Absence of fractionation of mercury isotopes during trophic transfer of methylmercury to freshwater fish in captivity

Significant biomagnification of methylmercury in songbird nestlings through a rice-based food web: Insights from stable mercury isotopes

To elucidate the sources and transfer of mercury (Hg) in terrestrial food chains, particularly in heavily Hg-contaminated rice paddy ecosystems, we collected rice leaves, invertebrates, and Russet Sparrow nestlings from a clear food chain and analyzed the dietary compositions and potential Hg sources using stable Hg isotopes coupled with a Bayesian isotope mixing model (BIMM). Our findings indicated that MeHg exposure is dominant through the dietary route, with caterpillars, grasshoppers, and katydids being the main prey items, while the less provisioned spiders, dragonflies, and mantises contributed the most of the Hg to nestlings. We found minimal MIF but certain MDF in this terrestrial food chain and identified two distinct MeHg sources of dietary exposure and maternal transfer. We firstly found that the dietary route contributed substantially (almost tenfold) more MeHg to the nestlings than maternal transfer. These findings offer new insights into the integration of Hg from the dietary route and maternal transfers, enhancing our understanding of fluctuating Hg exposure risk during the nestling stage. Our study suggested that Hg isotopes combined with BIMM is an effective approach for tracing Hg sources in birds and for gaining in-depth insight into the trophic transfers and biomagnification of MeHg in food chains.

Mercury stable isotopes in seabirds in the Ebro Delta (NE Iberian Peninsula): Inter-specific and temporal differences

Mercury (Hg) is a global pollutant, which particularly affects aquatic ecosystems, both marine and freshwater. Top-predators depending on these environments, such as seabirds, are regarded as suitable bioindicators of Hg pollution. In the Ebro Delta (NE Iberian Peninsula), legacy Hg pollution from a chlor-alkali industry operating in Flix and located ca. 100 km upstream of the Ebro River mouth has been impacting the delta environment and the neighboring coastal area. Furthermore, levels of Hg in the biota of the Mediterranean Sea are known to be high compared to other marine areas. In this work we used a Hg stable isotopes approach in feathers to understand the processes leading to different Hg concentrations in three Laridae species breeding in sympatry in the area (Audouin's gull Ichthyaetus audouinii, black-headed gull Chroicocephalus ridibundus, common tern Sterna hirundo). These species have distinct trophic ecologies, exhibiting a differential use of marine resources and freshwater resources (i.e., rice paddies prey). Moreover, for Audouin's gull, in which in the Ebro Delta colony temporal differences in Hg levels were documented previously, we used Hg stable isotopes to understand the impact of anthropogenic activities on Hg levels in the colony over time. Hg stable isotopes differentiated the three Laridae species according to their trophic ecologies. Furthermore, for Audouin's gull we observed temporal variations in Hg isotopic signatures possibly owing to anthropogenic-derived pollution in the Ebro Delta. To the best of our knowledge this is the first time Hg stable isotopes have been reported in seabirds from the NW Mediterranean.

Interspecific Variations in the Internal Mercury Isotope Dynamics of Antarctic Penguins: Implications for Biomonitoring

Mercury (Hg) biomonitoring requires a precise understanding of the internal processes contributing to disparities between the Hg sources in the environment and the Hg measured in the biota. In this study, we investigated the use of Hg stable isotopes to trace Hg accumulation in Adélie and emperor penguin chicks from four breeding colonies in Antarctica. Interspecific variation of Δ199Hg in penguin chicks reflects the distinct foraging habitats and Hg exposures in adults. Chicks at breeding sites where adult penguins predominantly consumed mesopelagic prey showed relatively lower Δ199Hg values than chicks that were primarily fed epipelagic krill. Substantial δ202Hg variations in chick tissues were observed in both species (Adélie: -0.11 to 1.13‰, emperor: -0.27 to 1.15‰), whereas only emperor penguins exhibited the lowest δ202Hg in the liver and the highest in the feathers. Our results indicate that tissue-specific δ202Hg variations and their positive correlations with % MeHg resulted from MeHg demethylation in the liver and kidneys of emperor penguin chicks, whereas Adélie penguin chicks showed different internal responses depending on their exposure to dietary MeHg. This study highlights the importance of considering intra- and interspecific variations in adult foraging ecology and MeHg demethylation when selecting penguin chicks for Hg biomonitoring.

