Humans have dramatically accelerated mercury (Hg) cycling in ecosystems across the globe due to anthropogenic activities. Depending on the speciation and concentration of Hg, exposure to this metal can cause a variety of health problems, which has led to significant research and regulation efforts to reduce the use and release of Hg. Despite these global and regional efforts, Hg will continue cycling at elevated levels for thousands of years due to its persistent nature. Continued cycling of Hg following its initial release is now of significant concern since around two-thirds of annual Hg emissions are from remobilized pools of Hg.

The goal of my research is to advance understanding of how mountain ecosystems cycle Hg and how that may change under a warming climate. First, I synthesized the recent literature on Hg cycling in mountain ecosystems with a focus on the U.S. Rocky Mountains. I identified primary Hg sources, storage, transformations, and losses, as well as impacts from climate change. I also highlighted important knowledge gaps and proposed future research priorities. Based on the gaps that I identified in my synthesis, I next characterized Hg inputs, storage, and bioaccumulation along an elevation gradient in the Colorado Rocky Mountains. Similar to past work conducted in the eastern U.S. Himalaya, and Tibetan Plateau, I found that elevation and tree cover are important drivers of Hg inputs and storage in mountain ecosystems. Unlike those past studies, however, I found that precipitation, rather than litterfall, dominated atmospheric inputs of Hg and was the primary driver of Hg bioaccumulation in terrestrial wildlife. Next, to assess the impact of climate change on the toxicity and bioavailability of Hg, I measured rates of methylmercury (MeHg) formation in alpine and subalpine wetlands under increasing sulfate concentrations. I found that MeHg formation in subalpine peatlands in the North Boulder Watershed of the Colorado Rocky Mountains are sulfate-limited. This watershed has experienced a 200 % increase in sulfate concentrations over the past 30 years—a pattern observed in over 100 high elevation watersheds globally—due to climate-driven sulfate weathering associated with thawing permafrost and rock glacier features. This increased export of sulfate may accelerate MeHg production and bioaccumulation in sulfate-limited high elevation watersheds. Finally, synthesizing information on Hg inputs, storage, and losses, I characterized the sink-source behavior of alpine and subalpine zones in the North Boulder Watershed to better constrain the role that mountain ecosystems play in cycling legacy Hg pools. I found that these regions act as sinks for Hg, however, major uncertainties exist with regards to losses via evasion. This factor is particularly relevant for the coniferous subalpine zone. My findings highlight the importance of better constraining losses of Hg in evasion from soil and snow surfaces to more accurately quantify the sink-source nature of mountains regions currently, as well as under future warming conditions. Altogether, the results of this body of research advance our understanding of the critical role that mountain ecosystems play in the global Hg cycle and underscore key priorities for future research to address remaining uncertainties, particularly in the context of climate change.