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 (S t. 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) un der various environmental conditions. Experiments took place during au tumn, winter, and spring and lasted from 2 weeks to 2 months. The bact erial response to the oil was assessed by recording the growth of tota l bacteria, viable heterotrophic bacteria, and oil-degrading bacteria. Some hydrocarbon ratios were calculated from gas chromatography in al iphatic and aromatic oil fractions and were used as biodegradation ind ices. A ''Combined Index of Biodegradation'' is proposed for assessing the overall biodegradation advancement. The winter period appeared cr itical for an oil spill in arctic/subarctic environments because of th e 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 env ironmental conditions (temperatures >O degrees C, effective chemical d ispersion, oil release, spring microalgal bloom), the bacterial degrad ation would significantly alter the dissolved/dispersed oil within a f ew 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 (dimethylp henanthrenes), depending on the contamination level. In microenvironme nts where oil residues accumulated with biological detritus (surface m icrolayer, settling matter), the oil biodegradation was naturally enha nced. In contrast, water-in-oil emulsions recovered at the surface of mesocosms were unaltered after one month exposure in autumn.