ENVIRONMENTAL-FACTORS INFLUENCING THE BIODEGRADATION OF PETROLEUM-HYDROCARBONS IN COLD SEAWATER

A group of five mesocosms (3.5 m(3) 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 degrees 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 >O degrees 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.