showed direct effects. Crustaceans were especially hard hit, but less 

 than a year later, recovery was well underway. Oil analyses of mussels, 

 Mytilus edulis , showed that oil had reached a larger area than was 

 visibly impacted. Extremely high oil levels were found in Mytilus after 

 the spill, and a year later Tsesis oil was still evidenced in the mussels. 



Sediment traps deployed in the area collected material strongly 

 contaminated with oil for the first weeks after the spill, demonstrating 

 that oil was rapidly transported to the benthos. A minimum sedimentation 

 of about 20 tons of oil was estimated. About two weeks after the spill, 

 when the first samples were taken on deeper (30m) soft bottoms, oil 

 impact was already extensive. At the most heavily affected station, 

 motile macrofauna were greatly reduced, possibly through emigration. 

 The sedentary species remained, and no increase in their mortality was 

 demonstrated. Macoma balthica showed high oil levels over a large area, 

 even at stations where no clear impact was shown by macrofauna community 

 composition. All meiofauna groups, except the nematodes, were also 

 reduced at the most affected station, and there was evidence of high 

 mortality of ostracods. A few months after the spill, the few remaining 

 gravid amphipod ( Pontoporeia af finis ) females at the most affected 

 station showed an increased frequency of abnormal eggs. The recovery of 

 the deep soft bottoms after oil damage proved slower than for other 

 systems studied. About ten months after the spill, neither macro- nor 

 meiofauna at this station showed any recovery, and oil levels in Macoma 

 balthica were higher than immediately after the spill. 



In June, seven months after the spill, the herring showed less 

 spawning and lower hatching success in the oiled area than in a refer- 

 ence area. This might be due to factors other than oil and needs con- 

 firmation. 



The chemical analysis by Gas Chromatography (GC) and Mass Spec- 

 trometry (MS) showed rapid weatheririg of the Tsesis oil, and all oil 

 found either in biota or in sediment trap material was altered in com- 

 position, but identifiable as Tsesis oil. Depuration in Mytilus and 

 Macoma showed more rapid elimination of the aliphatic than the aromatic 

 fractions. The trimethylbenzenes were identified as a fraction particu- 

 larly resistant to degradation. The oil analyses proved invaluable for 

 correct interpretation of the ecological data. 