Unveiling the Sources and Transfer of Mercury in Forest Bird Food Chains Using Techniques of Vivo-Nest Video Recording and Stable Isotopes

Knowledge gaps in mercury (Hg) biomagnification in forest birds, especially in the most species-rich tropical and subtropical forests, limit our understanding of the ecological risks of Hg deposition to forest birds. This study aimed to quantify Hg bioaccumulation and transfer in the food chains of forest birds in a subtropical montane forest using a bird diet recorded by video and stable Hg isotope signals of biological and environmental samples. Results show that inorganic mercury (IHg) does not biomagnify along food chains, whereas methylmercury (MeHg) has trophic magnification factors of 7.4-8.1 for the basal resource-invertebrate-bird food chain. The video observations and MeHg mass balance model suggest that Niltava (Niltava sundara) nestlings ingest 78% of their MeHg from forest floor invertebrates, while Flycatcher (Eumyias thalassinus) nestlings ingest 59% from emergent aquatic invertebrates (which fly onto the canopy) and 40% from canopy invertebrates. The diet of Niltava nestlings contains 40% more MeHg than that of Flycatcher nestlings, resulting in a 60% higher MeHg concentration in their feather. Hg isotopic model shows that atmospheric Hg0 is the main Hg source in the forest bird food chains and contributes >68% in most organisms. However, three categories of canopy invertebrates receive ∼50% Hg from atmospheric Hg2+. Overall, we highlight the ecological risk of MeHg exposure for understory insectivorous birds caused by atmospheric Hg0 deposition and methylation on the forest floor.

Isotopes and otolith chemistry provide insight into the biogeochemical history of mercury in southern flounder across a salinity gradient

Methylmercury (MeHg) continues to pose a significant global health risk to wildlife and humans through fish consumption. Despite numerous advancements in understanding the mercury (Hg) cycle, questions remain about MeHg sources that accumulate in fish, particularly across transitional coastal areas, where harvest is prominent and Hg sources are numerous. Here we used a unique combination of Hg and nutrient isotopes, and otolith chemistry to trace the biogeochemical history of Hg and identify Hg sources that accumulated in an economically important fish species across Mobile Bay, Alabama (USA). Fish tissue Hg in our samples primarily originated from wet deposition within the watershed, and partly reflected legacy industrial Hg. Results also suggest that little Hg was lost through photochemical processes (<10% of fish tissue Hg underwent photochemical processes). Of the small amount that did occur, photodegradation of the organic form, MeHg, was not the dominant process. Biotic transformation processes were estimated to have been a primary driver of Hg fractionation (∼93%), with isotope results indicating methylation as the primary biotic fractionation process prior to Hg entering the foodweb. On a finer scale, individual lifetime estuarine habitat use influenced Hg sources that accumulated in fish and fish Hg concentrations, with runoff from terrestrial Hg sources having a larger influence on fish in freshwater regions of the estuary compared to estuarine regions. Overall, results suggest increases in Hg inputs to the Mobile Bay watershed from wet deposition, turnover of legacy sources, and runoff are likely to translate into increased uptake into the foodweb.

Foraging behavior and sea ice-dependent factors affecting the bioaccumulation of mercury in Antarctic coastal waters

To elucidate the potential risks of the toxic pollutant mercury (Hg) in polar waters, the study of accumulated Hg in fish is compelling for understanding the cycling and fate of Hg on a regional scale in Antarctica. Herein, the Hg isotopic compositions of Antarctic cod Notothenia coriiceps were assessed in skeletal muscle, liver, and heart tissues to distinguish the differences in Hg accumulation in isolated coastal environments of the eastern (Chinese Zhongshan Station, ZSS) and the antipode western Antarctica (Chinese Great Wall Station, GWS), which are separated by over 4000 km. Differences in odd mass-independent isotope fractionation (odd-MIF) and mass-dependent fractionation (MDF) across fish tissues were reflection of the specific accumulation of methylmercury (MeHg) and inorganic Hg (iHg) with different isotopic fingerprints. Internal metabolism including hepatic detoxification and processes related to heart may also contribute to MDF. Regional heterogeneity in iHg end-members further provided evidence that bioaccumulated Hg origins can be largely influenced by polar water circumstances and foraging behavior. Sea ice was hypothesized to play critical roles in both the release of Hg with negative odd-MIF derived from photoreduction of Hg2+ on its surface and the impediment of photochemical transformation of Hg in water layers. Overall, the multitissue isotopic compositions in local fish species and prime drivers of the heterogeneous Hg cycling and bioaccumulation patterns presented here enable a comprehensive understanding of Hg biogeochemical cycling in polar coastal waters.

