Ephemeral wetlands are characterized by temporal changes in abiotic characteristics that could ameliorate or exacerbate contaminant effects on resident species. The goal of this study was to determine the effects of temperature and naturally occurring suspended solids and organic carbon on the response of Daphnia pulex, and the calanoid copepod, Diaptomus clavipes, to a copper reference toxicant. Organisms were exposed to copper at 10, 20 and 30 ∘C in 48-h static renewal tests with a diluent of either reconstituted laboratory water or water from a wetland that had elevated levels of both suspended solids and organic carbon. 48-h LC50 values were calculated for both total and free ion copper concentrations. When wetland water was used as the diluent, LC50 values based on total copper concentrations were significantly greater than free ion LC50s for both species. This difference was not as great in laboratory water, indicating that binding of the metal was greater in the wetland diluent and the free ion was largely responsible for toxicity. While D. clavipes was significantly less sensitive to the metal than D. pulex (48-h LC50 for total copper in laboratory water at 20 ∘C 607.4 μg/L vs. 10.7 μg/L, respectively), the copepod exhibited a much greater response to increasing temperature. When the Biotic Ligand Model was used to generate free ion concentrations, it was found that measured concentrations exceeded the predicted values at each test condition; however measured LC50 values for D. pulex were within a factor of two of the predicted LC50's at all temperatures and in both diluents.