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Evaluating the influence of seasonal stratification on mercury methylation rates in the water column and sediment in a contaminated section of a western U.S.A. reservoir
Citation:
Millard, G., Chris S. Eckley, Todd P. Luxton, D. Krabbenhoft, J. Goetz, J. McKernan, AND J. DeWild. Evaluating the influence of seasonal stratification on mercury methylation rates in the water column and sediment in a contaminated section of a western U.S.A. reservoir. ENVIRONMENTAL POLLUTION. Elsevier Science Ltd, New York, NY, 316(Part 1):120485, (2023). https://doi.org/10.1016/j.envpol.2022.120485
Impact/Purpose:
Mercury (Hg) methylation often occurs at the active redox boundary between oxic and anoxic conditions in sediment and the water column of lakes and reservoirs. Previous studies have suggested that sediment methylation rates are highest when the overlying water is oxygenated, and that the predominant methylation zone moves to the water column during periods of stratification. However, studies that simultaneously measured methylmercury (MeHg) production in the water column and sediment remain limited. Understanding the relative importance of sediment versus water column methylation and the impact of seasonal stratification on these processes has important implications for managing MeHg production. This study measured Hg methylation and demethylation rates using inorganic Hg and MeHg stable isotope tracers in sediments and water of the littoral zones and deeper central waters of a reservoir in California’s central coastal range. Measurements were conducted during well-mixed/oxygenated conditions (winter) as well as at the end of thermal stratification (late-summer). The results showed that the ambient MeHg concentrations were very high in the hypolimnetic waters (up to 7.5 ng/L; 79% MeHg/total Hg) during periods of stratification. During late summer, littoral sediments had higher methylation rates (2.4 %/day) compared to the profundal sediments (1.3 %/day). Anoxic water column methylation rates were of similar magnitude as rates in the sediment (3.0 %/day). Following turnover profundal sediment methylation rates did not change significantly, but water column methylation rates became insignificant. Sediment demethylation rates were much higher in profundal sediment (9.8%/hour), compared to the littoral sediments (2.2 %/d). When modelled, methylation rates in the water column could account for approximately 40% of the hypolimnetic MeHg. Our modelling results show that the remaining MeHg in the hypolimnion could originate from the profundal sediment. These results suggest that addressing methylation in the water column and profundal sediment are of equal importance to any remediation strategy.
Description:
Mercury methylation frequently occurs at the active oxic/anoxic boundary between the sediment bed and water column of lakes and reservoirs. Previous studies suggest that the predominant mercury methylation zone moves to the water column during periods of stratification and that high potential methylation rates (Km) in sediment require oxygenated overlying water. However, simultaneous measurements of methylmercury (MeHg) production in both the sediment and water column remain limited. Understanding the relative importance of sediment versus water column methylation and the impact of seasonal stratification on these processes has important implications for managing MeHg production. This study measured Km and potential demethylation rates (Kdm) using stable isotope tracers of unfiltered inorganic mercury and MeHg in sediments and water of the littoral and profundal zones of a shallow branch of the Nacimiento Reservoir in California's central coastal range.
URLs/Downloads:
DOI: Evaluating the influence of seasonal stratification on mercury methylation rates in the water column and sediment in a contaminated section of a western U.S.A. reservoir
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