THE BALTIC SEA 



281 



of the Danzig depression (113 m) varies, however, from 10-01 to 13-5% , that 

 of the Gotland one from 11-49 to 12-65% , and that of the Landsort one 

 (427 m, north of Gotland Island) from 9-83 to ll-08% o . The amplitude of the 

 salinity fluctuations decreases from 4 (Bornholm depression) to 1 • 1 5% as one 

 moves east. 



The instability of the saline conditions is very marked not only in the 

 western part of the Baltic Sea but also in the eastern. This is well illustrated by 

 Sv. Sagerstrale (1951 a) for the Gulf of Finland. In the western part of the 

 Gulf in 1927-49, at a depth of 5 m, the salinity varied from 4-29 to 6-80% , 

 and in its eastern part from 007 to 4-96% . 



Salinity fluctuations affect the distribution and biology of organic life. 

 Sv. Sagerstrale (1951 a) gives a number of interesting examples, among them 

 the differences in the time taken by the medusa, Amelia aurita, to reach 

 maturity. 



Gas conditions 



Saline and gas conditions off Gotland Island, shown on the diagrams (Figs. 

 124, 125), are most significant. The deep saline water is poor in oxygen and 



Fig. 124. Oxygen distribution in waters round Gotland Island in June 1922 (Schulz). 



rich in carbon dioxide. In the autumn there is a vigorous vertical circula- 

 tion which continues even in the winter ; but it embraces only the upper 60 or 

 70 m layer. The summer warming which follows penetrates deeper still but 



Fig. 125. Distribution of carbon dioxide in waters round Gotland Island in July 



1922 (Schulz). 



does not last long enough to warm the whole layer cooled during the winter. 

 This is the reason for the existence of an intermediate cold layer, between the 

 two warmer layers, at a depth of 40 to 60 m. The hydrological and hydro- 

 chemical conditions of Baltic waters in summer time off Gotland Island may 

 be illustrated by the data given in Table 120 (15 July 1922, west of Gotland 

 Island). 



