Assessing effects of dissolved organic carbon and water hardness on metal toxicity to Ceriodaphnia dubia using diffusive gradients in thin films (DGT)

We evaluated the ability of diffusive gradients in thin films (DGT) to assess the effects of water hardness and dissolved organic carbon (DOC consisting of humic acids) on Cu and Zn toxicity (i.e., 48 h LC50s) to Ceriodaphnia dubia. When DOC was high, Cu concentrations measured by DGT (DGT Cu) were significantly lower than Cu concentrations in water (dissolved Cu), and DGT LC50s were significantly lower than LC50s based on dissolved concentrations. When DOC was low, differences between dissolved Cu and DGT Cu were small, as were differences between dissolved LC50s and DGT LC50s. Differences between DGT and dissolved measurements of Zn were small compared with the differences observed for Cu, and DGT Zn LC50s were relatively similar to dissolved Zn LC50s. Humic acids formed strong organic-Cu complexes that were both inaccessible for biological uptake and excluded by DGT, which selected for free or weakly bound Cu. In contrast, Zn did not form strong complexes with DOC that greatly affected either toxicity or the measurement of Zn by DGT. The effects of hardness on DGT measurements of Cu and Zn were smaller and more complex than the effects of DOC. Large, statistically significant differences between DGT measurements of Cu in low and high DOC water accurately reflected the strong effects of DOC on Cu toxicity. However, the effects of DOC were inconsistent for Zn, and DGT provided less information about the toxic fraction of this metal.