Increased Contribution of Circumpolar Deep Water Upwelling to Methylmercury in the Upper Ocean around Antarctica: Evidence from Mercury Isotopes in the Ornithogenic Sediments

Upwelling plays a pivotal role in supplying methylmercury (MeHg) to the upper oceans, contributing to the bioaccumulation of MeHg in the marine food web. However, the influence of the upwelling of Circumpolar Deep Water (CDW), the most voluminous water mass in the Southern Ocean, on the MeHg cycle in the surrounding oceans and marine biota of Antarctica remains unclear. Here, we study the mercury (Hg) isotopes in an ornithogenic sedimentary profile strongly influenced by penguin activity on Ross Island, Antarctica. Results indicate that penguin guano is the primary source of Hg in the sediments, and the mass-independent isotope fractionation of Hg (represented by Δ199Hg) can provide insights on the source of marine MeHg accumulated by penguin. The Δ199Hg in the sediments shows a significant decrease at ∼1550 CE, which is primarily attributed to the enhanced upwelling of CDW that brought more MeHg with lower Δ199Hg from the deeper seawater to the upper ocean. We estimate that the contribution of MeHg from the deeper seawater may reach more than 38% in order to explain the decline in Δ199Hg at ∼1550 CE. Moreover, we found that the intensified upwelling may have increased the MeHg exposure for marine organisms, highlighting the importance of CDW upwelling on the MeHg cycle in Antarctic coastal ecosystems.

Reconsidering mercury sources and exposure pathways to bivalves: Insights from mercury stable isotopes

Identifying mercury (Hg) sources and exposure pathways to bivalves, particularly in relation to sediment, is important for expanding the utility of bivalves as a monitoring organism for sediment quality. Here we use Hg isotope ratios to decipher Hg sources accumulated into bivalves by conducting field studies and in situ experiments. In the first part of this study, we characterized Hg isotope ratios in individual geochemical fractions of riverine sediment, contaminated by liquid Hg in South Korea (Hyeongsan River; HS). Asian clams (Corbicula fluminea) were then deployed at the contaminated sites to evaluate the isotopic turnover. Over the two-month period, the isotope ratios of the clams shifted toward the labile/exchangeable Hg pools (F1, F2 fractions) of the sediment. Conversely, in the control site where sediment Hg is low, we observed similar Hg isotope ratios between Asian clams and the samples of precipitation and dissolved phase of water column. In East Fork Poplar Creek, (Oak Ridge) U.S., Asian clams also displayed similar Hg isotope ratios with the dissolved phase of water column, which have undergone substantial in-stream processing or input from Hg-contaminated groundwater from the hyporheic zones and riparian tributary during high hydrologic flow seasons. Our study demonstrates that the dissolved Hg phases within the water column, whether originating via sediment diffusion or derived externally, act as the primary source and exposure pathways to bivalves. The results of our study also shed new light to the prior Hg isotope measurement in bivalves collected from estuarine, lake, and coastal systems, which showed significant isotopic deviation from bulk sediment. The fact that bivalves are sensitive to in situ and external dissolved Hg phases provides additional insight into the existing biomonitoring program, which uses bivalves as a bioindicator for sediment quality.

