sediments which is dominated by higher boiling saturated hydrocarbons. 

 This phenomenon is unexplained and could represent selective accumula- 

 tion and/or retention of lighter aliphatics or more rapid metabolism and 

 excretion of, heavier aliphatics. The most likely explanation is that 

 oysters were being contaminated with hydrocarbons leaching from bottom 

 sediments into the water column. Lighter aliphatics, because of their 

 slightly higher aqueous solubility than heavy aliphatics, are desorbed 

 more readily from sediments and therefore are more available for uptake 

 by the oysters. Aliphatic hydrocarbon fractions of reference oysters 

 were more uniform (Table 7). Relative abundances of C^q to C32 aliphatics 

 were similar. 



There were no consistent differences in characteristics cf the ali- 

 phatic hydrocarbon fraction between reference oysters and oysters from 

 oil-polluted Aber Benoit and Aber Wrac'h (Table 8). With one exception 

 (April 1979), alkane/isoprenoid ratios were higher in oysters from 

 reference stations than in those from oil-polluted stations. Pristane/ 

 phytane ratios were quite variable and without pattern. All but two 

 carbon preference indices were near one indicating a petroleum origin 

 for the high molecular weight aliphatic fraction. 



Composition of the aromatic fraction of oysters, as determined by 

 gas chromatography /mass spectrometry, revealed a great deal about the 

 origin of the hydrocarbon contamination of the oysters (Tables 9-11) . 

 High concentrations of alkyl naphthalenes through alkyl dibenzothiophenes 

 are characteristic of samples contaminated with crude oil. Amoco Cadiz 

 oil was particularly rich in alkyl phenanthrenes and alkyl dibenzothio- 

 phenes. These were the most abundant aromatics/heterocyclics in oyster 

 samples from oil-contaminated Aber Benoit and Aber Wrac'h. Aromatic 

 hydrocarbon assemblages of crude oil origin are dominated by alkylated 

 species, whereas aromatic assemblages of pyrogenic origin are dominated 

 by the unalkylated parent compound (Neff, 1979). Thus we can conclude 

 that oysters from Aber Benoit and Aber Wrac'h at all five sampling times, 

 and reference oysters from the December 1978 and April 1979 collections 

 were heavily contaminated with crude oil, resembling the Amoco Cadiz oil. 

 The other three reference samples contained some oil, but it did not 

 resemble Amoco Cadiz oil. In oysters from the two Abers, there was a 

 general trend for the concentration of aromatics/heterocyclics in the 

 alkyl naphthalenes to alkyl dibenzothiophenes series to decrease 

 slowly with time. The February 1980 samples contained higher concentra- 

 tions of alkyl phenanthrenes and alkyl dibenzothiophenes than expected. 

 It is possible that winter storms in December and January resuspended 

 oil-contaminated sediments causing recontamination of resident oysters. 



293 



