The toxicity of road salt and copper to glochidia, assessing waterborne contaminants as potential threats to the recovery of endangered freshwater mussels

Recovery strategies have emphasized the need to determine the effect of environmental contaminants on imperiled freshwater mussels in Ontario. This study assessed the acute toxicity of chloride (Cl) and copper (Cu) to glochidia, the sensitive early life stage.  The Cu 24 h EC50s for the nine mussel species tested ranged from 7 to 36 µg Cu/L, with two of the endangered species experiencing 50% mortality at <10 µg Cu/L.  Water chemistry had a significant effect on Cu sensitivity.  Glochidia could withstand higher Cu concentrations in harder waters and in waters with elevated levels of dissolved organic carbon (DOC).  Current North American water quality regulations should protect freshwater mussels from acute copper toxicity but protection may only be marginal for sensitive endangered species particularly in soft and low DOC waters.  Toxicity tests with sodium chloride and road salt revealed that glochidia are more sensitive to chloride than previously tested aquatic organisms.  EC50s for chloride were 130 mg Cl/L for Lampsilis siliquoidea and 105 mg Cl/L for the endangered, Lampsilis fasciola.   Although there is no current Canadian water quality guideline for chloride, it is unlikely that the USEPA criteria (230 mg Cl/L) would protect glochidia from acutely toxic chloride exposure.  These results indicate that some waterborne contaminants could pose a threat to the recovery of freshwater mussels and that the impact would be contaminant, species, and site specific.