RECRUITMENT OF THE BALTIC FLORA - THE FUCUS-CERANOIDES ENIGMA

Sufficient time existed for the Atlantic flora to migrate into the Baltic Sea during the last marine episode, the Littorina Sea (7500 BP), and the question arises why is it that some euryhaline and estuarine algae from Atlantic shores have not found their way into the Baltic? Salinity requirements for growth and survival of Atlantic estuarine Fucus ceranoides have been measured. Vegetative apices have been cultured in a salinity range 1.5-45 parts per thousand in the laboratory at 10-degrees-C for 11 weeks. Thalli grew in all salinities; maximum growth in terms both of length and weight occurred in 6 and 12 parts per thousand; minimum growth occurred in 45 parts per thousand. Also, running Baltic seawater (6 parts per thousand) tank experiments were conducted in summer (10-17-degrees-C) and autumn (7-10-degrees-C) temperature ranges. Fucus ceranoides showed vigorous growth in both experiments and the growth rate was over double that of Baltic vesiculosus used for comparison. In the summer experiment, Fucus ceranoides plants approached fertility and the development of receptacles seemed to occur normally. Fucus ceranoides showed net photosynthesis over a wide range of salinities examined (1.5-102 parts per thousand). The optimum salinity was 34 parts per thousand and the species showed a clear decrease in O2 production under hypo- and hypersaline conditions. Measurements to quantify the main components which restore turgor (Na+, K+, Cl-, mannitol) were made. There were changes in ionic levels in response to changes of external salinities. The ability of Fucus ceranoides to maintain growth for long periods and to become fertile in low salinities should make it a perfect candidate for the Baltic flora: other possible reasons for its absence are discussed.