Environmental Origins of Methylmercury Accumulated in Subarctic Estuarine Fish Indicated by Mercury Stable Isotopes

Carbon, Nitrogen, and Mercury Isotope Evidence for the Biogeochemical History of Mercury in Hawaiian Marine Bottomfish

The complex biogeochemical cycle of Hg makes identifying primary sources of fish tissue Hg problematic. To identify sources and provide insight into this cycle, we combined carbon (δ¹³C), nitrogen amino acid (δ¹⁵N_Phe), and Hg isotope (Δ¹⁹⁹Hg, Δ²⁰¹Hg, δ²⁰²Hg) data for six species of Hawaiian marine bottomfish. Results from these isotopic systems identified individuals within species that likely fed from separate food webs. Terrestrial freshwater inputs to coastal sediments were identified as the primary source of tissue Hg in the jack species, Caranx ignobilis, which inhabit shallow marine ecosystems. Thus, coastal C. ignobilis were a biological vector transporting Hg from freshwater environments into marine ecosystems. Depth profiles of Hg isotopic compositions for bottomfish (excludung C. ignobilis) were similar, but not identical, to profiles for open-ocean pelagic fishes, suggesting that in both settings inorganic Hg, which was ultimately transformed to monomethylmercury (MeHg) and bioaccumulated, was dominantly from a single source. However, differences between pelagic fish and bottomfish profiles were attributable to mass-dependent fractionation in the benthos prior to incorporation into the food web. Results also confirmed that bottomfish relied, at least in part, on a benthic food web and identified the incorporation of deeper water oceanic MeHg sources into deeper water sediments prior to food web uptake and transfer.

Source tracing of natural organic matter bound mercury in boreal forest runoff with mercury stable isotopes

Terrestrial runoff represents a major source of mercury (Hg) to aquatic ecosystems. In boreal forest catchments, such as the one in northern Sweden studied here, mercury bound to natural organic matter (NOM) represents a large fraction of mercury in the runoff. We present a method to measure Hg stable isotope signatures of colloidal Hg, mainly complexed by high molecular weight or colloidal natural organic matter (NOM) in natural waters based on pre-enrichment by ultrafiltration, followed by freeze-drying and combustion. We report that Hg associated with high molecular weight NOM in the boreal forest runoff has very similar Hg isotope signatures as compared to the organic soil horizons of the catchment area. The mass-independent fractionation (MIF) signatures (Δ¹⁹⁹Hg and Δ²⁰⁰Hg) measured in soils and runoff were in agreement with typical values reported for atmospheric gaseous elemental mercury (Hg⁰) and distinctly different from reported Hg isotope signatures in precipitation. We therefore suggest that most Hg in the boreal terrestrial ecosystem originated from the deposition of Hg⁰ through foliar uptake rather than precipitation. Using a mixing model we calculated the contribution of soil horizons to the Hg in the runoff. At moderate to high flow runoff conditions, that prevailed during sampling, the uppermost part of the organic horizon (Oe/He) contributed 50-70% of the Hg in the runoff, while the underlying more humified organic Oa/Ha and the mineral soil horizons displayed a lower mobility of Hg. The good agreement of the Hg isotope results with other source tracing approaches using radiocarbon signatures and Hg : C ratios provides additional support for the strong coupling between Hg and NOM. The exploratory results from this study illustrate the potential of Hg stable isotopes to trace the source of Hg from atmospheric deposition through the terrestrial ecosystem to soil runoff, and provide a basis for more in-depth studies investigating the mobility of Hg in terrestrial ecosystems using Hg isotope signatures.

Transfer of marine mercury to mountain lakes

Stocking is a worldwide activity on geographical and historical scales. The rate of non-native fish introductions have more than doubled over the last decades yet the effect on natural ecosystems, in the scope of biologically mediated transport and biomagnification of Hg and Hg-isotopes, is unknown. Using geochemistry (THg) and stable isotopes (N, Sr and Hg), we evaluate natal origin and trophic position of brown trout (Salmo trutta fario), as well as mercury biomagnification trends and potential pollution sources to three high-altitude lakes. Farmed trout show Hg-isotope signatures similar to marine biota whereas wild trout shows Hg-isotope signatures typical of fresh water lakes. Stocked trout initially show Hg-isotope signatures similar to marine biota. As the stocked trout age and shifts diet to a higher trophic level, THg concentrations increase and the marine Hg isotope signatures, induced via farm fish feed, shift to locally produced MeHg with lower δ²⁰²Hg and higher Δ¹⁹⁹Hg. We conclude that stocking acts a humanly induced biovector that transfers marine Hg to freshwater ecosystems, which is seen in the Hg-isotopic signature up to five years after stocking events occurred. This points to the need of further investigations of the role of stocking in MeHg exposure to freshwater ecosystems.

Optimization of human, animal, and environmental health by using the One Health approach

Emerging diseases are increasing burdens on public health, negatively affecting the world economy, causing extinction of species, and disrupting ecological integrity. One Health recognizes that human, domestic animal, and wildlife health are interconnected within ecosystem health and provides a framework for the development of multidisciplinary solutions to global health challenges. To date, most health-promoting interventions have focused largely on single-sector outcomes. For example, risk for transmission of zoonotic pathogens from bush-meat hunting is primarily focused on human hygiene and personal protection. However, bush-meat hunting is a complex issue promoting the need for holistic strategies to reduce transmission of zoonotic disease while addressing food security and wildlife conservation issues. Temporal and spatial separation of humans and wildlife, risk communication, and other preventative strategies should allow wildlife and humans to co-exist. Upstream surveillance, vaccination, and other tools to prevent pathogen spillover are also needed. Clear multi-sector outcomes should be defined, and a systems-based approach is needed to develop interventions that reduce risks and balance the needs of humans, wildlife, and the environment. The ultimate goal is long-term action to reduce forces driving emerging diseases and provide interdisciplinary scientific approaches to management of risks, thereby achieving optimal outcomes for human, animal, and environmental health.

Future Impacts of Hydroelectric Power Development on Methylmercury Exposures of Canadian Indigenous Communities

Developing Canadian hydroelectric resources is a key component of North American plans for meeting future energy demands. Microbial production of the bioaccumulative neurotoxin methylmercury (MeHg) is stimulated in newly flooded soils by degradation of labile organic carbon and associated changes in geochemical conditions. We find all 22 Canadian hydroelectric facilities being considered for near-term development are located within 100 km of indigenous communities. For a facility in Labrador, Canada (Muskrat Falls) with planned completion in 2017, we probabilistically modeled peak MeHg enrichment relative to measured baseline conditions in the river to be impounded, downstream estuary, locally harvested fish, birds and seals, and three Inuit communities. Results show a projected 10-fold increase in riverine MeHg levels and a 2.6-fold increase in estuarine surface waters. MeHg concentrations in locally caught species increase 1.3 to 10-fold depending on time spent foraging in different environments. Mean Inuit MeHg exposure is forecasted to double following flooding and over half of the women of childbearing age and young children in the most northern community are projected to exceed the U.S. EPA's reference dose. Equal or greater aqueous MeHg concentrations relative to Muskrat Falls are forecasted for 11 sites across Canada, suggesting the need for mitigation measures prior to flooding.