Composition and distribution of the pelagic and sympagic algal assemblagesin the Laptev Sea during autumnal freeze-up

The phytoplankton and ice algal assemblages in the Siberian Laptev Sea duri ng the autumnal freeze-up period of 1995 are described. The spatial distrib ution of algal taxa (diatoms, dinoflagellates, chrysophytes, chlorophytes) in the newly formed ice and waters at the surface and at 5 m depth differed considerably between regions. This was also true for algal biomass measure d by in situ fluorescence, chlorophyll (Chl) a and taxon-specific carbon co ntent. Highest in situ fluorescence and Chi a concentrations (ranging from 0.1 to 3.2 mu g l(-1)) occurred in surface waters with maxima in Buor Khaya Bay east of Lena Delta. The algal standing stock on the shelf consisted ma inly of diatoms, dinoflagellates, chrysophytes and chlorophytes with a tota l abundance (excluding unidentified flagellates <10 mu m) in surface waters of 351-33 660 cells l(-1). Highest algal abundance occurred close to the L ena Delta. Phytoplankton biomass (phytoplankton carbon; PPC) ranged from 0. 1 to 5.3 mu g C l(-1) in surface waters and from 0.3 to 2.1 mu g C l(-1) at 5 m depth, and followed the distribution pattern of abundances. However, t he distribution of Chi a differed considerably from the distribution patter n shown by PPC. The algal assemblage in the sea ice, which could not be qua ntified due to high sediment load, was dominated by diatom species, accompa nied by dinoflagellates. Thus, already during the early stage of autumnal f reeze-up, incorporation processes, selective enrichment and subsequent grow th lead to differences between surface water and sea ice algal assemblages.