Ice covers the archipelago area of the tideless northern Baltic Sea regularly in winter and it is considered to be an important factor affecting the upper limit of Fucus vesiculosus L., the only large perennial algal species of the area. The potential of F. vesiculosus for vegetative regeneration after simulated ice scraping was tested experimentally, and untreated F. vesiculosus stands growing in archipelago areas of different wave exposures were monitored in autumn and winter to record their behaviour during the period of formation of ice cover. The capacity of Fucus individuals for vegetative regeneration after simulated ice damage was considerable. Even small parts of residual holdfast tissue in minor irregularities of the rock surface were able to support regeneration after removal of the holdfast. The amount of regeneration decreased with increasing severity of the treatment. All basal parts of the thallus were equally capable of regeneration, and the most important factor affecting the amount of regenerated biomass was the amount of residual biomass. Independently of the severity of the treatment 1 g of residual biomass produced on average 1.3 g of new biomass during the summer period. In autumn, well before the formation of ice cover, the untreated Fucus thalli lost their buoyancy and sank to the bottom. In winter, the persistence of this behaviour was confirmed by SCUBA diving under the ice cover. The sinking phenomenon was most conspicuous in sheltered archipelago areas where the thalli grow large and often reach the water surface. Two different strategies for survival from scraping by sea ice are proposed: in exposed archipelago areas one third of the winters are totally ice free, but when a permanent ice cover is formed wind can easily break it to form pack ice, which can destroy the perennial vegetation to a depth of several meters. When the intensity of the disturbance varies from year to year, effective vegetative regeneration can be a useful strategy to keep the occasionally scraped zone occupied. In sheltered archipelago areas, ice cover is formed every year and no pack ice occurs. The ice-scraped zone extends to a depth of half a metre and varies much less than in exposed areas. Vegetative regeneration is ineffective in a zone which is regularly cleared, because the thalli never have enough time to reach the size needed for reproduction. However, the stability of the disturbed zone makes the attachment of even large surface-reaching Fucus individuals possible just below the ice cover if they can avoid freezing into it. Loss of buoyancy and sinking to the bottom seems to be an adaptation to the winter conditions of sheltered archipelago areas.
