There is also a gradient in climate, from subarctic conditions and more 

 than six months of ice cover in the coastal zone of the extreme north, 

 to a more maritime climate with an average of only a month of coastal 

 ice in the south. Salinity is the most important controlling factor for 

 the ecology of the Baltic Sea. The organisms are present in an osmotic 

 environment, which allows only a limited number of euryhaline marine and 

 fresh-water organisms and a few brackish-water specialists to establish 

 themselves in the inner Baltic. The high osmotic stress, therefore, 

 results in a simple ecosystem with just a few dominant species. The 

 classic generalized "Remane's curve" (Remane , 1934) shows a marked 

 species minimum at 6-7 /oo S. For further information, the reader is 

 referred to the following general reviews of the ecology of the Baltic 

 Sea: Remane , 1934, 1940, 1958; Sergerstr9le , 1957; Zenkevitch, 1963; 

 Jansson, 1978. 



There has been an increase in tanker traffic in the Baltic Sea, 

 both with respect to the frequency of ships in the area and to tonnage. 

 From April 1976 the new buoyed-off fairway at Darss Sound at the en- 

 trance of the Baltic Sea permits tankers of up to 160,000 tons (dw) to 

 enter. The extreme risk of huge spillages when ships of this size enter 

 the Baltic is obvious. The shallow and imperfectly sounded waters 

 increase the risk of grounding. Also, the poor visibility due to prevail- 

 ing weather conditions during most of the year, as well as the high fre- 

 quency of ships sailing in the area, increases the risk of grounding and 

 collision. Totally, at least 1,000 spills with a calculated volume of 

 about 100,000 tons are reported each year (Engdahl , 1976). Accidents 

 involving spillage of oil are permanent phenomena in the Baltic. 



In heavily congested areas, such as the entrance of the Sodertalje 

 ship channel, spills are also very frequent. Less than 8 months before 

 the grounding of Tsesis , another accident resulting in a spill of 100-200 

 tons of medium grade (No. 5) fuel oil occurred about 5 km north of the 

 Tsesis rock, but most of the oil drifted north into Himmerf jarden, and 

 the area most heavily oiled by Tsesis was only lightly touched (as shown 

 also by the relatively low pre-Tsesis-spill hydrocarbon levels in Mytilus 

 in this area, see section 11). About four months after the grounding of 



30 



