Artisanal and small-scale gold mining (ASGM) poses a growing threat to tropical forest ecosystems. Operating in over 70 countries, often informally or illegally, ASGM accounts for roughly 20% of global gold production. While providing livelihoods, it causes deforestation, forest conversion to ponds, river sediment loading, and is the largest source of atmospheric mercury (Hg) emissions and freshwater Hg releases. Many ASGM sites are in biodiversity hotspots, causing biodiversity loss, species extinction, and high Hg exposure in humans and predators. ASGM releases an estimated 675–1000 tons of Hg annually into the atmosphere, shifting the primary atmospheric Hg emission source from the Global North to the Global South. The impact of these atmospheric Hg emissions and their deposition patterns on ASGM-affected landscapes is poorly understood. The Minamata Convention on Mercury (2017) addresses Hg from ASGM. In ASGM, liquid Hg is used to isolate gold, releasing gaseous elemental mercury (GEM; Hg°) when the amalgam is heated. Amalgam burning often lacks Hg capture, though mitigation efforts are underway. As of 2021, 132 countries (including Peru) signed the convention and started developing National Action Plans to reduce Hg in ASGM. These plans are urged to be inclusive, ongoing, and holistic, considering socioeconomic factors and environmental harm. Current Hg mitigation efforts largely focus on aquatic ecosystems, miners, those near amalgam burning, and communities consuming fish. However, terrestrial ecosystems are also at risk. Atmospheric Hg released as GEM from ASGM can reach terrestrial landscapes via three main pathways: (1) GEM sorbs to atmospheric particles and is intercepted by surfaces; (2) GEM is directly taken up by plants; (3) GEM oxidizes to Hg(II) species, dry deposited, sorbed to particles, or carried in rainwater. These pathways introduce Hg to soils through throughfall, litterfall, and rainfall. While Hg enrichment near ASGM is documented, the landscape-scale transport of Hg and the relative importance of these pathways remain unclear. This study examines how GEM concentrations and deposition pathways vary with proximity to ASGM and canopy leaf area, investigates the relationship between soil Hg storage and atmospheric inputs, and assesses Hg bioaccumulation in forest songbirds near ASGM activity.

Numerous studies document mercury enrichment in terrestrial and aquatic ecosystems near artisanal and small-scale gold mining (ASGM) activities. These studies highlight the significant impact of ASGM on the environment, with elevated mercury levels found in various components of these ecosystems. However, there is a lack of research specifically focusing on the fate of atmospheric mercury emissions from ASGM and the patterns of deposition and accumulation across ASGM-impacted landscapes, particularly in tropical ecosystems such as the Amazon rainforest. Existing literature predominantly focuses on mercury risks associated with ASGM near aquatic ecosystems, occupational exposure via inhalation, dietary exposure through fish consumption, and bioaccumulation in aquatic food webs. The relative contribution of various deposition pathways in tropical forests impacted by ASGM requires further investigation. The existing research provides a foundation for understanding the problem, but additional research is needed to fully understand the complex interactions between ASGM activities, atmospheric mercury deposition, and its consequences on diverse ecosystems. This study aims to address this gap in knowledge by investigating the influence of forest canopy structure on mercury deposition and its subsequent bioaccumulation in terrestrial food webs.

The study was conducted in southeastern Peru's Madre de Dios region, a region heavily impacted by ASGM. Five sites along the Madre de Dios River were selected: two remote sites (Boca Manu and Chilive) and three mining-impacted sites (Los Amigos, Boca Colorado, and Laberinto). The study employed a comprehensive sampling strategy to assess various aspects of mercury cycling and bioaccumulation. At each site, samples were collected from both forested and deforested areas. Gaseous elemental mercury (GEM) concentrations were measured using passive air samplers (PAS). Wet deposition was assessed using precipitation collectors. Throughfall and litterfall were also collected to quantify dry deposition. Foliar mercury concentrations were measured in *Ficus insipida* and *Inga feuillei* leaves. Soil samples were collected to determine mercury concentrations and pools. In addition, methylmercury (MeHg) concentrations were measured in precipitation, throughfall, litterfall, and soil samples. Finally, mercury concentrations were measured in the tail feathers of several songbird species to evaluate mercury bioaccumulation in the terrestrial food web. The data was analyzed to determine the relationship between mercury concentrations and proximity to mining activity, and to investigate the role of forest canopy structure in mercury deposition. The statistical analyses included ordinary least squares regression to examine the relationship between continuous variables, the Kruskal-Wallis test and the pairwise Wilcoxon test to make comparisons between sites, and the use of R software for data processing and visualization.