Mercury Isotope Values in Shoreline Spiders Reveal the Transfer of Aquatic Mercury Sources to Terrestrial Food Webs

The transfer of aquatic contaminants, including mercury (Hg), to terrestrial food webs is an often-overlooked exposure pathway to terrestrial animals. While research has implemented the use of shoreline spiders to assess aquatic to terrestrial Hg transfer, it is unclear whether Hg sources, estimated from isotope ratios, can be successfully resolved to inform site assessments and remedy effectiveness. To examine aquatic to terrestrial Hg transfer, we collected shoreline spiders (Tetragnatha spp.) and aquatic insect larvae (suborder Anisoptera) across a mosaic of aquatic and shoreline habitats in the St. Louis River and Bad River, tributaries to Lake Superior. The fraction of industrial Hg in sediments was reflected in the δ202Hg values of aquatic dragonfly larvae and predatory fish, connecting benthic Hg sources to the aquatic food web. Shoreline spiders mirrored these aquatic Hg source signatures with highly positive correlations in δ202Hg between tetragnathids and dragonfly larvae (r2 = 0.90). Further assessment of different spider taxa (i.e., araneids and pisaurids) revealed that differences in prey consumption and foraging strategies resulted in isotope differences, highlighting the importance of spider taxa selection for Hg monitoring efforts.

The mercury flow through a terrestrial songbird food chain in subtropical pine forest: Elucidated by Bayesian isotope mixing model and stable mercury isotopes

In order to comprehend the transfer of inorganic mercury (IHg) and methylmercury (MeHg) within food chains in terrestrial pine forests, we collected samples of Great Tit nestlings, common invertebrates, plants, and soil in a subtropical pine forest and used Bayesian isotope mixing model analysis, Hg daily intake, and stable Hg isotopes to elucidate the flow of MeHg and IHg in these food chains. Results indicate that caterpillars and cockroaches are the predominant prey items for nestlings, accounting for a combined contribution of 81.5%. Furthermore, caterpillars, cockroaches, and spiders were found to contribute the most (∼80%) of both IHg and MeHg that dietary accumulated in nestlings. The provisoned invertebrates tend to supply more IHg and diluting the proportion of MeHg as total Hg (MeHg%). Notably, nestling feathers displayed the highest Δ199Hg values but a relatively lower MeHg%, suggesting an imbalanced incorporation of Hg from maternal transfer and dietary accumulation during the nestling stage. This study highlights the efficacy of nestlings as indicators for identifying Hg sources and transfers in avian species and food chains. However, caution must be exercised when using Hg isotope compositions in growing feathers, and the contribution of maternally transferred Hg should not be ignored.

Aquatic methylmercury is a significant subsidy for terrestrial songbirds: Evidence from the odd mass-independent fractionation of mercury isotopes

In contrast to aquatic food chains, knowledge of the origins and transfer of mercury (Hg) and methylmercury (MeHg) in terrestrial food chains is relatively limited, especially in songbirds. We collected soil, rice plants, aquatic and terrestrial invertebrates, small wild fish, and resident songbird feathers from an Hg-contaminated rice paddy ecosystem for an analysis of stable Hg isotopes to clarify the sources of Hg and its transfer in songbirds and their prey. Significant mass-dependent fractionation (MDF, δ²⁰²Hg), but no mass-independent fractionation (MIF, ∆¹⁹⁹Hg) occurred in the trophic transfers in terrestrial food chains. Piscivorous, granivorous, and frugivorous songbirds and aquatic invertebrates were all characterized by elevated Δ¹⁹⁹Hg values. The estimated MeHg isotopic compositions obtained using linear fitting and a binary mixing model explained both the terrestrial and aquatic origins of MeHg in the terrestrial food chains. We found that MeHg from aquatic habitats is an important subsidy for terrestrial songbirds, even those that feed mainly on seeds, fruits, or cereals. The results show that MIF of the MeHg isotope is a reliable tool to reveal MeHg sources in songbirds. Because the MeHg isotopic compositions was calculated with a binary mixing model or directly estimated from the high proportions of MeHg, compound-specific isotope analysis of Hg would be more useful for the interpretation of the Hg sources, and is highly recommended for future studies.

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.

Mercury Isotopes in Deep-Sea Epibenthic Biota Suggest Limited Hg Transfer from Photosynthetic to Chemosynthetic Food Webs

