Elucidating sources of mercury in the west coast of Korea and the Chinese marginal seas using mercury stable isotopes

Biogeochemical dynamics and sustainable remediation of mercury in West African water systems

Pollution of environmental drinking water sources by mercury (Hg) in West Africa is challenging, with the need to develop strategies to understand its biogeochemical transformation and mitigation to provide clean drinking water void of Hg. This review evaluated the biogeochemical cycle of Hg in West African ecology and the mitigation of Hg contamination of drinking water sources in the West African region. The study revealed Hg-bearing mineral deposits and artisanal and small-scale gold mining as major sources of Hg in West African environment. West African countries must develop sustainable methods for removing Hg from water. However, bioremediation (including microbial and phytoremediation) and adsorption are promising methods for purifying Hg-contaminated environmental drinking water sources in West Africa. Microorganisms such as Arbuscular mycorrhizal, E. coli, Fusobacterium sp, Trichoderma viride, Gliocladium arborescens, Bascillus sp. and Brevibacterium cysticus have demonstrated the capacity to remediate Hg from the water system. Furthermore, plant species like Paspalum conjugatum, Cyperus kyllingia, and Lindernia crustacea revealed exciting capacity as phytoremediators of Hg. Activated carbon, clay and mineral clays are abundant resources in West Africa that can function as adsorbents for removing Hg during water treatment. However, future studies should focus on optimizing the field-scale application of bioremediation and adsorption methods as mitigation strategies and their long-term benefits in West Africa. It is essential that the government in West Africa fund initiatives and programmes that support the accomplishment of the Minamata Convention agreement, which favours the attainment of the sustainable development goal (SDG-6).

Impacts of dam construction on mercury methylation and bioaccumulation revealed by stable isotopes

Impacts of dam construction on mercury (Hg) sources, biogeochemical cycling, and bioaccumulation were investigated along the west coast of Korea, where large-scale national projects were initiated between 1978 and 1990 to build dam or weir at the interface between rivers and estuaries. Total Hg (THg) and methylmercury (MeHg) concentrations and Hg stable isotopes in estuarine sediment cores sampled downstream of dam/weir reveal 74 ± 3% reduction in THg, 536 ± 158% increase in MeHg, and shifts in Hg sources from riverine export to wet deposition (precipitation) as revealed by increases in Δ199Hg (by 0.13 ± 0.03‰) and Δ200Hg (by 0.10 ± 0.01‰). Stable carbon isotopes, n-alkanes, and four geochemical pools of Hg extracted from the sediment cores show enhanced proportion of labile (F1; 0.28 ± 0.21% points) and organically-bound (F2; 34 ± 12% points) Hg fractions and the ratio of marine-based organic matter relative to recalcitrant Hg bound to terrestrial organic matter at the downstream estuary. Dam/weir constructions have significant effects on Hg bioavailability and methylation, by enhancing the proportion of dissolved Hg delivered by precipitation and those subject to efficient assimilation by marine algae and microbial utilization. This is evident by the negative δ202Hg shifts in the sediment cores at the depths of MeHg peaks. Cessation of riverine Hg input is manifested by 96% reduction of THg in shellfish and enhanced methylation appears to cause 106% increase in fish THg at the downstream estuarine ecosystem. This study underscores the importance of considering complex and long-term biogeochemical modifications as a part of Environmental Impact Assessments.

Bivalves as a Mercury Bioindicator: A National Isotopic Survey along the Coast of South Korea

