Biogeochemical Cycling and Fate of Mercury and Methylmercury in the Arctic Alaskan Lakes

University Of Connecticut, Storrs CT

Investigators

Abstract

Prior work has identified two general areas, (a) monomethylmercury (MMHg) production and (b) photochemical sinks of mercury and MMHg, in which the cycling of mercury in arctic lakes appears exceptional and highly influential in controlling the fate of natural and anthropogenic inputs. Funds are provided to investigate the production of MMHg within arctic lakes. Well-constrained mass balances have confirmed the hypothesis that production of MMHg in sediments by bacteria is the dominant source to these lakes. These systems may be quite different, however, from those of temperate lakes and marine waters where the activity of sulfate-reducing bacteria (SRB) is the dominant biogeochemical process resulting in methylation of mercury. The arctic lakes are characterized by very low productivity, low sulfate, moderate to high dissolved organic matter (DOM), and high levels of iron. This biogeochemical setting could challenge conventional understanding of the role of SRB in controlling MMHg production and sediment concentrations. Field and laboratory investigations that examine the relationship between MMHg production and biogeochemical properties of sediments (abundance and diversity of SRB, mercury speciation in pore waters, temperature, and sulfate, iron, and organic contents) collected from lakes with varying physicochemical characteristics will be conducted. Given the abundance of Fe in the sediments, iron-reducing bacteria will be of particular interest as they may affect mercury methylation/demethylation processes. The second component of this research will be a mechanistic investigation of the substantial photochemical sinks for MMHg and mercury in arctic lakes. Photoreduction of mercury, leading to supersaturation and emission of a volatile form of mercury, represents a sink of ca. 20% of total mercury inputs to these lakes, and as a result lowers the amount of mercury available for methylation. Even more dramatic is the substantial photodegradation of MMHg, which can consume nearly all of the MMHg released from sediments, reducing or preventing its bioaccumulation. These processes are large enough to control concentrations and speciation of mercury in these lakes, and thereby control bioavailability of this toxic metal. The study of the mechanisms of mercury photochemistry will be expanded with a combination of laboratory and field studies, which will focus on the importance of DOM and Fe in facilitating mercury and MMHg photoreactions. This work is particularly important because of the bioaccumulation of MMHg in the organisms harvested by the subsistence hunter communities of the Arctic.