Comparisons of single-species, microcosm and field responses to a complex effluent

Abstract

Abstract Short-term, single-species toxicity tests are used to identify, regulate and monitor the discharge of problem effluents into receiving ecosystems. The ability of single-species tests to protect entire ecosystems has been questioned, and microcosm tests have been proposed as additional tools for reducing uncertainty in environmental risk assessment. We compared responses to a complex effluent in microcosms of indigenous macroinvertebrates and protozoans with responses observed in acute tests with Daphnia magna, Ceriodaphnia dubia and Pimephales promelas and in chronic survival and reproductive tests with C. dubia. The predictive utility of these various tests was then evaluated in relation to observed effects in the receiving stream. The acute LC50s (percent effluent) from the acute tests were 63.1 for P. promelas, 18.8 to 31.3 for D. magna and 54.7 for C. dubia. Results from 7-d chronic tests indicated that C. dubia survival was significantly (p ≤ 0.05) affected at 30% effluent and that reproduction was affected at concentrations ≥3% effluent. In the protozoan microcosms, community composition was significantly (p ≤ 0.05) changed at 1.0%, while protozoan species richness was significantly reduced at 3.0% effluent. In the macroinvertebrate microcosms, mayflies were the only macroinvertebrates adversely affected by the effluent. Four mayfly taxa showed significant (p ≤ 0.05) reductions in density in the 1.0% treatment and four others were significantly reduced at 10.0%; however, some chironomids had significant increases in density at 0.1% and above. Microcosm responses corresponded well with observed effects in the field. The microcosms correctly predicted which indigenous organisms would be lost and which would be stimulated at various ambient concentrations of the effluent.