INTRODUCTION Separated from the Atlantic by the Scandinavian Peninsula, the Baltic Sea penetrates far into the northern part of the European Continent, extending over an area of about 420,000 sq km (distance from Copenhagen, at the entrance, to the inner end of the Gulf of Bothnia, ca. 1500 km, greatest width ca. 300 km). The Baltic can be regarded as a northern equivalent of the Mediterranean in that it is enclosed by land and communicates with the Atlantic by narrow outlets at its western end. In many other respects, however, the Baltic is just the opposite of the Mediterranean Sea—e.g., regarding depth and temperature conditions. The main part of the Mediterranean basin lies more than 2000 m below the surface (maximum depth more than 4000 m), while the Baltic is a shallow area, where depths less than 100 m predominate (maximum depth 459 m). The temperature in the Mediterranean is fairly stable and high (never below 12°–13°C.), whereas in the Baltic, the northern end of which extends almost to the Arctic Circle, there are strong seasonal fluctuations, and in winter the inner parts are regularly covered with ice. However, the most striking difference between the basins concerns the salinity conditions. In the Mediterranean the strong evaporation balances the fresh-water influx and even increases the saltiness of the water in comparison with that of the Atlantic (to more than 39‰); the much lower evaporation from the Baltic, in combination with the great volume of fresh-water delivered by about...