The effect of amount of crude oil on extent of its biodegradation in open water‐ and sandy beach‐ laboratory simulations

We examined the biodegradation of varying amounts of artificially weathered Alaskan North Slope crude oil in laboratory microcosm test systems that use natural seawater and simulate spills in open water and on sandy beaches. The model bioremediation treatment consisted of periodic applications of marine bacteria, selected to degrade n-alkanes and a range of aromatic compounds, suspended in a salts solution that supplied inorganic nitrogen and phosphorous. Beach microcosms dosed with low and high oiling lost an average of 22.5% and 11.3% oil weight, respectively. Open-water microcosms dosed with high and low oiling lost 19.1% and 2.9% oil weight, respectively. Thus, the lower doses of oil were more efficiently degraded. The model bioremediation treatment also affected a greater number of selected analytical endpoints in the lower-oil-dose than higher-dose experiments and the former showed more substantial degradation of recalcitrant components. Above a certain threshold oil concentration, bioremediation did not effectively remove oil. Below this threshold the distinction between active bioremediation treatment and intrinsic biodegradation of the controls was less prominent; i.e., fewer of the oil components were statistically depleted by remediation treatment relative to controls. Furthermore, the oil-dose range over which bioremediation was realized in these systems occurred at very low oiling levels. Thus, under the environmental conditions simulated in these microcosms, the effectiveness of bioremediation peaked over a rather narrow low-dose oiling range.