The pelagic study covered only the acute phase of the spill, essen- 

 tially the first month after the grounding. Valid oil concentration 

 results were obtained from only two water column samples during this 

 period. These two samples suggested concentrations of around 60 (Jg/1 of 

 micro-dispersed oil droplets under a weathered and fully emulsified part 

 of the Tsesis oil slick. The phytoplankton biomass generally increased 

 in the affected area, but it could not be shown whether this was due to 

 increased growth rates or to decreased zooplankton grazing. Correspond- 

 ingly, primary production was elevated. Phytoplankton species composi- 

 tion showed little change. Zooplankton were found to be heavily contami- 

 nated (internally and externally) by oil (50% of the specimens were 

 visibly contaminated with oil droplets in the first week, 20% after 

 three weeks); although no reduction was found in the standing stock, 

 either for zooplankton or ciliates, except in the immediate vicinity of 

 the ship. Bacterial abundance increased in the spill area, but here, 

 too, it was inconclusive as to whether this was due to decreased grazing 

 pressure or to increased growth rates. 



The most interesting result of the pelagic study was the high oil 

 content (up to 0.7%) found in sediment trap material during the first 

 two weeks after the spill. This was true also for sediment traps de- 

 ployed in an area windward of the tanker, where no oil slicks were ever 

 observed and which had been thought to be little affected by the spill. 

 Within a week the oil in the sediment traps was significantly altered by 



weathering. The minimum extent of the impacted area can be estimated to 



2 

 be 42 km from oil analysis of sedimented material and of Mytilus and 



Macoma . By extrapolating from the sediment trap data it is possible to 



give a minimum estimate of 19 tons of oil reaching the sediment. An 



amount of 40 tons, which is not unlikely, would be 10% of the estimated 



total amount remaining after oil recovery operations had ceased. 



One month after the spill all parameters measured in the pelagic 



system were essentially normal (phytoplankton, zooplankton, bacteria, 



oil content of sedimenting material). This indicates the relatively 



short duration of oil impact from a moderate spill in the pelagic system. 



Acute effects on the fish could not be demonstrated. 



49 



