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



(4) Pontoporeia-\- Mesidothea biocoenosis. In the most northern part of the 

 Gulf of Bothnia, at a salinity of 3-5% , Macoma disappears and is replaced 

 by the Pontoporeia-f- Mesidothea biocoenosis in its pure form. This in its 

 turn passes at the shore-line into a mixed biocoenosis of Chironomidae and 

 Oligochaeta. 



In the Gulf of Finland we find a similar but somewhat different picture. As 

 has been mentioned above, this Gulf is not separated from the central part of 

 the Sea either by islands or a submarine ridge ; hence the way is open for 

 both more saline waters and their characteristic fauna to enter through the 

 deep channel. 



We shall consider the bottom fauna of the Gulf of Finland in greater detail, 

 from east to west, beginning at the Nevskaya Guba. 



Research organized by the State Hydrological Institute in 1923-24 and 

 1934-35 under the direction of Derjugin has shown that the so-called Nev- 

 skaya Guba, i.e. the area separated from the open sea by Kotlin Island and 

 the Oranienbaum shoal, has completely fresh-water conditions. A small 

 amount of salinity, evident in the western part of the Nevskaya Guba, has 

 no substantial influence on the fauna, which there has a true fresh-water 

 character with a preponderance of molluscs, oligochaetes and insect larvae. 

 To the west of the Oranienbaum shoal the typical Baltic relict community 

 mentioned above comes in full strength with a preponderance of Mesidothea 

 entomon, Pallasea quadrispinosa, Pontoporeia affinis, Mysis oculata and the 

 addition of some extremely euryhaline forms such as Gammarus zaddachi, 

 G. locusta f. reducta, G. duebeni, Neomysis vulgaris f. baltica, Zoarces viviparus, 

 and the fresh- water chironomids. In this area, to the south and west of Kotlin 

 Island, the deep-water layer has an unstable salinity with marked fluctuations. 

 Saline water frequently flows in at a lower layer from the west (the pheno- 

 menon of internal waves). Thus the distribution of salinity given in Table 

 133 was once observed in March 25 km west of Kotlin Island. 



Table 133 



In February the saline stratification somewhat to the east of this station 

 was even more strongly marked (Table 134). 



According to different observations, the deep-water salinity south of Kotlin 

 Island was at one time 3-44% , at another 0-52% . Without doubt the magni- 

 tude of these fluctuations may be even greater. If the surface salinity fluctua- 

 tions west of Kotlin Island can reach a magnitude of from 003 to l-28% and 

 probably more, then in the deep-water layer they may be from 0-5 to 5-00% . 



