Treatment of lakes and storage reservoirs with very low dosages of selenium to reduce methyl mercury in fish

The trials of the addition of selenium to three lakes indicate a rapid bioackumulation of selenium. The selenium concentrations in zooplankton increased, and these constitute the first and most important level in the aquatic ecosystem. The increase of selenium in zooplankton resulted in a reduction in the concentration of methyl mercury. The bio-magnification in the food chain led to increases in selenium concentrations and reductions in methyl mercury. This in turn led to reduced concentrations of mercury in crayfish and methyl mercury in younger fish. After a certain delay, the bio-magnification of selenium continued in the higher trophic levels. This resulted in increased selenium concentrations and reduced methyl mercury concentrations in older perch and pike several years after the zooplankton, young roach and young perch. When the selenium treatments stopped the concentrations of selenium in the water, aquatic moss, bentic invertebrates and the youngest age groups of roach and perch fell immediately. The reductions in selenium in these parts of the ecosystem are related to the water retention period of the lake or reservoir.