Mercury (Hg) is a contaminant that poses health risks for human populations relying on seafood consumption. To mitigate its impact, identifying and monitoring Hg sources have become priorities, notably under the Minamata Convention. Bivalves are commonly used as sentinels in contaminant biomonitoring but can accumulate Hg from diverse environmental media. To investigate their Hg sources, bivalves (blue mussel, Mytilus edulis, and Pacific oyster, Crassostrea gigas) and their associated sediment were sampled along the coast of South Korea and analyzed for Hg concentration and isotopic values. Oysters displayed low Δ199Hg (0.19 ± 0.19‰) and δ202Hg (-0.35 ± 0.55‰), highlighting a sedimentary source, whereas mussels exhibited higher values (0.72 ± 0.87 and 0.09 ± 0.72‰, respectively) indicating bioaccumulation from the water column. Sulfur, carbon, and nitrogen stable isotopes suggested that such a difference was not due to feeding niches. However, Hg isotopic trajectory analysis showed that environmental conditions controlling the desorption and remobilization of sediment Hg, notably via tidal flows, were likely to drive the observed source(s). While sediment Hg is not systematically reflected in biota, bivalves, which typically display Hg pools accumulated by benthic or pelagic food webs, appear valuable for Minamata Convention biomonitoring, though their fluctuating Hg sources require careful interpretation.

Study of mercury bioavailability using isotope dilution and BCR sequential extraction in the sediment of Yellow Sea and East China Sea, China

Mercury (Hg) emitted from East Asian has increased the risk of Hg in China Marginal Seas for decades. However, the speciation of Hg (especially the bioavailable Hg) in these regions remains unclear. To address this problem, we analyzed total Hg (THg) and methylmercury (MeHg) in the sediment and porewater of Yellow sea (YS) and East China Sea (ECS) and determined the speciation of Hg using both improved BCR sequential extraction and isotope dilution (ID) techniques. Nearshore areas of YS and ECS exhibited higher THg levels in sediments and porewater, suggesting the significant contribution of terrestrial inputs. The spatial distribution of MeHg showed similar trends with THg, but the sites with higher MeHg concentrations did not align with those of THg. The improved BCR sequential extraction method showed the residual fraction dominated Hg content (∼44 %) in both systems, with a minor bioavailable carbonate fraction (1 %). The Spearman correlation analysis indicates that Eh and pH are the two factors significantly affected Hg bioavailability in the sediment. The bioavailability of Hg (estimated by the BCR method) showed a significant positive correlation with MeHg levels in the sediment (R²=0.47, P < 0.05), suggesting that BCR can be used to estimate the potential of Hg methylation in the sediment. However, the extent of bioavailable Hg in BCR and ID method were 1.15 ± 0.38 % and 29.5 ± 14.8 %, respectively, implying that Hg bioavailability may be underestimated by BCR techniques compared to ID methods (T-test, P < 0.01).

Spatial and temporal variations in the pollution status and sources of mercury in the Jiaozhou bay

In the past few decades, mercury (Hg) discharged into the coastal bays of China has significantly increased; however, long-term trends regarding the pollution status and sources of Hg in these bays have yet to be clear. Focusing on this issue, surface sediments and core sediments were collected in the Jiaozhou Bay (JZB), a typical bay highly affected by human activities in China, to analyze the concentrations and stable isotopic composition of Hg. Total mercury (THg) concentrations in surface sediment varied from 7 to 163 ng/g, with higher levels located in the eastern JZB, possibly attributed to intensive industrial and population density. THg in sediment cores 14 and 20 displayed fluctuating increasing trends from 1936 to 2019, reflecting the deterioration of Hg pollution. In contrast, THg in sediment core 28 near the river mouth exhibited a declining trend, possibly due to the river dam construction. Using a stable isotope mixing model, contributions of various sources (atmospheric, riverine, and industrial emissions) to Hg in the JZB were estimated. The results showed that industrial emissions were the main source (over 50%) of mercury in the JZB in 2019. Sediment cores recorded an increase in industrial Hg due to early industrialization and Reform and Opening-up before 2000. In addition, sediment core 20 demonstrated a rise in the percentage of riverine Hg due to land reclamation at the bay's mouth during 2000-2007.

