<p>This dissertation addressed questions pertaining to mercury (Hg) fate and transport in aquatic ecosystems by applying stable Hg isotopes as a tracer. Mercury poses a public health burden worldwide. In parts of the developing world, Hg-use during artisanal and small-scale gold mining (ASGM) is pointed at as the source of elevated Hg in the environment. However, Hg from other sources including soil erosion associated with land cover and land-use change (LCLUC) may also contribute to local Hg pollution. </p><p> Stable Hg isotope profiles of sediment cores, surface sediments, and soils from two aquatic ecosystems in Amap&aacute;, Brazil, one downstream artisanal gold mining (AGM) and one isolated from AGM were assessed. Although previous studies attributed elevated environmental Hg levels in this area to AGM, stable Hg isotopic evidence suggests elevated Hg downstream of AGM sites is dominantly from erosion of soils due to LCLUC. </p><p> In contrast, the impact of Hg-use during small-scale gold mining (SGM) in the Southern Andean Region of Portovelo-Zaruma, Ecuador on Hg in the trans-boundary Puyango-Tumbes River was also investigated. By comparing preliminary isotopic Hg signatures from river sediment along the Puyango-Tumbes to soil and sediment from upstream locations along the Puyango tributaries, we suggest Hg-use during SGM in this region is likely responsible for elevated Hg downstream and into Peru. Technical and policy challenges in measuring and responding to gold mining-related cumulative impacts were also reviewed in the context of Portovelo-Ecuador. </p><p> Together, the findings not only answer questions of critical importance to preventing Hg pollution in two of the world's most vulnerable ecosystems but also provide information that can be used to better target interventions to reduce environmental Hg levels and subsequent human exposures. Furthermore, the validation and application of the stable Hg isotope method to trace Hg pollution from ASGM in different aquatic ecosystems represents a critical step to the application of stable Hg isotopes to trace pollution in other complex natural environments and to address public health-related questions. </p>