Toxic Substances Hydrology Program
In-Stream Toxicity Tests
U.S. Geological Survey (USGS) scientists and their colleagues conducted toxicity tests in two Montana streams to compare the survival of westslope cutthroat trout, which were exposed to dissolved metal concentrations that either exhibited the natural diel variation in streams or were held constant. The diel changes in cadmium and zinc concentrations in the two streams increased each night by as much as 61 and 125 percent, respectively, and decreased a corresponding amount the next day during the 4-day tests. Cadmium concentrations in the two streams ranged from approximately 2 to 4 micrograms per liter (mg/L), which spanned the site-specific acute water-quality criteria of 2.3 to 2.5 mg/L. On the other hand, zinc concentrations ranged from approximately 270 to 750 mg/L, which was above the site-specific acute water-quality criteria of approximately 140 mg/L.
Results of the Tests
Results for both streams showed that survival was significantly higher after exposure to natural diel-fluctuating metal concentrations than to a constant concentration about equal to the average metal concentration in the diel-fluctuating exposure. Survival likely was greater for the fish exposed to the diel fluctuations because the periods of lower metal concentrations provided some relief for the fish. In addition, toxicity during periods of higher metal concentrations was lessened by the simultaneous occurrence each night of lower water temperature, which reduces the rate of metal uptake. Based on this initial study, current water-quality criteria appear to be protective for streams with diel concentration cycles of cadmium and zinc. Water-resource managers and environmental regulators can potentially use this information to develop sound policies regarding stream fisheries and water-quality criteria.
Nimick, D.A., Harper, D.D., Farag, A.M., Cleasby, T.E., MacConnell, E., and Skaar, D., 2007, Influence of in-stream diel concentration cycles of dissolved trace metals on acute toxicity to one-year-old cutthroat trout (Oncorhynchus clarki lewisi): Environmental Toxicology and Chemistry, v. 26, no. 12, p. 2667-2678, doi:10.1897/07-265.