286 



BIOLOGY OF THE SEAS OF THE U.S.S.R, 



Fig. 132. Vertical distribution of phos- 

 phates and nitrates in the Gulfs of 

 Bothnia (A) and Finland (B), mg/m 3 

 (Gessner and Buch). 



at the head of the gulfs, especially the Gulf of Bothnia. Large amounts of 

 nitrogen, together with humus substances, are brought into the two large 



gulfs of the northern part of the 

 Baltic Sea from the mainland. Con- 

 trary to the ammonia nitrogen the 

 amount of nitrogen in the form of 

 nitrates increases with depth, since 

 the latter are consumed in the surface 

 layer by phytoplankton. Only in the 

 inner part of the Gulf of Bothnia'is 

 there a fairly high content of nitrates 

 in the surface layer (Fig. 132). In this 

 part of the Sea the plankton develop- 

 ment is very poor and, clearly, its 

 growth is not limited by the nitrates. 

 Nitrate content increases sharply in 

 the deeper layers below the thermo- 

 cline layer, where they are produced 

 mainly as a result of the nitrification 

 of organic matter. The reduced content of nitrate in the deep parts of the 

 Gulf of Bothnia is due to a very restricted inflow of deep waters from the 

 main basin of the Sea, owing to a shallow ridge which bars their entrance. 

 In the Gulf of Finland, which has no such ridge at its entrance, nitrate content 

 is the same as in the open parts of the Sea. The data on plankton distribution 

 accord fully with such a distribution of nitrates. 



The distribution of phosphates is somewhat similar to that of nitrates : they 

 are scarce on the surface, their number increases considerably in the depths. 

 Here too, however, the Gulf of Bothnia stands apart : its deep waters are poor 

 in phosphates ; this is, perhaps, the main factor limiting plankton develop- 

 ment. The difference between the two Gulfs is illustrated in Fig. 132. 



There are some considerable annual variations in the content of nutrient 

 salts in the depths of the Baltic Sea {Table 121). 



The hydrochemical conditions of the Baltic Sea are peculiar in that, 

 although it is connected with the ocean, there is no proper exchange of water 

 with the latter. As a result its whole biogenic cycle proceeds on account of its 

 own resources and of the inflow from the mainland. 



K. Buch (1931) represents as follows the nature of the processes of plant 

 food substances in the Baltic Sea. The current bringing the surface waters from 





