INVESTIGATION OF MERCURY BURIAL IN TWO SMALL LAKES UNDERGOING BROWNING IN NORTHEASTERN PENNSYLVANIA

In the past several decades, the “browning” of lakes has been observed in temperate regions of the Northern Hemisphere. Browning is likely caused by increased export of dissolved organic carbon (DOC) from catchment soils and is associated with a range of physical, chemical, and ecological changes to lakes. One possible but minimally studied impact of browning is the increased burial of mercury (Hg) in lake sediments. Mercury binds strongly with DOC and thus may be transported from catchment soils when browning occurs. Here, we investigate the relationship between browning and Hg burial in the sediments of two small seepage-fed lakes in northeastern Pennsylvania. These two lakes, Lake Lacawac and Lake Giles, are in forested watersheds protected from development. Both lakes have browned over the past three decades although Giles, which is a more transparent lake, has experienced a greater increase in water color and undergone more pronounced changes in response to browning. Downcore profiles of Hg in both lakes reveal a dramatic rise in the Hg content of sediments starting around the early 1990s. In Giles, Hg concentrations increase 6-fold over this time period reaching ~300 ppb in the most recent sediments. At Lacawac, there is an approximately 3-fold increase to Hg concentrations greater than 400 ppb. In both lakes, the highest concentrations of Hg were measured in the topmost (youngest) sediment interval. The timing and synchrony of increased Hg burial in both lakes is consistent with browning as the primary cause. Because these lakes are located in a region that has historically been impacted by anthropogenic Hg sources (e.g., coal fired power plants), their catchment soils may contain relatively large stores of “legacy” Hg that can be mobilized during the export of DOC. As browning continues, documenting the transport of legacy Hg may become increasingly important, especially in relatively "undisturbed" ecosystems.