Assessment of mercury and selenium interactions in freshwater

Additions of selenium (Se) have been shown to effectively lower mercury (Hg) concentrations in the muscle of fish in freshwater in some studies, but the responses of Hg to Se are highly variable, site specific, and concentration dependent.At low concentrations, Se is a nutrient, but at slightly elevated concentrations it can impair developrnent of fish larvae, which can result in failure to recruit.Therefore, the effectiveness of using Se additions to lower Hg concentrations in fish hinges on its ability to do so at low concentrations.Se concentrations may become depressed in newly flooded reservoirs where higher rates of methylation can produce volatile forms of Se.Reservoirs are known to have elevated concentrations of Hg in fish to which depressed Se concentrations could contribute.We surveyed reservoirs and lakes in Qubec and Ontario to assess natural background concentrations of Se and evaluate whether flooding caused depression of Se concentrations.'We also ran a mesocosm experiment based on a regression design to evaluate the effectiveness of low Se concentrations to lower Hg concentrations in yellow perch and the lower food web without imposing toxicity.In the reconnaissance suryey, concentrations of Se in fish, zooplankton, emerging insects, sediment, and water were not consistently depressed in reservoirs relative to reference lakes.Concentrations of Hg and Se in muscle of fish correlated inversely from the ELA and positively in fish from La Grande Complex, Qubec.The prey of fish met or exceeded dietary requirements of Se in the diet of fish, even in systems with low Se concentrations.Results from the mesocosm experiment indicate concentrations of spike Hg in muscle and liver of fish inversely correlated with Se concentrations in water after eight weeks of exposure.About 65Yo of the spike THg in muscle occurred as spike MeHg.Presumably Hg concentrations in muscle of fish would become lower with longer exposure.Increasing concentrations of Se in water from about 0.2 to 1.0 tglL resulted in Hg concentrations in muscle of fish that were 54o/olower relative to controls.The most conservative toxicity threshold for Se in tissue of fish is 7.9 Wglg@ry weight).At0.7 pgll-of Se in water for eight weeks, gonads of fish contained about 8 pglg (dry weight) of Se.This could increase with longer exposure.Thus, Se concentrations in water should be well below 0.7 tglL to prevent the possibility of Se toxicity.Also, there was a significant relationship between Se concentrations in muscle and gonads of yellow perch, which could enable non-lethal monitoring of this species for Se concentrations in gonads.Additions of Se did not affect partitioning of Hg in the physical- chemical environment, with the exception that concentrations of ambient MeHg in water were positively related to Se concentrations in water.In addition, the amount of Se present increased the partitioning of Sea* to particulate matter while it decreased the partitioning of Se* to this compartment.Evasion of Hg to the atmosphere was not rneasured, but we assume this to be the largest sink for spike Hg followed by sediment, water, and then particulate matter.While Se additions lowered Hg concentrations in yellow perch, it also elevated Se concentrations in gonads and thus we advise not adding Se to aquatic systems because of the risk of Se toxicity.Se addition is not advised as it would be difficult to maintain target Se concentrations in light of seasonal variability, winter stress in fish, accumulation in downstream habitats, and it may set an important precedent.In aquatic systems, Se may be chemically transformed, deposited, redistributed, bioconcentrated, and bioaccumulated.Selenium can be present as selenite, selenate, or diselenite in aerobic sediments (Masscheleyn et al. 1991) and as elemental Se or metal selenides (ie.FeSe) in anoxic settings (Masscheleyn and Patrick 1993; Peters et al. 1997).Elemental Se converts quickly to selenite on oxidation (Masscheleyn et al. 1990).Bioturbation causes acceleration of pore water exchange, physical mixing, andlocalized oxidation of sediments, and this may remobilize Se from sediments to the overlying water (reviewed in Peters et al. 1999a).Selenium can be methylated both abiotically and by microorganisms.Organic Se compounds, such as selenomethionine, can change to volatile Se forms in light and dark settings by abiotic mechanisms, cleavage or hydrolysis.Dimethyl selenide (DMSe) is the main volatile species found in water.Because DMSe constitutes only a srnall fraction of the products of abiotic methylation of organic Se, it may be formed as the breakdown product dimethyl diselenide (Amouroux et a|.2000).Selenium is an essential element for eukaryotes and bacteria (Guimaraes et al. 1996).Bacteria accumulate selenite and selenate.Microbial metabolites of Se are DMSe (Chasteen 1993), dimethyl diselenide (Chasteen 1993; Peters et al. 1999b), and dimethyl selenenyl sulphide (Chasteen 1993;Fan et al. 1997; Amouroux and Donard 1997; Fan and Higashi 1998; Van Fleet-Stalder et aL.2000).In addition, methanelelenol (CHSeH) was identified in microorganisms from reservoirs (reviewed in Chasteen and Bentley 2003).Chasteen and Bentley (2003) have extensively reviewed the mechanisms for Se biomethylation.Algal uptake of Se varies with cell volume suggesting enzpe mediated uptake rather than simple adsorption.At water concentrations of 3 to 10 pg/L of Se, selenite accumulation by algae increased at lower pH levels (Riedel and Sanders 1996) suggesting selenoamino acids could accumulate in algae more readily than charged species (Besser et al. 1993;Baines et al. 2001).In freshwater, uptake of Se(IV) by algae is proportional to irradiance and primary production (Baines et aL.2004).Community composition can greatly influence the concentration of Se in phytoplankton, and therefore the transfer of Se through aquatic food webs (Baines and Fisher 2001).Also, bacteria concentrate selenite, mainly as selenoamino acids, at concentrations up to l3 times greater than those in phytoplankton (Baines et a|.2004).Diet is the primary rout of exposure of Se to zooplankton and they concentrate selenite more readily than selenate, but selenomethionine, an organic form of Se, is the most easily accumulated species of Se (Bess er et al. 1993) and its production is essential for Se bioaccumulation.

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