Deep oceans receive mercury (Hg) from upper oceans, sediment diagenesis, and submarine volcanism; meanwhile, sinking particles shuttle Hg to marine sediments. Recent studies showed that Hg in the trench fauna mostly originated from monomethylmercury (MMHg) of the upper marine photosynthetic food webs. Yet, Hg sources in the deep-sea chemosynthetic food webs are still uncertain. Here, we report Hg concentrations and stable isotopic compositions of indigenous biota living at hydrothermal fields of the Indian Ocean Ridge and a cold seep of the South China Sea along with hydrothermal sulfide deposits. We find that Hg is highly enriched in hydrothermal sulfides, which correlated with varying Hg concentrations in inhabited biota. Both the hydrothermal and cold seep biota have small fractions (<10%) of Hg as MMHg and slightly positive Δ¹⁹⁹Hg values. These Δ¹⁹⁹Hg values are slightly higher than those in near-field sulfides but are 1 order of magnitude lower than the trench counterparts. We suggest that deep-sea chemosynthetic food webs mainly assimilate Hg from ambient seawater/sediments and hydrothermal fluids formed by percolated seawater through magmatic/mantle rocks. The MMHg transfer from photosynthetic to chemosynthetic food webs is likely limited. The contrasting Hg sources between chemosynthetic and trench food webs highlight Hg isotopes as promising tools to trace the deep-sea Hg biogeochemical cycle.

Trace mercury migration and human exposure in typical mercury-emission areas by compound-specific stable isotope analysis

Anthropogenic mercury (Hg) emissions have increased significantly since the Industrial Revolution, resulting in severe health impacts to humans. The consumptions of fish and rice were primary human methylmercury (MeHg) exposure pathways in Asia. However, the lifecycle from anthropogenic Hg emissions to human MeHg exposure is not fully understood. In this study, a recently developed approach, termed MeHg Compound-Specific Isotope Analysis (CSIA), was employed to track lifecycle of Hg in four typical Hg-emission areas. Distinct Δ¹⁹⁹Hg of MeHg and inorganic Hg (IHg) were observed among rice, fish and hair. The Δ¹⁹⁹Hg of MeHg averaged at 0.07 ± 0.15 ‰, 0.80 ± 0.55 ‰ and 0.43 ± 0.29 ‰ in rice, fish and hair, respectively, while those of IHg averaged at - 0.08 ± 0.24 ‰, 0.85 ± 0.43 ‰ and - 0.28 ± 0.68 ‰. In paddy ecosystem, Δ¹⁹⁹Hg of MeHg in rice showed slightly positive shifts (∼0.2 ‰) from those of IHg, and comparable Δ¹⁹⁹Hg of IHg between rice grain and raw/processed materials (coal, Hg ore, gold ore and sphalerite) were observed. Simultaneously, it was proved that IHg in fish muscle was partially derived from in vivo demethylation of MeHg. By a binary model, we estimated the relative contributions of rice consumption to human MeHg exposure to be 84 ± 14 %, 58 ± 26 %, 52 ± 20 % and 34 ± 15 % on average in Hg mining area, gold mining area, zinc smelting area and coal-fired power plant area, respectively, and positive shifts of δ²⁰²Hg_MeHg from fish/rice to human hair occurred during human metabolic processes. Therefore, the CSIA approach can be an effective tool for tracking Hg biogeochemical cycle and human exposure, from which new scientific knowledge can be generated to support Hg pollution control policies and to protect human health.

Can preserved museum specimens be used to reconstruct fish mercury burden and sources through time?

To evaluate the utility of preserved fishes for reconstructing historical and spatial patterns of mercury (Hg) exposure, we experimentally tested the stability of Hg concentrations and Hg stable isotope ratios under standard museum practices of specimen preservation. We found that loss of unidentified constituents during preservation increased Hg concentrations in fish muscle. Low-Hg fish reared in the laboratory were susceptible to exogenous contamination with inorganic mercury (iHg) when preservative fluids were intentionally spiked or iHg leached passively from contaminated wild fishes in the same container. This contamination impacted Hg isotope values of total Hg, but the conservative nature of methylmercury allows us to quantitatively correct for iHg contamination. Our findings validate the potential to use fishes from the world's museums to generate spatiotemporal baselines for the Minamata Convention on Mercury, but we recommend a set of precautions to maximize inference strength. Selecting the largest specimens of a target species helps dilute any iHg contamination. Specimens should be drawn from lots that were not comingled with fishes from other collections to minimize risk of iHg transfer among fish with different contamination histories. Finally, focusing on low-lipid species will enhance the comparability of Hg concentrations between historical and contemporary collections.

