Environmental factors influencing the biodegradation of petroleum hydrocarbons in cold seawater

A group of five mesocosms (3.5 m3 each) located at Pointe-au-Pere (St. Lawrence Estuary), Canada, was used to study the biodegradation of crude oil dispersed in cold and icy seawater (−1.8 to 5.5°C) under various environmental conditions. Experiments took place during autumn, winter, and spring and lasted from 2 weeks to 2 months. The bacterial response to the oil was assessed by recording the growth of total bacteria, viable heterotrophic bacteria, and oil-degrading bacteria. Some hydrocarbon ratios were calculated from gas chromatography in aliphatic and aromatic oil fractions and were used as biodegradation indices. A “Combined Index of Biodegradation” is proposed for assessing the overall biodegradation advancement. The winter period appeared critical for an oil spill in arctic/subarctic environments because of the reduced biodegradation under icy conditions. Crude oil adsorbed onto a substrate was found more degraded on its immersed part than on the emerged section exposed to winter conditions. Under more favorable environmental conditions (temperatures >0°C, effective chemical dispersion, oil release, spring microalgal bloom), the bacterial degradation would significantly alter the dissolved/dispersed oil within a few days. Under such conditions, half-life times of dissolved petroleum PAH ranged from 1.5–1.7 days (naphthalene) to 2.4–7.5 days (dimethylphenanthrenes), depending on the contamination level. In microenvironments where oil residues accumulated with biological detritus (surface microlayer, settling matter), the oil biodegradation was naturally enhanced. In contrast, water-in-oil emulsions recovered at the surface of mesocosms were unaltered after one month exposure in autumn.