Variability of the protective effect of sodium on the acute toxicity of copper to freshwater cladocerans

The acute biotic ligand model (BLM) is proposed by the U.S. Environmental Protection Agency (U.S. EPA) to incorporate bioavailability in calculating aquatic life criteria for Cu in freshwater. This approach currently assumes that the effects of water chemistry on acute Cu toxicity can be described with one single set of identical BLM parameters for all organisms. An important water characteristic is the concentration of Na, which protects aquatic organisms against Cu toxicity. Based on physiological considerations related to the mechanism of Cu toxicity and the possible role of Na therein, we hypothesized that an interspecies variability of the protective effect of sodium on Cu toxicity might exist among freshwater organisms. To test this hypothesis, acute 48-h toxicity assays with Cu were conducted with 16 field-collected cladoceran populations and a laboratory-reared clone of Ceriodaphnia dubia at Na concentrations of between 0.077 and 10 mM. Increased Na protected all but one population. Contrary to what the BLM predicts, however, an upper limit to this protective effect was observed for some populations at Na concentrations of greater than 4 mM. This may suggest that processes other than just Cu-Na competition at a single unidentate biotic ligand site may be involved in Cu toxicity. Between populations, conditional stability constants for binding of Na to the biotic ligand (log K-NaBL(*)), which quantify the protective effect of Na, varied between 2.2 and 4.4. Higher log K values generally were associated with more sensitive populations. Although a full mechanistic explanation for our observations is lacking, our data may potentially be used to refine the U.S. EPA procedure to establish site-specific water-quality criteria for Cu.