ABSTRACT
The potential short- and long-term responses of benthic river invertebrates to biotreated bleached kraft pulp mill effluent are illustrated in a review of two recent experimental and statistical analyses. To determine short-term responses, mayflies (Baetis tricaudatus Dodds) were exposed in artificial streams to control river water, 1% effluent, or 10% effluent while controlling periphyton food levels (low versus high food availability). After 2 wk, exposure to the effluent stimulated significant increases in growth and development of the experimental animals. This stimulation was greater than could be accounted for by differences in algal food availability. This suggests that the effluent may have (1) increased the nutritive value of the food, (2) stimulated an increase in mayfly feeding, and/or (3) directly stimulated increased mayfly growth by way of hormonal or other metabolic effects. Multivariate analysis (ordination) of the abundances of benthic invertebrates sampled over a 20-year period downriver of the pulp mill revealed that five families of stoneflies, caddisflies, and mayflies (including Baetidae) were more abundant in years when the mill output of suspended solids was greater. This positive correlation should be viewed with caution, however, until more direct evidence is available on the mechanism of the long-term effects of pulp mill effluent on river invertebrates.
