The Effect of Water Hardness on the Sensitivity of Freshwater Mussel Glochidia to Sodium Chloride and a Road Salt Alternative De-icing Product.

Freshwater mussels, one of the most imperiled groups globally, are very sensitive to sodium chloride. Long-term water quality monitoring indicates wild freshwater mussels, including Species at Risk, may be at risk of salt toxicity. Elevated water hardness reduces the toxicity of salt to biota including early life stage mussels. However, the response of mussels to salt and road salt alternatives in ion-dilute waters remained unknown. Standard methods were used to assess the acute salt sensitivity of freshwater mussel larvae (glochidia) in reconstituted waters of varying hardness (8.5-290 mg CaCO3/L). Lampsilis fasciola (Wavyrayed lampmussel) and Lampsilis siliquoidea (Fatmucket) glochidia were exposed to sodium chloride, with nominal chloride concentrations between 5 and 5000 mg Cl-/L depending on water hardness. Viability (ability to close valves), a survival surrogate, was assessed after 24 and 48 hr of exposure. Increasing water hardness resulted in reduced salt toxicity, up to a point (170-190 mg CaCO3/L) after which no further amelioration was observed. Lampsilis fasciola 48-hr EC50s ranged from 17.9 mg Cl-/L in very soft water (8.5 mg CaCO3/L) to 389.4 mg Cl-/L in hard water (172.5 mg CaCO3/L). L. siliquoidea 48-hr EC50s ranged from 13.7 mg Cl-/L in very soft water (16.5 mg CaCO3/L) to 221.8 mg Cl-/L in hard water (193.0 mg CaCO3/L). In exposures with a brine and beet-juice containing road salt alternative (Fusion 2300), L. siliquoidea was more sensitive in very soft water (13.1 mg CaCO3/L) than in harder waters (≥50 mg CaCO3/L), with 48-hr EC50s of 0.002% and 0.0055-0.0073% v/v, respectively. Acute chloride EC50s of both species, including one Species at Risk, were less than the long-term chloride Water Quality Guideline in very soft water (<30 mg CaCO3/L).