Acute and Chronic Toxicity of Nitrate to Brook Trout (Salvelinus fontinalis) Embryos and Larvae

Abstract

Increasing concentration of nitrate in groundwater is becoming an environmental health concern. Nitrate levels in pore spaces of groundwater upwelling zones of the Little Plover River, a cold, groundwater-fed stream located in central Wisconsin, frequently exceed 20 mg/L NO3-N (nitrate-nitrogen). Because brook trout, Salvelinus fontinalis, spawn within these zones developing embryos may be exposed to high concentrations of nitrate for extended periods of time. Therefore, the effect of nitrate exposure was examined in developing embryos and larvae of two domestic and one feral strain of brook trout at two levels of water hardness. The lowest observable effect concentration for feral brook trout was 6.25 mg/L NO3-N for both mortality in developing embryos (96-d test) and reduced growth of larvae (14-d test) in hard water. For the domestic strain, the lowest observable effect concentration was 12.50 mg/L NO3-N for embryo mortality (96- d test) and 100.00 mg/L NO3-N for biomass reduction (14-d test) of larvae. The 96-hour LC50 values for the domestic and feral brook trout larvae were 2151.4 and 2645.3 mg/L NO3-N, respectively. Pre-exposure of developing embryos to nitrate slightly decreased acute toxicity in 96 hour tests indicating the potential for brook trout to develop some tolerance to subsequent high-level exposures. These results show that under current conditions in central Wisconsin, increased embryo mortality and reduced growth in brook trout is possible, especially in areas with high agricultural uses, where groundwater nitrate levels are elevated.