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



larvae of Tcrihoptera, Turbellaria, Planaria lacustria, Poly cells nigra, the oligo- 

 chaetes Stylaria lacustrls, Nais ellnguis, and the Porifera Ephydatia fluviatilis. 



For the first time the microfauna of the coastal sands has been subjected to 

 examination on the model of the Baltic Sea (Remane, 1933, 1952). The 

 original interstitial fauna (Mesopsammon) was found to be abundant in the 

 following species: Turbellaria, Gastrotricha, Archiannelida, Tardigrada, 

 Ostracoda, Harpacticoida and Nematoda. Near Twerminn (southern Fin- 

 land) the number of microbenthos organisms in some cases is more than 

 100,000 specimens per 1 m 2 , mainly on account of nematodes and ostracodes. 



A. Schurin has carried out a comprehensive survey of the bottom fauna of 

 the Gulf of Riga (1961). Three characteristic features may be noted for bottom 

 fauna of the Gulf of Riga : (7) a general qualitative impoverishment, (2) de- 

 crease in size of all the main components, probably as a result of lower salinity 

 which makes this fauna completely accessible to local fish as food, and (5) a 

 rise in the levels of vertical distribution of biocoenoses and of individual 

 forms. Whereas in the open parts of the Sea the replacement of the shallow- 

 water mollusc benthos by the deep-water one, with a preponderance of crus- 

 taceans, takes place at depths of 50 to 100 m, in the Gulf of Riga this change 

 occurs at 10 to 20 m (Fig. 160). 



On the actual shores of the Gulf of Riga the biocoenosis of the macrophyte 

 overgrowth is well represented, with abundant settlements of small amphi- 

 pods (Leptocheirus pilosus, Gammarus locusta, and others) and mysids 

 (Praunus inermis and P.flexuosus). In the sublittoral zone (2 to 20 m) there is a 

 very marked preponderance of bivalves (over 95 per cent) and especi- 

 ally Macoma baltica, My a arenaria and Cardium edule, but at a depth of 10 

 to 20 m the molluscs are greatly reduced in numbers, while the crustaceans 

 and worms increase ; the latter, and above all Pontoporeia affinis, are markedly 

 dominant at 20 to 40 m. The number of Pontoporeia affinis may reach 

 7,000 specimens per 1 m 2 . Among the other organisms the most significant 

 are Mesidothea entomon, Pontoporeia femorata, Halicryptus spinulosus and 

 Mysis mixta. Macoma baltica is still found in small numbers. Below 40 m 

 (and down to 60 m) in the stagnant zone of the central depression five species 

 in all have been found: Pontoporeia femorata, Pont, affinis, Mesidothea entomon, 

 Mysis mixta and M. oculata f. relicta. The molluscs and worms are entirely 

 absent. Total biomass of benthos in the Bay of Riga is about 670,000 tons 

 (Shurin, 1961). 



General characteristics of productivity. The data on the qualitative distribution 

 of the Baltic Sea (Fig. 161) can therefore be summarized as follows. At the 

 start there are the biocoenoses of the Danish Belt and Oresund which are 

 diversified and rich in biomass (200 to 300 g, sometimes kilogrammes, in the 

 case of Modiolaria and Mytilus). Then, as one moves to the east and north, 

 an ever greater impoverishment is observed, both in quality and quantity ; 

 this continues until it finds its extreme expression in the uniformity of the 

 inner parts of the Gulfs of Bothnia and Finland, where at every step one finds 

 almost pure populations of Macoma baltica on the gyttja in the bays. Starting 

 from the Darss ridge itself we find, in effect, only one biocoenosis — Macoma 