Mercury isotope fractionation during methylmercury transport and transformation: A review focusing on analytical method, fractionation characteristics, and its application

Methylmercury (MeHg), a potent neurotoxin, can be formed, migrated and transformed in environmental compartments, accompanying with unique mass-dependent and mass-independent fractionation of mercury (Hg). These Hg isotope fractionation signals have great potential to probe the transformation and transport of MeHg in aquatic environments. However, the majority of studies to date have focused on total Hg isotopic composition, with less attention to the isotopic fractionation of MeHg due to technical difficulties in analysis, which severely hinders the understanding of MeHg isotopic fractionation and its applications. This review a) evaluates the reported analytical methods for Hg isotopic composition of MeHg, including online and offline measurement techniques; b) summarizes the extent and characteristics of Hg isotopic fractionation during MeHg transport and transformation, focusing on methylation, demethylation, trophic transfer and internal metabolism; and c) briefly discusses several applications of MeHg isotopic fractionation signatures in estimating the extent of photodemethylation, tracing the source of Hg species, and diagnosing reaction mechanisms. Additionally, the existing problems and future directions in MeHg isotope fractionation are highlighted to improve the analytical protocol for Hg isotope fractionation and deepen our understanding of Hg isotope fractionation in the biogeochemical cycling of MeHg.

Combining of C, N and specific Hg stable isotopes to track bioaccumulation of monomethylmercury in coastal and freshwater seafood

Human exposure to monomethylmercury (MMHg) through seafood consumption is a global concern. This study investigates the potential sources and processes of MMHg in seafood of coastal and freshwater areas through combing of δ¹³C, δ¹⁵N, and specific Hg (including MMHg and inorganic Hg (IHg)) isotopes. The results showed that δ¹³C and δ¹⁵N values exhibit different patterns in coastal and freshwater species. Δ¹⁹⁹Hg_MMHg/δ²⁰²Hg_MMHg values suggested that coastal and freshwater seafood undergo similar aqueous MMHg photodegradation processes. The Δ¹⁹⁹Hg_MMHg values could distinguish that, coastal fish absorb MMHg from water column whereas coastal shellfish absorb MMHg mainly from sediment. The positive values of Δ¹⁹⁹Hg_IHg in seafood could reflect in vivo MMHg demethylation and IHg reabsorption. Positive correlation between δ¹⁵N and Δ¹⁹⁹Hg_IHg indicated that aquatic organisms in various trophic levels may have different MMHg demethylation efficiency. We proposed that combining of multiple isotopes can provide overall profiles on aquatic MMHg biogeochemical cycle and bioaccumulation.

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.

Internal Dynamics and Metabolism of Mercury in Biota: A Review of Insights from Mercury Stable Isotopes

Monitoring mercury (Hg) levels in biota is considered an important objective for the effectiveness evaluation of the Minamata Convention. While many studies have characterized Hg levels in organisms at multiple spatiotemporal scales, concentration analyses alone often cannot provide sufficient information on the Hg exposure sources and internal processes occurring within biota. Here, we review the decadal scientific progress of using Hg isotopes to understand internal processes that modify the speciation, transport, and fate of Hg within biota. Mercury stable isotopes have emerged as a powerful tool for assessing Hg sources and biogeochemical processes in natural environments. A better understanding of the tissue location and internal mechanisms leading to Hg isotope change is key to assessing its use for biomonitoring. We synthesize the current understanding and uncertainties of internal processes leading to Hg isotope fractionation in a variety of biota, in a sequence of better to less studied organisms (i.e., birds, marine mammals, humans, fish, plankton, and invertebrates). This review discusses the opportunities and challenges of using certain forms of biota for Hg source monitoring and the need to further elucidate the physiological mechanisms that control the accumulation, distribution, and toxicity of Hg in biota by coupling new techniques with Hg stable isotopes.

