Diversity and biomass of ciliates and heterotrophic dinoflagellates were analysed at six stations on a south–north transect (mimicking a time span of months in the biological succession during the Arctic spring–summer) from open water through drift ice and into fast ice (72°30′N 76°32′N) during spring 1993 in the open Barents Sea. A pycnocline was observed beneath the sea ice at 40–50 m. A diatom spring bloom beneath the ice with chlorophyll-a maximum >10 μg l−1 and a diverse protozoan community with a peak biomass of 34 μg C l−1 was associated with this bloom. Maximum biomass of tintinnids (1 μg C l−1), athecate dinoflagellates (8 μg C l−1) and thecate dinoflagellates (26 μg C l−1) were found associated with the chlorophyll-a maximum in the upper 10 m of the water column beneath the sea ice at the northern stations. In contrast, the protozooplankton community was dominated by naked ciliates at the southern open water stations. Here chlorophyll-a was low (<1 μg l−1) and the maximum biomass of protozooplankton was 10 μg C l−1 of which naked ciliates accounted for >50%.

Cell sizes and estimated carbon content of cells >11 μm, as well as depth by depth biomass of 12 species/types of naked ciliates, 12 tintinnids, 12 athecate dinoflagellates and 24 thecate dinoflagellates, are presented. The community of naked ciliates was dominated by Strombidium spp. and Strobilidium spp., the tintinnids were dominated by Parafavella spp., Ptychocylis, Leprotintinnus, Acanthostomella and Tintinnopsis. The very small gyro-/gymnodinoid cells and Gyrodinium cf. spirale dominated the athecate dinoflagellates and the thecate dinoflagellates by the heterotroph Protoperidinium spp. generally accounted for the major part of the protozooplankton biomass along the transect. The Margalef diversity index revealed lowest diversity of ciliates and heterotrophic dinoflagellates in the open water and higher at ice-associated stations. The overall diversity was coupled with prey availability in terms of food concentration, but already saturated at 0.1 μg chlorophyll-a l−1.