ABSTRACT 



On October 26, 1977, the Soviet tanker Tsesis struck a rock in the 

 fairway while inbound through the archipelago off Sodertalje, Sweden 

 (northern Baltic proper). During the next few days she released about 

 1100 tons of oil, mostly a No. 5 fuel oil, but also some bunker oil. 

 Due to a quick and efficient response by the Swedish Coast Guard, and 

 unusually favourable circumstances, the clean-up operations recovered 

 most of the oil, leaving only about 400 tons in the enclosed archi- 

 pelago, visibly oiling an area of 34 km . 



Within a day after the grounding, a cooperative international 

 scientific investigation was launched to cover important aspects of the 

 spilled oil's ecological effects on plankton, benthos, fish, littoral 

 and supralittoral communities, as well as the chemical and biochemical 

 processes of weathering, bioaccumulation and depuration. The area was 

 one which had been relatively well studied in the past as the site of 

 several marine biological programs. The spill study was initiated 

 rapidly enough to have some littoral sites surveyed immediately before 

 impact by oil--an example of the "ideal" oil spill study. 



The direct effects on the plankton included decreased zooplankton 

 biomasses in the immediate vicinity of the tanker. Within days, in- 

 creased phytoplankton biomass and primary production, as well as bac- 

 terial abundance, were noted in a larger area surrounding the tanker. 

 Here zooplankton abundance and biomass were apparently not affected in 

 spite of considerable contamination of zooplankton by oil droplets. No 

 contamination or harmful effects on pelagic fish could be demonstrated. 

 In a small bay near the spill site, oil concentrations of 60 [Jg/1 were 

 found in the water below a weathered oil slick. After one month, all 

 parameters measured in the pelagic zone had returned to normal values. 



Damage to bird life and the littoral zone was alleviated by the 

 season of the spill. The supralittoral zone showed little damage when 

 surveyed the following summer. In the littoral zone, no effects on the 

 algae were seen, but the fauna of the most heavily oiled coastline 



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