Water-effect ratio (WER) testing, examining the relative toxicity of copper in various dilutions of effluent and stream water as compared with laboratory dilution water, was conducted during January, April, and June, 1995, to assess the potential of seasonal and flow effects on site water quality and toxicity of copper to the fathead minnow. Total organic carbon (TOC) and dissolved solids were significantly related to effluent concentration in the site water (R-2 = 0.92, p < 0.01), resulting in higher copper median lethal concentrations (LC50s) and higher WERs as effluent contribution in site water increased. Total recoverable copper LC50s were similar to dissolved copper LC50s in laboratory and upstream water tests (p > 0.10), while effluent and stream water tests had significantly higher total recoverable than dissolved copper LC50s (p < 0.01), suggesting more solids or complexing agents when effluent was present. The LC50 and WER for upstream water were significantly higher in April than in January (p < 0.05), consistent with the higher TOC, alkalinity, and lower stream flow observed during April. The WER tests, using different proportions of upstream water and effluent (tests of additivity) in both January and April, indicated that total recoverable copper WER was predictable under a variety of stream flow conditions. Dissolved copper WERs were less predictable, in part because effluent and upstream water WERs were similar, particularly in April. Low-flow stream conditions (when effluent concentration was greatest) appeared to be the limiting condition in terms of greatest copper toxicity at this site. Furthermore, winter low-flow conditions appeared to be more limiting (less water effect on copper toxicity) than similar or even lower flows in spring (April) or summer (June). This was probably due to the higher TOC and dissolved solids present in upstream water in the warmer seasons. All analyses indicated that copper was at least five times less toxic in the effluent-influenced stream water than in typical laboratory test dilution water. Our data indicate that seasonal effects on water quality, as well as stream flow, can be important in determining limiting conditions on copper toxicity in effluent-dominated stream systems.