The Hg behaviors in mangrove ecosystems revealed by Hg stable isotopes: a case study of Maowei mangrove

As one of the most productive marine ecosystems in the tropics and subtropics, mangroves are an important part of the global mercury (Hg) cycling. The environmental processes and effects of Hg in mangroves are complex and affect human Hg exposure, and it is crucial to understand Hg behaviors in the mangrove ecosystem. However, clarifying Hg behaviors in the mangrove ecosystem remains difficult because of an insufficient understanding of the dominant pathways. In this study, measurements of mercury (Hg) concentration and isotope ratios in sediment and plant tissues from a mangrove wetland were used to investigate Hg isotope fractionation in mangrove plants and sediments. Spatial patterns in Hg concentration and isotope signatures indicate that Hg re-emission in the sediment was suppressed by mangrove plants. The ratio of Δ¹⁹⁹Hg/Δ²⁰¹Hg was 0.93 for all sediments, indicating that Hg mass-independent fractionation in the mangrove ecosystem was primarily affected by photoreduction, while the ratios of Δ¹⁹⁹Hg/Δ²⁰¹Hg and Δ¹⁹⁹Hg/δ²⁰²Hg for plant tissues suggested that natural organic matter reduction of Hg(II) was occurred in the plants. The distinct positive Δ¹⁹⁹Hg values found in mangrove plants were supposed to be the results of the unique physiological characteristics of mangroves. The exterior Hg sources from atmosphere and seawater emphasize the role of mangrove ecosystems in the global Hg biogeochemistry. Our study highlights the distinct Hg isotope signatures in the mangrove from that in forests and indicates unique Hg behaviors in the mangrove ecosystem.

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

Nearshore systems play an important role as mercury (Hg) sources to the open ocean and to human health via fish consumption. The nearshore system along East Asia is of particular concern given the rapid industrialization, which contributes to significant anthropogenic Hg emissions and releases. We used Hg stable isotopes to characterize Hg sources in the sediment and fish along the west coast of Korea, located at the northeast of the East China Sea. The Hg isotope ratios of the west coast sediments (δ²⁰²Hg; -0.89 to -0.27‰, Δ¹⁹⁹Hg; -0.04 to 0.14‰) were statistically similar with other nearshore sediments (δ²⁰²Hg; -0.99 to -0.30‰, Δ¹⁹⁹Hg; -0.04 to 0.19‰) and overlapped with the industrial Hg source end-member (δ²⁰²Hg; -0.5‰, Δ¹⁹⁹Hg; 0.01‰) estimated from the Chinese marginal seas. Using a ternary mixing model, we estimated that industrial Hg sources contribute 83-97% in the west coast of Korea, and riverine and atmospheric Hg sources play minor roles in the Korean west coast and the Chinese marginal seas. The comparison between Hg isotope ratios of the sediment and nearshore fish revealed that the fish in the most west coast sites are exposed to MeHg produced in the sediment. At a few southwest coast sites, external MeHg produced in rivers and the open ocean water column appears to be more important as a source in fish. This is supported by much higher δ²⁰²Hg (0.74‰; similar to oceanic fish) and lower δ²⁰²Hg (-0.71‰; similar to riverine sources) compared to fish collected from other west coast sites influenced by sedimentary MeHg. The substantial Hg contributions from industrial activities suggest the national policies regulating anthropogenic Hg releases can directly mitigate human Hg exposure originating via local fish consumption. This study contributes to the growing regional and global inventories of Hg fluxes and sources exported into coastal oceans.

Using carbon, nitrogen, and mercury isotope values to distinguish mercury sources to Alaskan lake trout

Lake trout (Salvelinus namaycush), collected from 13 remote lakes located in southwestern Alaska, were analyzed for carbon, nitrogen, and mercury (Hg) stable isotope values to assess the importance of migrating oceanic salmon, volcanic activity, and atmospheric deposition to fish Hg burden. Methylmercury (MeHg) bioaccumulation in phytoplankton (5.0 - 6.9 kg L^-1) was also measured to quantify the basal uptake of MeHg to these aquatic food webs. Hg isotope values in lake trout revealed that while the extent of precipitation-delivered Hg was similar across the entire study area, volcanic Hg is likely an important additional source to lake trout in proximate lakes. In contrast, migratory salmon (Oncorhynchus nerka) deliver little MeHg to lake trout directly, although indirect delivery processes via decay could exist. A high level of variability in carbon, nitrogen, and Hg isotope values indicate niche partitioning in lake trout populations within each lake and that a complex suite of ecological interactions is occurring, complicating the conceptually linear assessment of contaminant source to receiving organism. Without connecting energy and contaminant isotope axes, we would not have understood why lake trout from these pristine lakes have highly variable Hg burdens despite consistently low water Hg and comparable age-length dynamics.

