Plankton community structure and carbon cycling on the western coast of Greenland during the stratified summer situation. II. Heterotrophic dinoflagellates and ciliates

The structure and ecological role of protozooplankton (ciliates and heterot rophic dinoflagellates) were studied in Disko Bay, West Greenland, from mid -July to mid-September 1994. Heterotrophic dinoflagellates dominated the pr otozooplankton biomass. When diatoms formed subsurface blooms, heterotrophi c dinoflagellates averaged 70 % of the integrated protozooplankton biomass. After the diatoms declined, large heterotrophic dinoflagellates became les s abundant. Ciliates followed the same vertical distribution as heterotroph ic dinoflagellates. However, in contrast to heterotrophic dinoflagellates, maximum ciliate biomasses developed in surface water after the diatom bloom s, when the large copepods (Calanus spp.) migrated to the deeper water. Mix otrophic oligotrich ciliates, mainly Laboea strobila, contributed on averag e 20% to the integrated ciliate biomass. Growth experiments revealed simila r specific maximum growth rates for ciliates and heterotrophic dinoflagella tes. Maximum growth rates were allometrically related to cell volume. Cilia tes and heterotrophic dinoflagellates appear to be key organisms in the cyc ling of organic matter in this Arctic pelagic ecosystem.