324 



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



Harmothoe sarsi, Halicryptus spinulosus, Pontoporeia femorata, Idothea 

 granulosa and /. viridis) becomes poorer still ; there is a further drop of salin- 

 ity to 1% ; this is the extreme northern limit of its distribution. It is practically 

 never found north of the Aland Islands, where Pontoporeia affinis, Mesidothea 

 and Chironomidae begin to appear in considerable numbers. 



The quantitative relationship of the main forms of benthos given in Table 

 131 is characteristic of the area of the Aland Islands. 



In the open sea to the west of the Aland Islands, at depths greater than 40 m, 

 the benthos biomass reaches 44-36 g/m 2 and 91 per cent of the benthos con- 

 sists of Macoma baltica. Deeper down a picture typical of the whole of the 

 Gulf of Bothnia is established : the biomass is reduced to 10 g/m 2 owing to the 

 decrease of M. baltica (23 per cent) ; Pontoporeia affinis becomes the dominant 

 form, comprising half of this fauna. Chr. Hessle (1924) suggests that these two 

 forms (M. baltica and P. affinis) are either competitors for food, or that cray- 

 fish destroys the Macoma larvae. Hessle tries in this way to find an explana- 

 tion for the peculiar bathymetric distribution of both forms and, chiefly, for 

 the fact that Macoma baltica disappears with increasing depth in the areas 

 north of Gotland Island, that is, in the areas of mass development of Ponto- 

 poreia affinis in the deeper layers. Off Aland Island, and to some extent off Got- 

 land, the populations of Macoma baltica are very abundant at depths of 100 to 

 140 m (i.e. in water which is very poor in oxygen) ; but Pontoporeia affinis does 

 not grow in large numbers there. Its place is taken by P. femorata, with which 

 M. baltica can exist without harm to itself. Mesidothea entomon chiefly 

 inhabits the deep waters of the Aland Sea and of the Gulfs of Bothnia and 

 Finland, existing at the expense of Pontoporeia affinis, which is its basic food. 



On the soft soils of the northern part of the central area the polychaetes 

 Nereis diversicolor in shallower places, and Harmothoe sarsi in deeper ones 

 (down to 200 m), are added to the three main fauna forms — Mesidothea and 

 the two species of Pontoporeia, which form the basic food of fish in the area. 



Towards the south Harmothoe sarsi increases in numbers at lesser depths, 

 limited by a salinity of about 7% ; simultaneously it becomes more important 

 as fish food. Pygospio elegans and Halicryptus spinulosus, though not as 

 important, are also significant on the sandy bottoms of the central area. 



The same benthic biocoenoses which were already formed in the area of 



