A seawater field study of crude and fuel oil depletion in Northern Norway at two different seasons - Chemistry and bacterial communities.

After marine oil spills, natural processes like photooxidation and biodegradation can remove the oil from the environment. However, these processes are strongly influenced by environmental conditions. To achieve a greater understanding of how seasonal variations in temperature, light exposure and the bacterial community affect oil depletion in the marine environment, we performed two field experiments during the spring and autumn. Field systems equipped with a thin oil film of Statfjord, Grane or ULSFO were deployed in northern Norway. Depletion of the total extractable matter was faster during the spring than during the autumn. Statfjord showed faster depletion of n-alkanes during spring, while depletion of polycyclic aromatic hydrocarbons varied between the seasons based on the degree of alkyl-substitutions. ULSFO displayed the overall slowest depletion. Biodegradation of the oils was associated with high abundances of unassigned bacteria during the spring but was governed by Alcanivorax, Cycloclasticus, Oleibacter and Oleispira during the autumn.

Comparison of two field systems for determination of crude oil biodegradation in cold seawater.

Marine oil spills have devastating environmental impacts and extrapolation of experimental fate and impact data from the lab to the field remains challenging due to the lack of comparable field data. In this work we compared two field systems used to study in situ oil depletion with emphasis on biodegradation and associated microbial communities. The systems were based on (i) oil impregnated clay beads and (ii) hydrophobic Fluortex adsorbents coated with thin oil films. The bacterial communities associated with the two systems displayed similar compositions of dominant bacterial taxa. Initial abundances of Oceanospirillales were observed in both systems with later emergences of Flavobacteriales, Alteromonadales and Rhodobacterales. Depletion of oil compounds was significantly faster in the Fluortex system and most likely related to the greater bioavailability of oil compounds as compared to the clay bead system.

Acute toxicity of dispersed crude oil on the cold-water copepod Calanus finmarchicus: Elusive implications of lipid content.

In this investigation, acute toxicity data were used from two previously reported studies where cold-water copepods were exposed to mechanically dispersed (MD) and chemically (CD) dispersed oil. In one of these studies, concentration-dependent mortality was observed, whereas no apparent relationship between exposure concentration and mortality was found in the other. The only marked difference between the studies is that copepods in the first experiment displayed a lower lipid sac volume (on average) than in the second one. In this study additional biometric data on lipid content were utilized and observed effects and toxicokinetics modeling applied in order to investigate whether differences in sensitivity between copepod cohorts might be explained by differences in lipid content. Results suggest that although a considerable lipid sac might retard toxicokinetics, the observed differences in lipid volume are not sufficient to explain differences in toxicity. Further, there are no apparent indications that acute toxic stress leads to lipid depletion, or that acute increased mortality rate selectively affects lipid-poor individuals. It is conceivable that other potential explanations exist, but the causal relationship between lipid content and increased mortality frequency remains elusive.

Oil droplet ingestion and oil fouling in the copepod Calanus finmarchicus exposed to mechanically and chemically dispersed crude oil.

The rates of ingestion of oil microdroplets and oil fouling were investigated in the zooplankton filter-feeder Calanus finmarchicus (Gunnerus, 1770) at 3 concentrations of oil dispersions ranging from 0.25 mg/L to 5.6 mg/L. To compare responses to mechanically and chemically dispersed oil, the copepods were exposed to comparable dispersions of micron-sized oil droplets made with and without the use of a chemical dispersant (similar oil droplet size range and oil concentrations) together with a constant supply of microalgae for a period of 4 d. The filtration rates as well as accumulation of oil droplets decreased with increasing exposure concentration. Thus the estimated total amount of oil associated with the copepod biomass for the 2 lowest exposures in the range 11 mL/kg to 17 mL/kg was significantly higher than the approximately 6 mL/kg found in the highest exposure. For the 2 lowest concentrations the filtration rates were significantly higher in the presence of chemical dispersant. Furthermore, a significant increase in the amount of accumulated oil in the presence of dispersant was observed in the low exposure group.