Effects of a controlled under-ice oil spill on invertebrates of an arctic and a subarctic stream

Abstract

The short-term drift of macroinvertebrates is documented following two controlled oil spills placed under ice in aan arctic (Imnavait Creek) and subarctic (Poker-Caribou Creek) stream just as ice covered the water in early winter. No mortality was observed, but several species responded by differentially drifting from the oil-impacted areas during the following days. In the arctic stream, Trichotanypus posticalis (Diptera) showed a significant increase in drift for the first few days. There was also an overall increase in drift of total organisms post spill. Phaenospsectra sp. 1, the numerical dominant, decreased its nocturnal drifting compared with the upstream control station in the 5 days post spill. In the subarctic stream, Skwala sp. 1 (Plecoptera), Prosimulium sp. 1 (Simulidae) and Pseudo-diamesa sp. 1 showed significant increase in drift post spill. Among the species of benthic invertebrates sampled with a Hess sampler (WILDCO, Saginaw, Mich.), only the density of Nemoura sp. 1 declined significantly post spill. Polar ordinations using per cent difference showed that the oil-treated stations separated from the control stations in both the drift and the Hess bottom samples. Colonisation of artificial substrates in Imnavait Creek during the winter following the spill was almost non-existent. In Poker-Caribou Creek much colonisation took place over the winter with significantly more occurring on unoiled rocks as compared with oiled rocks. Trapped under a layer of ice, the oil injected should have had a reduced rate loss of the volatile, toxic aromatic components, exposing the invertebrates to them for a longer period of time, albeit at a cold temperature. The cold temperature lowered the body metabolism, thus reducing the effect of soluble oil fractions on invertebrates. In addition, the large-sized, over-wintering instars present had a low surface area per unit volume, which reduced absorption of volatiles, reducing the probability of any massive mortality. Key species in each system began drifting, presumbly as a behavioural response to the oil, compared with upstream control sites.