Identifying the external N and Hg inputs to the estuary ecosystem based on the triple isotopic information (δ15NNO3, Δ17ONO3 and δ18ONO3)

The supply and sources of N and Hg in the Geum estuary of the western coast of Korea were evaluated. Triple isotope proxies (δ15NNO3, Δ17ONO3 and δ18ONO3) of NO3- combined with conservative mixing between river and ocean waters were used to improve isotope finger-printing methods. The N pool in the Geum estuary was primarily influenced by the Yellow Sea water, followed by riverine discharge (821 × 106 mol yr-1) and atmospheric deposition (51 × 106 mol yr-1). The influence of the river was found to be greater for Hg than that of the atmosphere. The triple isotope proxies revealed that the riverine and atmospheric inputs of N have been affected by septic wastes and fossil fuel burning, respectively. From the inner estuary towards offshore region, the influence of the river diminishes, thus increasing the relative impact of the atmosphere. Moreover, the isotope proxies showed a significant influence of N assimilation in February and nitrification in May.

New insight into biomagnification factor of mercury based on food web structure using stable isotopes of amino acids

Although many attempts have been carried out to elaborate trophic magnification factor (TMF) and biomagnification factor (BMF), such as normalizing the concentration of pollutants and averaging diet sources, the uncertainty of the indexes still need to be improved to assess the bioaccumulation of pollutants. This study first suggests an improved BMF (i.e., BMF') applied to mercury bioaccumulation in freshwater fish from four sites before and after rainfall. The diet source and TP of each fish were identified using nitrogen stable isotope of amino acids (δ15NAAs) combined with bulk carbon stable isotope (δ13C). The BMF' was calculated normalizing with TP and diet contributions derived from MixSIAR. The BMF' values (1.3-27.2 and 1.2-27.8), which are representative of the entire food web, were generally higher than TMF (1.5-13.9 and 1.5-14.5) for both total mercury and methyl mercury, respectively. The BMF' implying actual mercury transfer pathway is more reliable index than relatively underestimated TMF for risk assessment. The ecological approach for BMF calculations provides novel insight into the behavior and trophic transfer of pollutants like mercury.

Mercury Sources and Fate in a Large Brackish Ecosystem (the Baltic Sea) Depicted by Stable Isotopes

Identifying Hg sources to aquatic ecosystems and processes controlling the levels of monomethylmercury (MMHg) is critical for developing efficient policies of Hg emissions reduction. Here we measured Hg concentrations and stable isotopes in sediment, seston, and fishes from the various basins of the Baltic Sea, a large brackish ecosystem presenting extensive gradients in salinity, redox conditions, dissolved organic matter (DOM) composition, and biological activities. We found that Hg mass dependent fractionation (Hg-MDF) values in sediments mostly reflect a mixing between light terrestrial Hg and heavier industrial sources, whereas odd Hg isotope mass independent fractionation (odd Hg-MIF) reveals atmospheric inputs. Seston presents intermediate Hg-MDF and odd Hg-MIF values falling between sediments and fish, but in northern basins, high even Hg-MIF values suggest the preferential accumulation of wet-deposited Hg. Odd Hg-MIF values in fish indicate an overall low extent of MMHg photodegradation due to limited sunlight exposure and penetration but also reveal large spatial differences. The photodegradation extent is lowest in the central basin with recurrent algal blooms due to their shading effect and is highest in the northern, least saline basin with high concentrations of terrestrial DOM. As increased loads of terrestrial DOM are expected in many coastal areas due to global changes, its impact on MMHg photodegradation needs to be better understood and accounted for when predicting future MMHg concentrations in aquatic ecosystems.

Mercury stable isotopes in the ocean: Analytical methods, cycling, and application as tracers

