Fish Exposed to Daily Cycles of Dissolved Metals Show Lower Mortality Rates

Fish exposed to daily changes in levels of dissolved toxic metals in mining-contaminated streams showed significantly lower mortality rates than fish exposed to constant average concentrations. U.S. Geological Survey (USGS) scientists and their colleagues, reported these finding in Environmental Toxicology and Chemistry. The lower mortality rate found in streams with cycling concentrations is an important finding considering that water-quality criteria for streams are based on constant concentrations.

Are Water-Quality Criteria Protective?

Elevated concentrations of metals in streams can be toxic to fish and other aquatic life. To protect aquatic life, agencies such as the U.S. Environmental Protection Agency have established water-quality criteria. These criteria usually are based on how well test organisms survive in laboratory experiments in which organisms are exposed to specific but constant metal concentrations. These laboratory tests do not replicate all the conditions typically found in streams, and therefore may indicate different toxicity when compared to toxicity in actual streams. Many contaminated and uncontaminated streams undergo daily variations that are caused by daily changes in temperature, sunlight, and other factors. The large daily change (diel) in metal concentrations observed in streams is one of the natural processes not accounted for in standard laboratory toxicity tests. USGS scientists and their colleagues conducted toxicity tests in two Montana streams to compare the survival of fish that were exposed to dissolved metal concentrations that were allowed to vary (diel changes) or were held constant.

Based on this initial study, current water-quality criteria appear to be protective for streams with diel concentration cycles of cadmium and zinc. Environmental regulators and water-resource managers can potentially use this information to develop sound policies regarding stream water-quality criteria, fisheries, and monitoring programs.

Reference

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:10.1897/07-265.1.

More Information

• Diel Cycling of Trace Metals in Streams
• Diurnal Metal Variations in Streams Photo Gallery
• Biofilms in Streams Help Create Daily Variations in Metal Concentrations
• Importance and Causes of Daily Variation in Metal Concentrations in Streams
• Daily Variation of Metals in Streams Can Confound Scientific Studies
• Mercury Concentrations in Streams Found to Go Through Daily Cycles
• Watershed Contamination from Hard-Rock Mining Investigation

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