Comparative Toxicity of Nitrate to Common and Imperiled Freshwater Mussel Glochidia and Larval Fishes.

Growing human populations and increasingly intensive agriculture have resulted in widespread aquatic nitrate pollution. Freshwater mussel populations have been in decline for decades but often are underrepresented in data used for the development of ambient water quality criteria and acute toxicity of nitrate to mussel glochidia has not yet been established. Additionally, toxicity testing with aquatic species often is limited to a few model species; however, relatively little is known about how representative model species are of imperiled species. Therefore, to better define the acute toxicity of nitrate to common and rare aquatic species, we conducted 24-h nitrate acute toxicity tests with glochidia of four species of freshwater mussels, including a federally threatened species (Hamiota altilis) and 7-day tests with larval fish of three species: fathead minnow (Pimephales promelas), tricolor shiner (Cyprinella trichroistia), and tilapia (Oreochromis spp.), across a range of water hardness. Median effective concentrations (EC 50 s) in freshwater mussel glochidia ranged from 524 to 904 mg/L NO 3 -N in moderately hard water. In fish, median lethal concentrations (LC 50 s) ranged from 228 to 1725 mg/L NO 3 -N and varied with water hardness. Of the species tested, generally sensitivity of the common species was similar to the rare species, although relative sensitivity varied with water hardness. Based on these results, we can conclude that acute lethal effects are unlikely for the fish and mussel species considered here at current environmental levels, but the results of these standardized tests are useful for the development of ambient water quality criteria and other regulatory and management decisions regarding acute nitrate exposures.