Determining the Importance of Sediment Methylation to Methylmercury Concentrations in the Nacimiento Reservoir, California

Abstract:

Mercury methylation typically occurs along the redox boundary between oxic and anoxic conditions in the sediment and water column of surface waters. Previous studies have suggested that the dominant zone of methylation in lakes and reservoirs shifts from the sediment into the water column during periods of anoxic stratification, however, studies measuring methylmercury (MeHg) production in both sediment and the water column simultaneously remain limited. Understanding the importance of sediment methylation on water column MeHg concentrations has implications for management strategies aimed at reducing MeHg production. In this study, sediment (top 4 cm) and water column samples were collected from profundal and littoral zones of the Nacimiento Reservoir in the summer (2016) and winter (2015). Microbial methylation and demethylation rates were measured using mercury isotope tracers in triplicate microcosms. Microbial methylation rates in the sediment ranged from 0.03%/hr to 0.10%/hr, with demethylation ranging from 2%/hr to 15%/hr. Water column methylation rate ranged from 0.03%/hr to 0.12%/hr, with demethylation ranging from 0.5%/hr to 1.9%/hr. Simple rate models estimated that only 40% of hypolimnic MeHg concentrations could be explained by internal loading while sediment-to-water column loading could contribute the remaining 60%. This highlights the importance of examining both methylmercury production and loading when developing management strategies because even under anoxic conditions the sediment of Lake Nacimiento contributed a significant portion of hypolimnic MeHg. This suggests MeHg reduction strategies in this system need to address both sediment and water column methylation.