton assemblages in surface waters. The combined biomass of protozooplankton has been reported to comprise from < 7 to > 75% of the total nano- and microplankton biomass depending on season and location. Protozoans are also found in sea ice communities where their abundances exceed those typically found in the plankton. Several protozoan species occupy both ice and water habitats, suggesting that seasonally melting sea ice may be the source of ice-edge protozooplankton assemblages. The feeding rates of protozooplankton in Antarctic waters are poorly documented. Consumption estimates based on clearance rates and some preliminary grazing experiments, however, indicate that the protozooplankton should be capable of utilizing a significant proportion of the daily primary and bacterioplankton production. Protozoans may contribute to vertical flux, but present evidence suggests that their contribution will be lower than from other sources.