The response of two remote, temperate lakes to changes in atmospheric mercury deposition, sulfate, and the water cycle

Abstract

Atmospheric deposition is the ultimate source of Hg to many remote lakes, but the response to depositional change remains uncertain because of potential buffering by historical pools of Hg in sediments and soils. We investigated the responsiveness of two lakes over time scales ranging from weeks to decades. For waterborne Hg, results indicated that a precipitation-dominated seepage lake and a wetland-dominated drainage lake responded similarly to depositional change, despite high loadings of terrigeneous matter to the drainage lake. For methylmercury (meHg), the response was more complicated, reflecting the influence of multiple factors on in-lake methylation. In the seepage lake, waterborne meHg was correlated with the atmospheric deposition of Hg(II) and SO4, and there was an indirect dependence on the water cycle. In the drainage lake, although wetland export was always the dominant external source of meHg, mass balance indicated that net in-lake methylation was four- to seven-fold greater than loading from the wetland. However, in-lake meHg production was related to the export of Hg(II), SO4, dissolved organic carbon, and P from the wetland to the lake. The results suggest that atmospheric Hg deposition, weather, and microbial activity interactively effect the aquatic mercury cycle in ways that can be independent of historical pools of Hg in catchments.