HOW DOES FUCUS-VESICULOSUS SURVIVE ICE SCRAPING

Ice covers the archipelago area of the tideless northern Baltic Sea re gularly in winter and it is considered to be an important factor affec ting the upper limit of Fucus vesiculosus L., the only large perennial algal species of the area. The potential of F. vesiculosus for vegeta tive regeneration after simulated ice scraping was tested experimental ly, and untreated F. vesiculosus stands growing in archipelago areas o f different wave exposures were monitored in autumn and winter to reco rd their behaviour during the period of formation of ice cover. The ca pacity of Fucus individuals for vegetative regeneration after simulate d ice damage was considerable. Even small parts of residual holdfast t issue in minor irregularities of the rock surface were able to support regeneration after removal of the holdfast. The amount of regeneratio n decreased with increasing severity of the treatment. All basal parts of the thallus were equally capable of regeneration, and the most imp ortant factor affecting the amount of regenerated biomass was the amou nt of residual biomass. Independently of the severity of the treatment 1 g of residual biomass produced on average 1.3 g of new biomass duri ng the summer period. In autumn, well before the formation of ice cove r, the untreated Fucus thalli lost their buoyancy and sank to the bott om. In winter, the persistence of this behaviour was confirmed by SCUB A diving under the ice cover. The sinking phenomenon was most conspicu ous in sheltered archipelago areas where the thalli grow large and oft en reach the water surface. Two different strategies for survival from scraping by sea ice are proposed: in exposed archipelago areas one th ird of the winters are totally ice free, but when a permanent ice cove r is formed wind can easily break it to form pack ice, which can destr oy the perennial vegetation to a depth of several meters. When the int ensity of the disturbance varies from year to year, effective vegetati ve regeneration can be a useful strategy to keep the occasionally scra ped 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. Vege tative regeneration is ineffective in a zone which is regularly cleare d, because the thalli never have enough time to reach the size needed for reproduction. However, the stability of the disturbed zone makes t he attachment of even large surface-reaching Fucus individuals possibl e 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 w inter conditions of sheltered archipelago areas.