Newswise — Concentrations of heavy metals and many other contaminants fluctuate in the environment; however, toxicity testing used to set safe concentrations typically use constant concentrations. Researchers in a new study compared the results of toxicants exposed constant versus fluctuating metal concentrations. Results suggested that current water-quality criteria are protective of fish in streams. The study is published in the latest issue of Environmental Toxicology and Chemistry.

Mining sites release metals to surface waters; surface water concentrations of metals from mining sites fluctuate as a result of these discharges. Fluctuating exposures can be caused by cyclic variations in discharge, stream flow or in-stream biogeochemical processes, commonly on a diel time scale. Fluctuations in metal concentrations raise questions about the applicability of results from standard toxicity bioassays conducted in laboratory settings, where metal concentrations are constant, compared to the field, where metal concentrations may not be constant. Considering these issues, studies that compare the relative effect of exposure to constant versus fluctuating metal concentrations needed to be conducted.

Researchers in the current study conducted in situ experiments with cutthroat trout in streams that receive mining wastes. Trout were exposed to either a fluctuating or a constant concentration of several metals. In the streams, cadmium and zinc concentrations increased by 61 and 125 percent, respectively, at night. Trout exposed to natural fluctuations in metal concentrations had increased survival compared to trout exposed to constant metal concentrations.

Fluctuating metal concentrations were not more toxic than constant concentrations, and some data suggest that fluctuating concentrations may be less toxic. Results indicate that current water quality criteria, which are generally based on exposures to constant concentrations, are protective of fish in these streams.

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Environmental Toxicology and Chemistry is the monthly journal of the Society of Environmental Toxicology and Chemistry (SETAC). For more information about the Society, visit

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Environmental Toxicology & Chemistry (ol. 26(12))