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



(1949). He points out that in the changes of Baltic Sea flora and fauna account 

 should be taken not only of the qualitative factors — the appearance of a 

 formerly unknown form, but also the quantitative ones — a rare form can be- 

 come predominant. 



Among the species formerly unknown in the Baltic Nikolaev notes the 

 diatom algae Coscinodiscus granii and the mullet Mugil capito, and among the 

 forms which have migrated into the eastern and northern areas of the Sea : 

 Sagitta elegans baltica ; the amphipod Bathyporeia pilosa ; and among the 

 fish : anchovy (Engraulis encrassicholus), marine pike (Belone belone), mackerel 

 {Scomber scomber) and the marine turbot (Onos cimbrius). 



Quantitative biocoenotic distribution of benthos. As one moves farther into 

 the Baltic Sea an impoverishment is observed, both in species and in the 

 variety of bottom communities. 



Petersen established eight benthic biocoenoses in a small area of the 

 Skagerrak; in the German Belt there are only two of these, the 'Abra bio- 

 coenosis' and the ' Macoma baltica biocoenosis'. All the rest of the compara- 

 tively huge area of the Baltic Sea bottom is occupied by only one community, 

 the Macoma baltica. 



Data for an estimate of the qualitative and quantitative distribution of 

 the bottom communities of the Baltic Sea are given in the works of A. Hag- 

 meier (1926, 1930), G. Thulin (1922), Chr. Hessle (1924), S. Sagerstrale (1923), 

 A. Remane (1933, 1940, 1955), F. Gessner (1933, 1940, 1957), and K. 

 Demel and his collaborators (1935, 1951, 1954). The quantitative biocoenotic 

 distribution of the bottom fauna of the Baltic Sea presents a fairly simple 

 picture in consequence of the qualitative impoverishment of the population 

 and the two important factors of the medium — lower oxygen content in the 

 deeper layers and the gradual fall of salinity from west to east ; this general 

 picture is fully brought out by the researches mentioned above. The distri- 

 bution of the main bottom communities throughout the Baltic Sea is given in 

 Fig. 149. The data refer to the average benthos biomass in g/m 3 . 



In general, moving from west to east, we can distinguish in the Baltic Sea 

 four main biocoenoses : (7) Cyprina + Astarte (a modification of Petersen's 

 'Abra biocoenosis') in the German Belt (Kiel and Mecklenburg Bays and the 

 adjacent sea areas) ; (2) Macoma calcarea (Arcona and Bornholm depressions 

 and the adjacent sea areas) ; (5) Macoma baltica and Astarte borealis (most of 

 the Baltic Sea and the Gulfs of Bothnia and Finland) ; and (4) Pontoporeia+ 

 Mesidothea (the northern part of the Gulf of Bothnia). 



(1) Cyprina-\- Astarte biocoenosis. According to the results of Petersen's work 

 in the deeper parts of the southern Kattegat, the dominant forms are Abra 

 alba, Macoma calcarea, and Cyprina islandica, while in the shallower Kiel 

 and Mecklenburg Bays Cyprina islandica and Astarte borealis become markedly 

 preponderant ; they provide, at some stations, a biomass of up to 450 g/m 2 in 

 the first of these bays, and 190 g/m 2 in the second. The average biomass of the 

 whole of this area is 176-6 g/m 2 . 110-2 g of this consists of Cyprina islandica 

 and 32 g of Astarte borealis. All the rest provides only 34-4 g/m 2 (see Fig. 150). 



