Seasonal effects on the degradation potential of oil recovered from a 50 year old shipwreck

Two different oils, a Bunker C heavy fuel oil and a marine diesel-type oil, were recovered from the M/V Schiedyk 53 years after sinking. Microcosm experiments to quantify their degradation potential, potential toxicity and the microbial response to oiling were conducted in 4 seasons. Incubations lasted 21 or 28 d without additional nutrients and with low (<200 ppm) oil concentrations. The recovered oils showed no indication of prior weathering. Abiotic degradation rate constants indicate faster removal of polycyclic aromatic compounds (PACs) (0.037 d^-1 ± 0.024 (n = 237)) compared to n-alkanes (0.006 d^-1 ± 0.002 (n = 60)). Biodegradation rate constants could rarely be calculated (n = 53 total). Abiotic losses of n-alkanes from marine diesel were 15× higher than with Bunker C. Losses of parent PACs (0.64% ± 1.70) were lower than losses of alkylated PACs (10.85% ± 19.51), which had lowest losses in winter. Potential toxicity of the oils remained high throughout the incubations, with higher toxicity associated with marine diesel (76.9% ± 3.2) compared to Bunker C (65.7% ± 4.3). Although little biodegradation was observed and no bloom of prokaryotes occurred, taxa associated with aerobic degradation of both n-alkanes and PACs increased. The identity of the taxa varied with season, reflecting differences in temperature and nutrient status. The lack of detectable biodegradation is likely associated with nutrient limitation. Overall, the results suggest oil recovered from shipwrecks after half a century may have limited weathering and some susceptibility to abiotic degradation with biodegradation depending on the local temperature and nutrient concentrations.