THE BALTIC SEA 



317 



especially in the shallower parts of the Sea and, in general, as depth increases 

 it gradually disappears. 



(2) Macoma calcarea-\-Astarte borealis biocoenosis. In the deeper northern 

 half of the southwestern part of the Baltic Sea (below 40 m) the benthos com- 

 position undergoes a change — Macoma baltica decreases markedly in num- 

 bers, or disappears altogether, and is replaced by Macoma calcarea and 

 Astarte borealis, the former being more abundant in the Bornholm depression 

 and the latter in the Arcona depression (Fig. 150). 

 In some places in the Arcona depression Astarte borealis forms very dense 



Fig. 151. Graphs of density indices (Zenkevitch). 

 A For the mesomixed community of the Arcona depres- 

 sion ; В For the oligomixed community of the inside part 

 of the Gulf of Bothnia. For A : Macoma baltica, Tere- 

 bellides stromi, Halicryptus spinulosus, Astarte borealis, 

 etc. ; for В : Pontoporeia affinis, Macoma baltica, Mesi- 

 dothea entomon. 



populations with a biomass of 177 g/m 2 and 346 specimens per 1 m 2 . Besides 

 the two mollusc forms, crustaceans are represented there at depths of 100 to 

 150 m by Pontoporeia femorata, Diastylis rathkei and the worms by Harmothoe 

 sarsi, Scoloplos armiger, Aricidea suecica, Terebellides stromi, Priapulus cau- 

 datus and Halicryptus spinulosus. The Astarte borealis community occupied 

 the Arcona and Bornholm depressions, extending to the east right up to the 

 entrance of the Bay of Danzig. The average benthos biomass for the Bornholm 

 region is about 102-5 g/m 2 . 



(3) Macoma baltica biocoenosis. A little to the east of Mecklenburg Bay the 

 typical Macoma baltica biocoenosis begins; it remains almost unchanged 

 right up to the Bay of Danzig through the southern, shallower parts of the 

 Sea. The average biomass of this whole area may be taken as about 48T5 g/m 2 

 (see Fig. 150). Macoma baltica begins here to become the dominant benthos 

 form. Some other forms, however, are well represented still : Cardium edule, 

 Mytilus edulis and Mya arenaria, Macoma calcarea, Astarte borealis and 



