Ice algal accumulations were recognised by their vertical distribution in the ice, as surface, interior and bottom assemblages. The latter were quantitatively thc most important in the Barents Sea and in particular the sub-ice assemblage floating towards, or attached to, the under-surface of thc sea ice. Colonisation of the ice takes place by a "sieving" of the water between closely spaced platelets on thc ice under-surface. Once associated with the ice, the assemblage undergoes a succession terminated by the dominance of ice specialists. In a horizontal S-N section through thc ice, three distinct zones may be recognised: at the ice edge the recently colonised ice has a layer of algae up to a few millimeters in thickness consisting primarily of planktonic species. Further into older first year ice the algal layer becomes thicker and is typically dominated by the pennate diatom Nitzschia frigida Grunow. Below multi-year ice in the central polar basin decimetre-thick mats of algae are found, consisting almost exclusively of the centric diatom Melosira arctica (Ehrenberg) Dickie and a few associated, mostly epiphytic, species. Thc predominantly planktonic sub-ice assemblages at the ice edge can grow under stable conditions as soon as the light becomes adequate in the spring, and they arc able to multiply actively for one to two months before planktonic growth is possible. Thc sub-ice plankton assemblage thus forms an inoculum released to the stabilising water when the ice starts melting. This may explain how a phytoplankton bloom can develop explosively at the ice edge as soon as the ice melting commences, at a time when the number of algal cells in the water column is still very low.
