Stimulation of increased short‐term growth and development of mayflies by pulp mill effluent

Abstract

AbstractTo determine the food‐dependent effects of biologically treated, bleached‐kraft pulp mill effluent on mayfly (Baetis tricaudatus Dodds) growth and development, mayflies were exposed to effluent for 2 weeks within artificial streams arranged in a 2 × 3 factorial design (two periphyton food levels ‐ low, high; three concentrations ‐ control river water, 1% effluent, 10% effluent). Although survival was not affected, the effluent had a significant stimulatory effect on growth (resulting in 20 to 50% greater body weights) and development of the wing pads. Furthermore, the initial growth and development trajectories suggest that effluent‐exposed mayflies may emerge sooner and at a larger size than nonexposed individuals. A key result was that the stimulatory effects occurred within both the low‐ and high‐food treatments. Thus, the effluent‐exposed mayflies grew‐faster than even the high‐food control animals, which already had access to more food than they could eat throughout the experiment. This shows that the stimulatory effect of the effluent on the mayflies involved more than just an increase in food‐availability due to enhanced algal growth in response to nutrients in the effluent. Three possible mechanisms for this growth‐enhancement effect are that the effluent (a) increased the nutritive value of the food, (b) enhanced the palatability of the periphyton, inducing increased mayfly feeding, and/or (c) directly stimulated increased mayfly growth via hormonal or other growth‐stimulation effects. Further work is required to determine whether the growth stimulation occurred at the expense of future reproductive output.