296 



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



W 



Melasma 



■ Chaetocero damcus 



■ Chlorophyceae* C/iroocnccacea 



• » » • • 



Infusoria 

 Marine Parafnuella 8. Tintinnopsis 

 — Brackish Water 



Fig. 139. Transition from 'marine' forms 



of plankton to ' brackish- water ' forms as 



one passes from the Belt into the Baltic. 



A Seaweeds; В Infusora (Brandes, 1939). 



2° to 5° and in the northern part of the Sea. The Boreal Arctic community 

 develops most at temperatures from 3° to 4° to 8° to 10°, both in the spring 

 and in the autumn. The two communities are characterized by their broad 

 euryhalinity. I. Nikolaev (1951) has pointed out that 'there is a break between 

 the Arctic region of distribution and the Baltic Sea in the case of these forms'. 



Table 123 



Arctic forms 



Boreal Arctic forms 



Melocira arctica 

 Achnanthes taeniata 

 Fragilaria cylindricus 

 Navicula grand 

 Navicula Vanhoffeni 

 Nitzschia frigida 

 Goniaidax catenata 



Chaetoceras gracilis 

 Chaetoceras holsaticus 

 Chaetoceras wighami 

 Sceletonema costatum 

 Thalassiosira baltica 

 Nitzschia longissima 

 Paridinium achromaticum 

 Dinobryon pellucidum 



Plankton development in various parts of the Sea. The Baltic Sea plankton is 

 poorer both qualitatively and quantitatively than that of the North Sea and 

 the parts of the Atlantic Ocean adjacent to it. As has been pointed out by 

 F. Gessner, this results in the greater transparency of the Baltic Sea waters 

 as compared with those of the North Sea. Organic matter is accumulated in 

 the deep depressions of the Baltic, which are poor in oxygen and rich in car- 

 bon dioxide. The occurrence of such deep depressions in a body of water 

 causes a more or less inadequate development of its plankton life, especially 

 as geologically the Baltic basin was fed by melt waters, poor in nutritive 