Atmospheric GEM concentrations were significantly higher near ASGM activity, particularly near towns where amalgam burning is common. In contrast to GEM concentrations in clearings, total Hg concentrations in throughfall were strongly influenced by both proximity to ASGM and forest canopy structure. The intact mature forest at Los Amigos Conservation Concession had the highest Hg concentrations in throughfall, exceeding reported values from other sites contaminated by cinnabar mining and coal combustion. Throughfall Hg fluxes at Los Amigos were significantly higher than at other mining sites or remote sites. Litterfall was also a significant pathway for Hg deposition near ASGM activities. Foliar Hg concentrations at all mining sites were higher than values reported for temperate, boreal, and alpine forests. The estimated litterfall Hg flux was highest at Los Amigos, likely due to higher litterfall mass. Preliminary estimates of total annual atmospheric Hg fluxes at Los Amigos showed that forested areas had fluxes more than 15 times greater than deforested areas. The majority (94%) of total Hg deposition in the conserved forest at Los Amigos occurred via dry deposition. Deforested areas had lower Hg loading. Soil Hg concentrations were highest at Los Amigos and a deforested site near Boca Colorado. A strong correlation was found between Hg concentrations in surficial soil and Hg concentrations in throughfall at forested sites, but not deforested sites. Measurable MeHg concentrations were found in Amazonian soils near ASGM, extending beyond aquatic ecosystems. Hg concentrations in songbirds from Los Amigos were 2–3 times higher than at a remote site, regardless of feeding habits. Some songbirds exceeded mercury concentrations associated with decreased reproductive success.

The study's findings demonstrate that intact forests near ASGM act as significant sinks for atmospheric mercury, capturing substantial amounts of mercury from the atmosphere. This sequestration reduces mercury risks to nearby aquatic ecosystems and the global atmospheric mercury pool. However, the high mercury accumulation in forest soils and biomass, and the elevated mercury levels observed in songbirds, pose a significant threat to the biodiversity of these ecosystems. The results highlight the vulnerability of these complex ecosystems to mercury pollution and the potential for long-term impacts on wildlife and human populations. The strong correlation between leaf area index and mercury deposition via throughfall indicates the importance of forest canopy structure in controlling mercury deposition. Furthermore, the finding of elevated MeHg concentrations in soils suggests that the risk of mercury exposure in terrestrial food webs may be higher than previously anticipated. This highlights the importance of comprehensive risk assessment and management strategies that consider both aquatic and terrestrial mercury exposure pathways.

This study provides novel evidence of high mercury deposition and accumulation in intact Amazonian forests near ASGM. Intact forests effectively capture atmospheric mercury but also accumulate high levels, with potential for future mobilization. Mercury bioaccumulation in songbirds suggests significant risks to terrestrial food webs. Reducing Hg releases from ASGM is crucial, as forest sequestration alone is insufficient to prevent widespread mercury pollution. Future research should investigate MeHg dynamics in diverse habitats and assess the ecological consequences of mercury accumulation in terrestrial food webs.

The study's relatively short sampling period (three seasonal campaigns) might not fully capture the long-term dynamics of mercury cycling. The lack of extensive data on the exact locations and intensity of amalgam burning could influence the interpretation of the spatial patterns of mercury deposition. The study area may not fully represent the diverse conditions across the Amazon basin. Further studies with longer-term monitoring and more detailed information on ASGM activities are needed to fully comprehend the complex interactions between ASGM, mercury deposition, and ecosystem responses.