pounds were, however, noted at depths below the minimum Eh 20-26 months 

 after the oil spill. It is difficult to rule out exposure to oxygen in 

 these subsurface muddy- sediment environments. The degree of storm 

 driven mixing and/or irrigation by sediment fauna are unknown and might 

 be significant over a 12-18 month time course. More than two years 

 after the AMOCO CADIZ spill many well resolved aromatic compounds per- 

 sisted (e.g., phenanthrenes) ; however, the slow disappearance of some 

 aromatic hydrocarbons (e.g., naphthalenes) from subsurface muds may 

 indicate that these resolved compounds will not persist indefinitely. 



14 

 The potential biodegradation of hydrocarbons measured using C- 



labelled hydrocarbons was much lower under anaerobic than under aerobic 

 conditions. For example, 33% of added [1- C] -hexadecane was converted 

 to C-gases aerobically, whereas only 3% was converted anaerobically 

 after 233 days in Aber Wrac'h sediment (Table 4). Due to the expected 

 high oxygen demand of the surface sediments, it is likely that 0„ deple- 

 tion occurred rapidly in vials incubated aerobically. Thus, it is got 

 surprising that complete conversion of added C-hydrocarbons to C- 

 gases did not occur in long term aerobic incubations. Since the added 

 radiolabelled hexadecane (4.2 |jg/sample) exceeded the indigenous hexa- 

 decane measured in these sediments (16-174 ng/sample), potential rates 

 of aerobic and anaerobic metabolism can be calculated by multiplying 

 the percentage conversion by the amount of hexadecane added and divid- 

 ing by the number of days incubation and dry weight of the sample. The 

 maximum level of C-gases produced under aerobic conditions was de- 

 tected at the earliest analysis time (66 days). This leads to a cal- 

 culated rate of 13.8 ngm/gm-dry weight/day. The true potential rate is 

 probably greater due to incomplete exposure of the entire sample to 

 oxygen and to oxygen depletion. Calculations using rates of oxygen 

 consumption for European coastal sediments suggest that oxygen should 

 have been completely consumed within the first ten days of incubation. 

 The corresponding rate would be 91 ngm/gm-dry weight/day, approximately 

 one-fifth of the potential rate reported by Atlas and Bronner (1981). 

 C-gases increased with time during anaerobic incubation. The corre- 

 sponding potential anaerobic rate of hexadecane metabolism after 233 

 days incubation of 0.3 ngm/gm-dry weight/day is 46 times slower than 

 the measured potential aerobic rate and over 300 times slower than the 

 aerobic rate calculated from reasonable assumptions about the condi- 

 tions under which aerobic controls were run. These results are similar 

 to other reports which demonstrate the severe limitations on hydrocar- 

 bon metabolism imposed by reduced amounts or lack of oxygen (Ward and 

 Brock, 1978; Hambrick, et al, 1980; DeLaune, et al, 1981). It was in- 

 teresting that no evidence was obtained for anaerobic naphthalene oxi- 

 dation. Obvious problems of naphthalene volatility may have decreased 

 the amount actually added to vials thereby lowering the sensitivity for 

 detecting its oxidation. 



As in earlier studies (Ward and Brock, 1978), it was not possible 

 to eliminate metabolism of hydrocarbons under stringent anaerobic con- 

 ditions. It was important to investigate the possibility that slow 

 anaerobic oxidation might occur in order to predict whether or not hy- 

 drocarbons buried in permanently anoxic sediments persist indefinitely. 

 Controls were run against the possibility of initial accidental inclu- 

 sion of oxygen, photosynthetic oxygen production, oxygen leakage into 



184 



