THE BALTIC SEA 



299 



concentrated as a result of winter vertical circulation. At that time Copepoda 

 is the dominant form. Phytoplankton begins to develop rapidly with the first 



10000a: 



5000 



MONTHSW ШК1ЛШ1ПШП¥Ш 



z 

 < 



о 



О 



U- 



о 



Й 



00 



Z 



D 

 Z 



Fig. 141. Alterations in the quantity of plankton 

 and nutrient substances with the months in the sur- 

 face layer of the Arcona depression (Gessner, 1940). 



rays of spring sunshine, using all the nutrient salts and thus killing off the 

 plankton. As early as 1908 C. Apstein, working on the quantitative data of 



2000 



WOO 



500 



Fig. 142. Plankton bio- 

 mass in the Northern 

 (//) and Baltic (III) 

 Seas and in the straits 

 (/), in cc in the water 

 column of 1 m 2 section 

 (Apstein). 



Baltic Sea plankton, noted its huge development in May in the Beltsee, the 

 straits between the North and Baltic Seas (Fig. 142). Moreover he had found 

 that plankton growth in the Baltic is considerably poorer than in the North 

 Sea. The quantitative indices of plankton even in the most productive 