Mercury (Hg) has seven stable isotopes that can be utilized to trace the sources of Hg and evaluate the importance of transport and transformation processes in the cycling of Hg in the environment. The ocean is an integral part of the Earth and plays an important role in the global mercury cycle. However, there is a lack of a systematic review of Hg stable isotopes in marine environments. This review is divided into four sections: a) advances in Hg stable isotope analysis, b) the isotope ratios of Hg in various marine environmental matrices (seawater, sediment, and organisms), c) processes governing stable Hg isotope ratios in the ocean, and d) application of Hg stable isotopes to understand biotic uptake and migration. Mercury isotopes have provided much useful information on marine Hg cycling that cannot be given by Hg concentrations alone. This includes (i) sources of Hg in coastal or estuarine environments, (ii) transformation pathways and mechanisms of different forms of Hg in marine environments, (iii) trophic levels and feeding guilds of marine fish, and (iv) migration/habitat changes of marine fish. With the improvement of methods for seawater Hg isotope analysis (especially species-specific methods) and the measurement of Hg isotope fractionation during natural biogeochemical processes in the ocean, Hg stable isotopes will advance our understanding of the marine Hg cycle in the future, e.g., mercury exchange at the sea-atmosphere interface and seawater-sediment interface, contributions of different water masses to Hg in the ocean, fractionation mechanisms of Hg and MeHg transformation in seawater.

Ecological Traits of Fish for Mercury Biomonitoring: Insights from Compound-Specific Nitrogen and Stable Mercury Isotopes

We coupled compound-specific isotopic analyses of nitrogen (N) in amino acids (δ¹⁵N_Glu, δ¹⁵N_Phe) and mercury stable isotopes (δ²⁰²Hg, Δ¹⁹⁹Hg) to quantify ecological traits governing the concentration, variability, and source of Hg in largemouth bass (LB) and pike gudgeon (PG) across four rivers, South Korea. PG displayed uniform Hg concentration (56-137 ng/g), trophic position (TP_corrected; 2.6-3.0, n = 9), and N isotopes in the source amino acid (δ¹⁵N_Phe; 7-13‰), consistent with their specialist feeding on benthic insects. LB showed wide ranges in Hg concentration (45-693 ng/g), TP_corrected (2.8-3.8, n = 14), and δ¹⁵N_Phe (1.3-16‰), reflecting their opportunistic feeding behavior. Hg sources assessed using Hg isotopes reveal low and uniform Δ¹⁹⁹Hg in PG (0.20-0.49‰), similar to Δ¹⁹⁹Hg reported in sediments. LB displayed site-specific δ²⁰²Hg (-0.61 to -0.04‰) and Δ¹⁹⁹Hg (0.53-1.09‰). At the Yeongsan River, LB displayed elevated Δ¹⁹⁹Hg and low δ¹⁵N_Phe, consistent with Hg and N sourced from the atmosphere. LB at the Geum River displayed low Δ¹⁹⁹Hg and high δ¹⁵N_Phe, both similar to the isotope values of anthropogenic sources. Our results suggest that a specialist fish (PG) with consistent ecological traits and Hg concentration is an effective bioindicator species for Hg. When accounting for Hg sources, however, LB better captures site-specific Hg sources.

Investigating the dynamics of methylmercury bioaccumulation in the Beaufort Sea shelf food web: a modeling perspective

High levels of methylmercury (MeHg) have been reported in Arctic marine biota, posing health risks to wildlife and human beings. Although MeHg concentrations of some Arctic species have been monitored for decades, the key environmental and ecological factors driving temporal trends of MeHg are largely unclear. We develop an ecosystem-based MeHg bioaccumulation model for the Beaufort Sea shelf (BSS) using the Ecotracer module of Ecopath with Ecosim, and apply the model to explore how MeHg toxicokinetics and food web trophodynamics affect bioaccumulation in the BSS food web. We show that a food web model with complex trophodynamics and relatively simple MeHg model parametrization can capture the observed biomagnification pattern of the BSS. While both benthic and pelagic production are important for transferring MeHg to fish and marine mammals, simulations suggest that benthic organisms are primarily responsible for driving the high trophic magnification factor in the BSS. We illustrate ways of combining empirical observations and modelling experiments to generate hypotheses about factors affecting food web bioaccumulation, including the MeHg elimination rate, trophodynamics, and species migration behavior. The results indicate that population dynamics rather than MeHg elimination may determine population-wide concentrations for fish and lower trophic level organisms, and cause large differences in concentrations between species at similar trophic levels. This research presents a new tool and lays the groundwork for future research to assess the pathways of global environmental changes in MeHg bioaccumulation in Arctic ecosystems in the past and the future.