THE BALTIC SEA 



325 



Gotland Island, Pontoporeia-Mesidothea-Macoma, is very clearly repre- 

 sented in the Gulf of Bothnia, with a tendency for the biomass to be consider- 

 ably less. 



The benthos biomass decreases markedly as one moves northwards (except 

 along the very shores of the bays). In the south of the Gulf of Bothnia (Bot- 

 tensee) in the shallow zone of the open sea the biomass is 30 to 40 g/m 2 down 

 to a depth of 40 m ; but with increasing depth it is reduced to a few grammes 

 on account of the decrease of the number of specimens and the reduced 

 size of Macoma baltica and Mesidothea entomon. The benthos composition 

 at depths above and below 40 m is given in Fig. 1 50. For the southern part 

 of the Gulf of Bothnia the same decrease with depth is given in Table 132. 



The northern part of the Gulf of Bothnia with a salinity of no more than 

 4% , with the latter falling as one moves northwards, has an extremely im- 

 poverished benthos with an average yield of 2-384 g/m 2 . Pontoporeia affinis is 

 the dominant form here; it is followed by Mesidothea entomon, Macoma 

 baltica and finally by the oligochaetes (see Fig. 151). M. baltica moves north- 

 wards only up to a salinity of 3-5% , disappearing when the water is less saline 

 than this. The graph of the indices of the community density is given in Fig. 

 151. Of the 22 stations surveyed by Hessle in the northern part of the Gulf of 

 Bothnia, Macoma was found at only two, Pontoporeia at 19. The highest 

 quantitative indices for the latter are 2,160 specimens per m 2 at a weight of 

 4-3 g. The average for all the Bottenwiek stations is 505 specimens and 1-32 

 g/m 2 . The corresponding data for Mesidothea are 2-7 specimens and 0-32 

 g/m 2 , and for Macoma 15 specimens and 0-80 g/m 2 . The biomass decrasese 

 somewhat with depth. For depths of less than 10 m it equals, on an average, 

 3-32 g/m 2 , while at 1 1 to 50 m it is 2-44 g/m 2 . In the bays of the off-shore zone 

 of the Gulf of Bothnia the fauna is undoubtedly much richer in numbers, 

 primarily on account of Chironomidae and Oligochaete larvae. 



However, there are so far no quantitative data on this part of the Gulf. 

 On the basis of his own researches Hessle comes to the conclusion that 

 Bottenwiek is very poor in benthos biomass, mainly as a result of a consider- 

 able admixture of iron oxides in the sea-bed. A considerable area of the floor 

 of the Gulf is covered with these non-productive red sands. As for the number 

 of specimens, here too it is at times high — up to 1,000 specimens of Ponto- 

 poreia affinis per 1 m 2 . 



