The effect of natural dissolved organic carbon on the acute toxicity of copper to larval freshwater mussels (glochidia).

The present study examined the effect of dissolved organic carbon (DOC), both added and inherent, on Cu toxicity in glochidia, the larvae of freshwater mussels. Using incremental additions of natural DOC concentrate and reconstituted water, a series of acute copper toxicity tests were conducted. An increase in DOC from 0.7 to 4.4 mg  C/L resulted in a fourfold increase (36-150 µg Cu/L) in the 24-h median effective concentration (EC50) and a significant linear relationship (r² = 0.98, p = 0.0008) between the DOC concentration and the Cu EC50 of Lampsilis siliquoidea glochidia. The ameliorating effect of added DOC on Cu toxicity was confirmed using a second mussel species, the endangered (in Canada) Lampsilis fasciola. The effect of inherent (i.e., not added) DOC on Cu toxicity was also assessed in eight natural waters (DOC 5-15 mg C/L). These experiments revealed a significant relationship between the EC50 and the concentration of inherent DOC (r² = 0.79, p = 0.0031) with EC50s ranging from 27 to 111 µg Cu/L. These laboratory tests have demonstrated that DOC provides glochidia with significant protection from acute Cu toxicity. The potential risk that Cu poses to mussel populations was assessed by comparing Cu and DOC concentrations from significant mussel habitats in Ontario to the EC50s. Although overall mean Cu concentration in the mussel's habitat was well below the acutely toxic level given the concentration of DOC, episodic Cu releases in low DOC waters may be a concern for the recovery of endangered freshwater mussels. The results are examined in the context of current Cu water quality regulations including the U.S. Environmental Protection Agency's (U.S. EPA) biotic ligand model.