THE BALTIC SEA 327 



Table 134 



It must be perfectly clear from the above why in winter this saline, deep 

 water has a higher temperature than the surface layers, and why it retains its 

 reduced amount of oxygen, thereby destroying its winter homohalinity and 

 homothermia. The arrival of this water in summer also destroys the summer 

 homothermia and homohalinity. 



Moving from the Nevskaya Guba to the west, we observe a gradual change 

 in the composition of the mass forms. According to the researches of S. Sager- 

 strale (1923), in the Pellinge area (coast of Finland, marked approximately 

 centrally on the chart — Fig. 121 — of the Gulf of Finland) at a salinity of 5 to 

 6% the main components of the benthos are again Macoma baltica, Mesidothea 

 entomon, Pontoporeia affinis and Chironomidae, with the addition of Cardium 

 edule. In other words we already have here the Macoma baltica biocoenosis. 



Farther to the west, where the Gulf of Finland opens into the Sea, in the 

 Twerminn area (see Fig. 129), with the deep-water salinity slightly above 

 6% and the annual variations of salinity of not more than l-7% , the dominant 

 form is again Macoma baltica ; it is followed by Chironomidae, Pontoporeia 

 affinis, Cardium edule, Mesidothea entomon, with the addition of Halicryptus 

 spinulosus and Mytilus edulis. The biomass of the Gulf of Finland increases 

 considerably from 25-75 to 60-206 g/m 2 (the average for Pellinge is 55 and 

 for Twerminn 1 19 g/m 2 ) as one moves from the centre of it to its exit. 



The first area of S. Sagerstr ale's work (Pellinge, 1932) has a soft mud bottom 

 rich in organic matter, the so-called gyttja or sapropel. The composition of the 

 bottom communities of this area are illustrated in Fig. 158. The first is at a 



48-24 61-58 



%2 



Ш з 



]' 



Fig. 158. Composition of bottom communities of 

 shores of Finland (Pellinge) (Sagerstrale, 1932). 

 Mean biomass, g/m 2 , is shown above the circles. 

 1 Macoma baltica ; 2 Mesidothea entomon ; 3 Ponto- 

 poreia femorata ; 4 Chironomidae ; 5 Others. 



