6.4 Discussion 



6.4.1 Sediment samples 



No Tsesis oil hydrocarbons could be identified with certainty in 

 any of the sediment samples. Nevertheless, we can state with certainty 

 that oil must have reached the sediment surface, since 1) the sediment- 

 ing material in the water column contained large concentrations (up to 

 0.7%) of oil, especially in the first week after the spill; 2) large 

 Macoma balthica collected not only at station 20 but also at other 

 stations (e.g., 5, 6 and 8) contained considerable quantities of oil 

 hydrocarbons within about a month after the spill, when the first Macoma 

 samples for oil analysis were collected (see Section 11 for details); 

 and 3) a smell of oil was detected from several grab samples at Station 

 20 in November 1977. 



The failure to find oil in the sediment samples can probably be 

 explained by several interacting factors. Newly sedimented oil will be 

 concentrated in the uppermost flocculent surface layer of the sediment, 

 and this layer is not well sampled by gravity corers such as those used 

 in the present investigation (cf. Mclntyre 1971, Elmgren 1973). The 

 sampling efficiency of the Asko corer for meiofauna has been tested by 

 Ankar and Elmgren (1976). For the non-nematode meiofauna, which tend to 

 live in the floccular top sediment, the efficiency was only 38-61 / o . The 

 Kajak corer has been shown to be even less efficient (Hallberg, BSgander 

 and Elmgren, unpublished). After the sampler had been brought to the 

 surface, it is highly likely that there was even further loss as the 

 water, with its suspended load of oil-contaminated surface floe, was 

 poured off from above the sediment, prior to extrusion and sectioning of 

 the cores . 



Finally, the sediment contained fairly high concentrations of 

 biogenic hydrocarbons and in some cases unidentified anthropogenic 

 hydrocarbons as well and this tended to swamp the signal from the newly 

 sedimented oil hydrocarbons. This is especially likely since sections 

 as thick as 2 cm were used for all early sediment cores. 



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