Journal of Aquatic Ecosystem Stress and Recovery 6: 141–157, 1998. © 1998 Kluwer Academic Publishers. Printed in the Netherlands. 141 Use of bioassay-based whole efﬂuent toxicity (WET) tests to predict benthic community response to a complex industrial efﬂuent Helen C. Sarakinos ∗ & Joseph B. Rasmussen Department of Biology, McGill University, 1205 Docteur Penﬁeld, Montr´ eal, QC, H3A 1B1, Canada ( ∗ Current address: Redpath Museum, McGill University, 859 Sherbrooke St. O., Montr´ eal, QC, H3A 2K6) Received 3 April 1998; accepted 30 July 1998 Key words: benthic invertebrates, community response, complex efﬂuent, ecotoxicology, ﬁeld validation, MATC, pulp and paper, toxicity tests, WET, whole efﬂuent toxicity Abstract Whole efﬂuent toxicity (WET) tests are a useful monitoring tool because they provide a rapid and replicable measure of the potential ecotoxicological effect of efﬂuents. Although WET tests have been incorporated into toxicity-based efﬂuent limits to protect receiving systems from adverse effects, few studies have attempted to quantitatively ﬁeld-validate laboratory-derived toxicity thresholds. In this study, we examine the ability of WET tests to predict response thresholds of an invertebrate community to a paper mill efﬂuent discharged into the Nicolet-SW River, Québec, Canada. We quantiﬁed invertebrate community structure and density in the river and detrended for the effects of physical/chemical variables. This allowed examination of direct correlation between invertebrate community structure and efﬂuent concentration. There was a signiﬁcant decrease in taxonomic rich- ness at an efﬂuent concentration of 16%, but signiﬁcant changes in the density of invertebrates occurred between 0% and 2% efﬂuent. This suggests that although most taxa returned to the river downstream of the efﬂuent, they did so at lower densities. Calculated ﬁeld thresholds were compared to laboratory thresholds for the efﬂuent using chronic WET tests with algae, cladocerans and ﬁsh. The WET tests produced a mean MATC of 3.6%. Thus, standard WET tests overestimated response thresholds of the invertebrate community in the receiving environment and impacts were observed in areas where no impact was expected. Abbreviations: CCA = canonical correlation analysis; HC 5 = concentration hazardous to 5% of a community or population; LOEC = lowest observed effect concentration; MATC = maximum allowable toxicant concentra- tion; NOEC = no observed effect concentration; WE T= whole efﬂuent toxicity; EEM = environmental effects monitoring 1. Introduction Concern for the impact of chemical pollution on the quality of aquatic ecosystems has stimulated over 30 years of research on the biological effects of pollu- tants. Quantifying the ecotoxicological effects of pollutants is critical to the protection of aquatic ecosystems (Birge et al., 1989). The U.S. Clean Water Act requires that “the discharge of toxic pollutants in toxic amounts be prohibited” (Section 101(a)(3)) and the U.S. Environmental Protection Agency (EPA) has incorporated toxicity-based discharge limits into its National Pollutant Discharge Elimination System permits since 1984 (U.S. EPA, 1984). Establishment of toxicity-based limits relies on the use of standard- ized laboratory toxicity tests that can assess the potential effect of efﬂuents on aquatic life in the receiving system. Since efﬂuents often contain com- plex mixtures of chemicals that are poorly charac- terized, a suite of acute and chronic toxicity tests (termed whole efﬂuent toxicity, or WET, testing) is used to measure aggregate toxicity of chemicals in an efﬂuent (U.S. EPA, 1991).