Recent evolutionary changes in the algae of the Baltic Sea

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Abstract
A decrease in salinity and temperature over the past 3000 yr has presented the marine algae of the Baltic Sea with considerable problems in adaptation. The effects of salinity upon a number of Baltic algae were measured. The results showed cell mortality to be severe in 0, 68 and 102.permill., and minimal in 6 and 11.permill.; there was most variation in tolerance to 34 and 51.permill.. The salt tolerances of Baltic marine algae have proved more hyposaline than those of British intertidal algae. Water uptake and loss in tissues of Chorda filum and Fucus vesiculosus from Baltic and British populations were measured in response to salinity changes. The results revealed significant population differences in both live and killed tissues. Receptacle development and oogonial maturation were observed in Baltic and British F. vesiculosus, and found to differ in seasonality. Some observations were associated with local sea temperatures but differences in the timing of receptacle initiation and in oogonial size were not. The depauperate thallus, commonly ascribed to the effects of low salinity, is a complicated phenomenon, comprising numerous attributes which are combined differently in different taxa. The morphological differences between Baltic and British marine algae were usually striking. The marine algae of the Baltic Sea have diverged in a number of ways from their North Atlantic counterparts. The naturally high variability of these taxa has enabled them to survive the period of increasingly strong selection pressure which followed the Littorina Sea episode. Divergence seems not to have advanced to the point where speciation may be said to have occurred. The Baltic may therefore be contrasted with the much older Mediterranean Sea, which contains a large number of endemic species. Nevertheless, the Baltic is a site of considerable evolutionary importance.