Seasonal variation and spatial distribution of phyto- and protozooplankton in the central Barents Sea

Seasonal and geographical variations of suspended single-celled organisms on a transect across the western part of the Barents Sea in March and May 1998 and in June–July 1999 revealed that pico- and nanoplankton flagellates and monads (<2 and 2–20 μm, respectively) entirely dominated total algae and protozoa numbers and biomass in March and in June–July, but in May, microplankton (>20 μm) prevailed in total biomass. In general, spring bloom progresses independently of the southern part of the Atlantic Water (AW) and follows the receding ice edge in the Arctic Water (ArW) to the north. The blooms started almost simultaneously and had similar composition (small diatom Chaetoceros socialis dominated total phytoplankton biomass) in both localities, so the share of resting spores, indicating the age of the bloom, differed markedly. As for underwater rise—the Sentralbanken (SBW) altered this pattern, and the spring bloom spreads from north to the south from the rise to the trench. The next stage of the bloom was dominated by the large diatoms Thalassiosira antarctica var. borealis above the Sentralbanken, in the Polar Front (PF) and in the ice-edge areas. In the southern part of transect, this stage of the spring bloom had a delay or was absent due to low stability of water column and/or due to grazing impact. The presence of ribbon-shaped forming species indicated the earlier stage of bloom in Marginal Ice Zone (MIZ). In May 1998 as well as in June/July 1999, at the ice-covered stations, early spring conditions—rather similar to the conditions in March 1998—were observed. Summer conditions at most of the stations in June–July 1999 were characterized by high species diversity of diatoms and dinoflagellates. High abundance of heterotrophic dinoflagellates and protozoans indicated the active functioning of the microbial loop in the nutritive chains.