THE BALTIC SEA 



287 



the inner parts of the Sea towards the straits carries with it the living matter 

 produced in those inner parts. As they die off, the organisms must sooner or 

 later sink into the depths; the organic matter oxidizes, turns into mineral 

 matter and is accumulated on the bottom. The deep current moving in the 



P, N mg/m 3 

 5 10 15 



2 4 6 8 10 12 14 16 18 20 



t\Of,S°/oo 



Fig. 133. Vertical distributions of phosphorus, 

 nitrogen, density, temperature and salinity in the 

 Arcona depression in August 1932 (Buch from 

 Gessner). a t indicates density at any given 

 temperature. 



opposite direction carries back into the inner parts of the Sea the decomposed 

 nutritive substances. 



The Gulf of Bothnia, however, has its own independent hydrochemical life. 

 As a result of this isolation of the separate parts of the Sea, and the obstacles 

 to the movements of organic substances to the southwestern part of the Sea, 

 these areas are poorer in nutritive matter than the northern ones ; this is con- 

 firmed by the data given by Buch for the Arcona depression (Fig. 133). 



IV. THE GEOLOGICAL PAST 



The composition of the fauna of the Baltic Sea, its ecological characteristics 

 and its distribution — more so than in the case of any other sea — cannot be 

 properly understood without taking account of its geological past. In that 

 respect the Baltic Sea is undoubtedly the best-studied Sea in the world (Fig. 

 134). From its last glaciation period, i.e. for the last 15,000 years, the history 

 of the Baltic Sea has been thoroughly studied. A sufficiently complete and 

 reliable history of this period and even its chronology can be found in the 

 works of the geologists, botanists and zoologists of primarily Sweden, Nor- 

 way and Finland. The study of this history of the Baltic Sea is linked with the 



