The aim of this text is to present a critical overview of Hg research in the Amazon along the last 30 years, discussing some of the lessons learned and the unique challenges that the complex Amazonian environment can place to researchers working on mercury. The description provided here is based on our long-term research with mercury in this tropical rainforest environment and may be particularly relevant for those initiating mercury studies in the tropics.

The largest source of global mercury (Hg) anthropogenic inputs to the environment is derived from artisanal and small-scale gold mining (ASGM) activities in developing countries. While our understanding of global Hg emissions from ASGM is growing, there is limited empirical documentation about the levels of total mercury (THg) and methylmercury (MeHg) contamination near ASGM sites. We measured THg and MeHg concentrations in soil (n = 119), sediment (n = 22), and water (n = 25) from four active ASGM villages and one non-ASGM reference village in Senegal, West Africa. Nearly all samples had THg and MeHg concentrations that exceeded the reference village concentrations and USEPA regulatory standards. The highest median THg concentrations were found in huts where mercury-gold amalgams were burned (7.5 μg/g), while the highest median MeHg concentrations and percent Hg as MeHg were found in river sediments (4.2 ng/g, 0.41%). Median river water concentrations of THg and MeHg were also elevated compared to values at the reference site (22 ng THg/L, 0.037 ng MeHg/L in ASGM sites). This study provides direct evidence that Hg from ASGM is entering both the terrestrial and aquatic ecosystems where it is converted in soils, sediment, and water to the neurotoxic and bioavailable form of MeHg.

Marine fog water samples were collected over two summers (2014–2015) with active strand collectors (CASCC) at eight coastal sites from Humboldt to Monterey counties in California, USA, and on four ocean cruises along the California coastline in order to investigate mercury (Hg) cycling at the ocean-atmosphere-land interface. The mean concentration of monomethylmercury (MMHg) in fog water across terrestrial sites for both years was 1.6 ± 1.9 ng L -1 (<0.01–10.4 ng L -1 , N = 149), which corresponds to 5.7% (2.0–10.8%) of total Hg (HgT) in fog. Rain water samples from three sites had mean MMHg concentrations of 0.20 ± 0.12 ng L -1 (N = 5) corresponding to 1.4% of HgT. Fog water samples collected at sea had MMHg concentrations of 0.08 ± 0.15 ng L -1 (N = 14) corresponding to 0.4% of HgT. Significantly higher MMHg concentrations in fog were observed at terrestrial sites next to the ocean relative to a site 40 kilometers inland, and the mean difference was 1.6 ng L -1 . Using a rate constant for photo-demethylation of MMHg of -0.022 h -1 based on previous demethylation experiments and a coastal-inland fog transport time of 12 hours, a mean difference of only 0.5 ng L -1 of MMHg was predicted between coastal and inland sites, indicating other unknown source and/or sink pathways are important for MMHg in fog. Fog water deposition to a standard passive 1.00 m 2 fog collector at six terrestrial sites averaged 0.10 ± 0.07 L m -2 d -1 , which was ∼2% of typical rainwater deposition in this area. Mean air-surface fog water fluxes of MMHg and HgT were then calculated to be 34 ± 40 ng m -2 y -1 and 546 ± 581 ng m -2 y -1 , respectively. These correspond to 33% and 13% of the rain fluxes, respectively.

Analyses of mercury (Hg) isotope ratios in fish tissues are used increasingly to infer sources and biogeochemical processes of Hg in natural aquatic ecosystems. Controlled experiments that can couple internal Hg isotope behavior with traditional isotope tracers (δ 13 C, δ 15 N) can improve the applicability of Hg isotopes as natural ecological tracers. In this study, we investigated changes in Hg isotope ratios (δ 202 Hg, Δ 199 Hg) during bioaccumulation of natural diets in the pelagic Pacific bluefin tuna ( Thunnus orientalis ; PBFT). Juvenile PBFT were fed a mixture of natural prey and a dietary supplement (60% Loligo opalescens , 31% Sardinops sagax, 9% gel supplement) in captivity for 2914 days, and white muscle tissues were analyzed for Hg isotope ratios and compared to time in captivity and internal turnover of δ 13 C and δ 15 N. PBFT muscle tissues equilibrated to Hg isotope ratios of the dietary mixture within ∼700 days, after which we observed a cessation in further shifts in Δ 199 Hg, and small but significant negative δ 202 Hg shifts from the dietary mixture. The internal behavior of Δ 199 Hg is consistent with previous fish studies, which showed an absence of Δ 199 Hg fractionation during Hg bioaccumulation. The negative δ 202 Hg shifts can be attributed to either preferential excretion of Hg with higher δ 202 Hg values or individual variability in captive PBFT feeding preferences and/or consumption rates. The overall internal behavior of Hg isotopes is similar to that described for δ 13 C and δ 15 N, though observed Hg turnover was slower compared to carbon and nitrogen. This improved understanding of internal dynamics of Hg isotopes in relation to δ 13 C and δ 15 N enhances the applicability of Hg isotope ratios in fish tissues for tracing Hg sources in natural ecosystems.