Selenium: an insidious and persistent toxin
The long-term effects of selenium on aquatic wildlifeSelenium, an essential nutrient for humans and animals, occurs as a trace element in most soils. Selenium can move into water systems when soil is disturbed by various types of development, or as a byproduct of fossil fuel combustion. When it occurs in higher than normal levels in water, selenium has been shown to cause malformations in fish and other aquatic wildlife.
In an article in the April issue of Aquatic Toxicology, a USDA Forest Service researcher warns that the impacts of selenium on freshwater fish populations may become more widespread as human disturbance increases -- and that long-term effects may be underestimated.
Dennis Lemly, Forest Service research biologist at the Southern Research Station, has spent the last two decades studying the fish in Belews Lake, North Carolina. Created in 1973 to provide cooling water for a large coal-fired power plant, the lake was contaminated by selenium in discharged wastewater.
"Belews Lake represents one of the most extensive and prolonged cases of selenium poisoning of freshwater fish in the United States," said Lemly. "It provides an excellent case study of the insidious and persistent toxicity of selenium to aquatic ecosystems."
As selenium accumulated in Belews Lake, the 20 resident species of fish started showing deformities to the spine, head, fins, and eyes. In 1986, the power plant stopped discharging wastewater into the lake. Natural recovery began, but long-term studies by Lemly and other researchers show that ill effects persist long after the source of pollution is gone.
"Selenium poisoning in fish can be invisible for a time," said Lemly, "because the primary impact is on the egg, which receives the toxin from the mother's diet. When the eggs are hatched, the developing fish metabolize the selenium. Some fish are visibly deformed, but others grow into adult fish that appear healthy yet fail to reproduce. Because there is no apparent fish kill, species can disappear before you can do anything about it."
Belews Lake was the first site to provide conclusive evidence that exposure to elevated levels of selenium -- in this case only 10 to 20 times those in nearby uncontaminated reservoirs -- causes deformities in natural populations of freshwater fish. Nineteen out of the 20 species of fish in Belews Lake disappeared over the course of four years: by 1978, only the selenium-tolerant mosquito fish remained.
Lemly conducted a follow-up study of Belews Lake in 1996, a decade after the plant quit discharging selenium-laden wastewater into the lake. He found selenium still present at a moderate risk level in the sediment of the lake. The element continued to gradually move from the sediment through the food chain, accumulating to toxic levels in fish eggs.
"Bioaccumulation in food chains causes otherwise harmless concentrations of selenium to reach toxic levels," said Lemly. "Selenium in contaminated sediments can be cycled into food chains for decades. Lessons from Belews Lake provide the foundation for protecting aquatic ecosystems as new selenium issues emerge."
At the time Belews Lake was contaminated, the irrigation of selenium-rich soils in arid regions of the American West was the other major source of selenium poisoning. New sources of selenium pollution include phosphate mining, animal feedlot waste, landfill disposal of ash from coal-fired plants, and the use of constructed wetlands to treat wastewater.
More information about the effects of selenium on the aquatic environment, as well as methods to evaluate and control it, can be found in Lemly's new book, "Selenium Assessment in Aquatic Ecosystems: A Guide for Hazard Evaluation and Water Quality Criteria," published in April 2002 by Springer-Verlag.
Source: Southern Research Station - USDA Forest Service. April 2002.
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