Fate and effects of dispersed crude oil under icy conditions simulated in mesocosms

Abstract

The fate of the chemically dispersed crude oil Forties, the effects on natural phytoplankton and bacteria and the hydrocarbon biodegradation were studied in mesocosms during a two-week period in winter. Five double-walled stainless steel tanks (3 m height; 3·5 m 3) were used. Each tank was equipped with a cooling system maintaining entrapped seawater below zero (−1·6 ± 0·2°C) and under an ice coverage, simulating winter conditions prevailing in boreal/sub-boreal environments. A flow-through cascade set-up between three tanks simulated the progressive dilution of the oil in the water mass. A total oil concentration of 7·5 mg liter −1 was measured in the water column of the oiled cascading tanks, four days after the addition of 435 g (nominal concentration of 124 mg liter −1) of dispersed crude oil. At this time, most of the oil (> 90%) initially introduced into mesocosms had escaped the water column, mainly toward the surface. A non-negligible portion of the oil (5·7%) was settling during the course of the experiment. The low efficiency of the dispersant mixture used and the oil settling seemed to be due to the very low seawater temperature tested here. In the first days following the oil addition, a slight decline of viable heterotrophic bacteria was noted in the tanks, and then the oil improved the heterotrophic activity by the end of the experiment. The marked increase of both density and proportion of oil-degrading bacteria observed two days after the oil addition gave evidence of the potential capability of the indigenous bacterial community to adapt to an oil spill event. However, under such extreme icy conditions, the biodegradation of the dispersed oil was reduced and only the aliphatic hydrocarbon fraction, mainly recovered as dispersed oil droplets, was slightly degraded. The aromata hydrocarbon fraction, on the other hand, essentially recovered as dissolved/colloidal compounds (< 0·7 μm), was not altered after two weeks. Oil biodegradation in the material collected in sediment traps progressed more rapidly than in the water column. At the end of the experiment, about 1–2 mg liter −1 of oil remained in the water column and still induced a chl-a inhibition, but no significant change of the specific composition of the phytoplankton was noted, microflagellates remaining dominant throughout the experiment (40·5–76% in number).