THE BALTIC SEA 345 



Three investigators, I. Valikangas (1933), A. Remane (1935) and S. Sager- 

 strale (1957), have given a more detailed estimate of the brackish waters of the 

 Baltic Sea. The first of them has established three marked limits of qualitative 

 change of Baltic Sea fauna as we move from west to east : (7) the area lying 

 between the Kattegat on one side and the Belt and Oresund on the other, with 

 salinity fluctuations of 1 5 to 20% o ; (2) the outlets from these straits into the 

 Baltic Sea with a salinity of 8 to 10% o ; and (J) a zone of much reduced salinity, 

 which differs somewhat for various groups: 3 to 3-5% salinity for the brown 

 and red algae and a little higher for the molluscs. 



Valikangas, on the basis of Redeke's scheme, divides the marine waters into 

 the oligohaline (0-2 to 2-0% o ), mesohaline (2-0 to 16-5% ) and polyhaline 

 (>16-5% )asin Table 139. 



Remane approached this problem in a rather different way. He took as a 

 basis the natural distribution of organisms and counted the number of species 

 of different genesis in waters of varying salinity. Thus Remane was the first 

 to apply, in the classification of the brackish zone, an indirect quantitative 

 method. He established that in the Baltic Sea marine forms more than 50 per 

 cent were at a salinity of 30 to 17% , while in the range 17 to 8% the propor- 

 tion fell from 50 to 30 per cent. As a result Remane gives the following sub- 

 divisions for the Baltic Sea according to its salinity : 



I. Purely marine zone 35 to 15% 



II. Brackish- water zone 15 to 3% 

 (7) Brackish-marine mixed zone with a preponderance of 



marine forms 15 to 10% o 



(2) True brackish-water zone with a maximum develop- 

 ment of specific brackish- water fauna 10 to 5% 



(3) Brackish-fresh-water mixed region with a preponder- 

 ance of fresh-water elements 5 to 3% 



///. Fresh- water zone <3% 



Moreover the limits for various groups of organisms may be different. 

 Remane's system is applicable to benthos; for plankton the fresh-water 

 elements are already dominant at a salinity of 5 to 7% . Remane distinguished 

 the four following groups of organisms : 



(7) Euryhaline fresh-water forms 

 (2) Euryhaline marine forms 



