Cumulative effects of multiple effluent and low dissolved oxygen stressors on mayflies at cold temperatures

Abstract

Effluents produced by pulp mills and sewage plants on northern rivers have the potential for a variety of interacting effects on downstream benthic invertebrates via increased levels of toxicants and nutrients and decreased levels of dissolved oxygen (DO) in the substratum. We experimentally measured the combined effects of these stressors at low temperature (4.5°C) on a common northern river invertebrate, the mayfly Baetis tricaudatus Dodds. Mayflies were exposed in laboratory artificial streams to one of two DO levels (low (5 mg·L-1) versus high (11 mg·L-1)) in the presence and absence of pulp mill and sewage effluent from an Alberta, Canada, mixed-effluent outfall (control river water versus 1% effluent); the DO and effluent treatments bracketed typical field concentrations. In the low-DO treatment, grazing intensity was reduced by 80%, and after 2 weeks of exposure, survival was reduced by 60-90%. Furthermore, 250-350% more mayflies in the low-DO treatment moved upward into regions of greater current velocity close to the surface of the artificial streams, a behavior that would likely make them more susceptible to fish predation in the field. In contrast, the 1% effluent treatment increased mayfly survival (possibly due, in part, to stimulation of increased mayfly grazing intensity by the effluent), although this effect only partly compensated for the pronounced negative impact of low DO levels.