An acute, short-term synthetic crude oil spill into a northern brown-water river in Alberta, Canada, was simulated by dipping limestone bricks into the oil and then examining their microbial and macroinvertebrate colonisation under conditions of light and dark. A reduction to <0·3% of surface radiation in the dark resulted in a non-significant reduction of algae as compared with samples in the light. On oiled bricks in the light compared with the controls there was an increase in bacterial numbers by a factor of 9, an increase in chlorophyll a by a factor of 9, an increase in diatoms by a factor of 5·6 and an increase in blue-green algae by a factor of 0·7. Significant increases were also noted in the trichopteran Lepidostoma, the Chironomidae and the Naididae. On oiled bricks in the dark the only significant changes were an increase in numbers of bacteria and Tanypodinae and a reduction in Gastropoda and Baetis sp.; all other parameters did not differ significantly from the control. It therefore appears that synthetic crude oil has a stimulatory action upon bacterial, algal and macroinvertebrate benthic colonisation but one that is not apparent under conditions of low light. The traditional approach of assessing the impact of a pollutant through the use of ‘indicator species’ needs to be reconsidered to include the supporting food web.