(Larsson, unpubl.), even in the heavily eutrophicated innermost parts of 

 the Himmerfjard, where bad oxygen conditions are normal in late summer 

 and early autumn. Second, oxygen deficiency would not have resulted in 

 a decrease in Harmothoe sarsi , which normally is the macrofauna species 

 most tolerant towards oxygen deficiency in the northern Baltic proper 

 (e.g., Cederwall, 1978). Finally, since there is no question of any 

 oxygen deficiency occurring after the breakdown of the thermocline in 

 early October 1977 or before late summer 1978, one would have expected 

 considerable re-invasion by Pontoporeia and Harmothoe from the surround- 

 ing areas during the 9-10 months of good oxygen conditions. There is 

 little evidence of such an immigration to station 20, the variations 

 noted being more likely due to the natural patchiness of the benthos 

 (since Macoma balthica which is sedentary also varied). The continued 

 presence of repellent oil hydrocarbons in the sediment indicated by 

 continued high hydrocarbon levels in Macoma balthica from station 20 

 could, on the other hand, easily explain the absence of immigration 

 during the first ten months following the spill. 



The impact of the oil spill on the macrobenthic community at sta- 

 tion 20 was clear, but not totally catastrophic. While total abundance 

 declined drastically, due to the reduction in abundance of Pontoporeia 

 (both species) and Harmothoe , there was little change in biomass, since 

 this is dominated by Macoma balthica , which did not decrease. The abun- 

 dance of the two macrofauna species without swimming ability, and with 

 generally low mobility, Macoma and Halicryptus , seem to have been little 

 influenced by the spill, even though Macoma , at least, showed consider- 

 able contamination by oil hydrocarbons (see Section 11). The relatively 

 low sensitivity of Macoma balthica to oil pollution, coupled with its 

 ability to take up hydrocarbons from the sediment, makes it an excellent 

 indicator of the level of oil pollution to which a soft bottom station 

 has been subjected. This supports the value of M. balthica as an indi- 

 cator of oil pollution, as suggested by Shaw et al. 1976, 1977, and 

 supported also by Taylor and Karinen, 1977. 



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