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Methylmercury production and degradation under light and dark conditions in the water column of western US reservoirs.
Citation:
Eckley, C., T. Luxton, D. Krabbenhoft, AND J. DeWild. Methylmercury production and degradation under light and dark conditions in the water column of western US reservoirs. International Conference on Mercury as a Global Pollutant, Krakow, Lesser Poland, POLAND, September 08 - 13, 2019.
Impact/Purpose:
The abstract and presentation highlights the impact of dissolved organic matter sources on elevated levels of mercury in fish
Description:
Previous studies have identified that reservoirs tend to have elevated fish methylmercury (MeHg) concentrations relative to natural lakes and free flowing rivers. There are several factors and processes that may contribute to this phenomenon, such as a shift from a lotic to lentic foodweb, increased sedimentation of mercury (Hg) and organic carbon, thermal stratification/development of an anoxic hypolimnion, among other processes. This study focuses on understanding one aspect of Hg cycling in a lake: the production and degradation of methylmercury (MeHg) in the water column. The primary study location is the Hells Canyon Complex of three lakes that serve as drinking water and irrigation reservoirs? on the Snake River along the Idaho-Oregon (USA) border. The degradation of MeHg was assessed as a biotic and abiotic process as well as under light and dark conditions. The study utilized stable isotope tracers of both inorganic Hg and MeHg in short-term (3 ng/L and >60% of the total-Hg present), the methylation rates in the anoxic hypolimnion of the water column were relatively low (on average 0.1%/hour), but were significantly higher compared to time-zero incubations and samples collected from the oxic epilimnetic waters. Water-column methylation also varied seasonally, between reservoirs, and locations within the reservoirs. This variation was mostly driven by the presence of anoxic conditions, which varied between years depending on snowmelt and other climatological conditions. Biotic demethylation rates in the dark were typically low and mostly below detection, whereas abiotic photodemethylation rates were much higher (around 1%/hour) in surface waters. Photodemethylation rates decreased to below detection 2m below the surface where there was reduced light penetration. Comparisons of photodemethylation rates of ambient MeHg relative to the added isotopic MeHg tracer showed that the ambient MeHg had significantly lower demethylation rates suggesting that the dissolved compounds/elements that MeHg is bound to can affect photodegradation. Photodemethylation rates between water samples collected in different reservoirs or different depths within a reservoir (e.g. epilimnion versus hypolimnion) were not significantly different. Overall, the results from this study identify the main areas within lakes where MeHg production and degredation occur in the water column and can be used to help optimize lake management opportunities aimed at reducing MeHg levels.