6.4.2 Macrofauna 



6.4.2.1 Macrofauna community response 



The drastic reduction in macrofauna abundance at station 20 after 

 the spill left little doubt that this was a direct effect of the oil. 

 The three species which decreased in abundance were all vagile forms 

 that may have emigrated from the area most heavily influenced by the 

 oil. Initially it was thought this a more likely explanation of their 

 virtual disappearance than direct mortality, since very few remains of 

 dead animals were found in the benthos samples taken 16 days after the 

 grounding of the Tsesis . Assuming that oil would take a couple of days 

 to reach the bottom in quantity and that mortality would not have been 

 instantaneous (that would probably have required oil levels high enough 

 to kill Halicryptus and Macoma also), any animals killed by the oil 

 would have had to disappear almost without a trace in about one week, 

 which was not very likely at a bottom water temperature of 7 C, at least 

 not for the cuticle-covered amphipods. Yablonskaya, (1947, quoted in 

 Winberg 1971) found that Chironomus larvae were still recognizable after 

 30 days at 5 C, and after 12 days at 10 C. Later simple aquarium experi- 

 ments with dead Pontoporeia af finis, kept in natural sediment at field 

 temperature, have indicated that 1-2 weeks might indeed be enough time 

 for the dead animals to disappear almost completely (Sundelin, unpubl.). 

 That amphipods actively avoid oil-contaminated sediments has been shown 

 experimentally by Percy (1977) for Onisimus affinis and the same is true 

 for Pontoporeia affinis (unpublished experiments by M. Notini, pers. 

 comm.) and probably also for P. femorata (Atlas et al., 1978). At 

 present it is therefore impossible to say with certainty whether most of 

 the amphipods emigrated or died. 



The only natural environmental perturbation that could have been 

 expected to give a reduction of the macrofauna as drastic as that found 

 at station 20 would have been a period of oxygen deficiency during the 

 autumn or late summer preceding the spill. The reduced sediment surface 

 and fairly low oxygen values found at station 20 in summer 1978 (post- 

 spill) could be taken as indicating that even lower oxygen values might 

 have occurred the year before. However, several lines of evidence 

 contradict this hypothesis. First, bottom oxygen values at the three 

 permanent stations in the "Himmerf jard" were unusually good during 1977 



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