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 Québec 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, Québec.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 Seó* 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.bioaccumulation.This project involved two separate studies.The first part was a reconnaissance survey to evaluate concentrations ofSe in hydroelectric reservoirs and lakes in Québec.Before adding Se it is necessary to know how much Se is already present to determine how close background concentrations are to toxicity thresholds.Hg concentrations are typically high in fish from environments that are high in organic matter and low in oxygen, such as new reservoirs.Under these conditions, Se may also be lowered by losses resulting from the formation of volatile Se species.In new reservoirs, Hg concentrations in fish are known to be elevated.If Se lowers Hg in fish, the absence of Se may contribute to elevated Hg concentations in fish.In this hypothesis, additions of Se to reservoirs would restore Se and may be more acceptable than additions beyond what is deemed required or natural.The second phase of our approach was a mesocosm study.'We evaluated the effectiveness of low concentrations of Se to lower Hg bioaccumulation and associated risks of Se toxicity.Finding the lowest effective Se concentration prevents excessive Se additions and potential Se toxicity.Our research builds on past studies by lowering the range of Se concentrations tested, 0.1 to 1.6 ¡tglL of Se with no Se added to the controls.Past studies only assessed acute Se toxicity.Assessing chronic toxicity, which is currently known to manifest through elevated Se concentrations in gonads and to affect reproduction, gives resource managers more information with which to make decisions about Se additions.t2 In summary the null hypotheses were: 1. there is no effect on concentrations of Se in environmental compartments of reservoirs, 2. changes in the concentrations of Se caused by flooding do not affect Hg concentrations in environmental compartments, 3. Hg partitioning among environmental compartments is not related to Se concentrations in water, 4. MeHg concentrations in environmental compartments are not related to Se concentrations in filtered water, 5. there is no effect of Se on Hg concentrations in fish, 6. concentrations of Se in gonads of fish will be below the toxicity threshold of 10 ¡rg/g after Se additions.In hypotheses 3 to 6 we manipulated Se concentrations in mesocosms, knowing it would accumulate through the food web and possibly cause Se toxicity.We also rnanipulated Hg concentrations using stable isotopes for the purpose of tracing the transfer of Hg through the food web to assess the impacts of adding different Se concentrations.We expected our highest Se treatment (1.6 ¡tglL) to have lower Hg concentrations in fish because in past studies I pglL of Se lowered Hg concentrations in fish.We were uncertain if we would observe elevated Se concentrations in gonads of fish because our water concentrations were lower than those that caused developmental deformities in Belews Lake (2 ¡tglL;Lemly 2002).Each chapter of this thesis will be submitted for publication to the following journals: l A review of mercury and selenium interactions in freshwater fish toward lowering mercury in fish will be submitted to Science of the Total Environment, 2. Are concentrations of selenium lower in reservoirs and does selenium affect mercury cycling?Results from a reconnaissance survey will be submitted to Environmental Pollution, 3. Fates and mass balances of selenium and isotopic mercury added to mesocosms will be submitted to Environmental Science and Technology, 4. Selenium additions lowered mercury concentrations in fish and increased selenium concentrations in gonads will be submitted to Environmental Science and Technology.

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