Foraging plasticity diversifies mercury exposure sources and bioaccumulation patterns in the world's largest predatory fish

Large marine predators exhibit high concentrations of mercury (Hg) as neurotoxic methylmercury, and the potential impacts of global change on Hg contamination in these species remain highly debated. Current contaminant model predictions do not account for intraspecific variability in Hg exposure and may fail to reflect the diversity of future Hg levels among conspecific populations or individuals, especially for top predators displaying a wide range of ecological traits. Here, we used Hg isotopic compositions to show that Hg exposure sources varied significantly between and within three populations of white sharks (Carcharodon carcharias) with contrasting ecology: the north-eastern Pacific, eastern Australasian, and south-western Australasian populations. Through Δ²⁰⁰Hg signatures in shark tissues, we found that atmospheric Hg deposition pathways to the marine environment differed between coastal and offshore habitats. Discrepancies in δ²⁰²Hg and Δ¹⁹⁹Hg signatures among white sharks provided evidence for intraspecific exposure to distinct sources of marine methylmercury, attributed to population and ontogenetic shifts in foraging habitat and prey composition. We finally observed a strong divergence in Hg accumulation rates between populations, leading to three times higher Hg concentrations in large Australasian sharks compared to north-eastern Pacific sharks, and likely due to different trophic strategies adopted by adult sharks across populations. This study illustrates the variety of Hg exposure sources and bioaccumulation patterns that can be found within a single species and suggests that intraspecific variability needs to be considered when assessing future trajectories of Hg levels in marine predators.

ENSO Climate Forcing of the Marine Mercury Cycle in the Peruvian Upwelling Zone Does Not Affect Methylmercury Levels of Marine Avian Top Predators

Climate change is expected to affect marine mercury (Hg) biogeochemistry and biomagnification. Recent modeling work suggested that ocean warming increases methylmercury (MeHg) levels in fish. Here, we studied the influence of El Niño Southern Oscillations (ENSO) on Hg concentrations and stable isotopes in time series of seabird blood from the Peruvian upwelling and oxygen minimum zone. Between 2009 and 2016, La Niña (2011) and El Niño conditions (2015-2016) were accompanied by sea surface temperature anomalies up to 3 °C, oxycline depth change (20-100 m), and strong primary production gradients. Seabird Hg levels were stable and did not co-vary significantly with oceanographic parameters, nor with anchovy biomass, the primary dietary source to seabirds (90%). In contrast, seabird Δ¹⁹⁹Hg, proxy for marine photochemical MeHg breakdown, and δ¹⁵N showed strong interannual variability (up to 0.8 and 3‰, respectively) and sharply decreased during El Niño. We suggest that lower Δ¹⁹⁹Hg during El Niño represents reduced MeHg photodegradation due to the deepening of the oxycline. This process was balanced by equally reduced Hg methylation due to reduced productivity, carbon export, and remineralization. The non-dependence of seabird MeHg levels on strong ENSO variability suggests that marine predator MeHg levels may not be as sensitive to climate change as is currently thought.

Hg concentrations and stable isotope variations in tropical fish species of a gold-mining-impacted watershed in French Guiana

The aim of the study was to determine if gold-mining activities could impact the mercury (Hg) concentrations and isotopic signatures in freshwater fish consumed by riparian people in French Guiana. Total Hg, MeHg concentrations, and Hg stable isotopes ratios were analyzed in fish muscles from different species belonging to three feeding patterns (herbivorous, periphytophagous, and piscivorous). We compared tributaries impacted by gold-mining activities (Camopi, CR) with a pristine area upstream (Trois-Sauts, TS), along the Oyapock River. We measured δ¹⁵N and δ ¹³C to examine whether Hg patterns are due to differences in trophic level. Differences in δ ¹⁵N and δ ¹³C values between both studied sites were only observed for periphytophagous fish, due to difference of CN baselines, with enriched values at TS. Total Hg concentrations and Hg stable isotope signatures showed that Hg accumulated in fish from both areas has undergone different biogeochemical processes. Δ¹⁹⁹Hg variation in fish (-0.5 to 0.2‰) was higher than the ecosystem baseline defined by a Δ¹⁹⁹Hg of -0.66‰ in sediments, and suggested limited aqueous photochemical MeHg degradation. Photochemistry-corrected δ²⁰²Hg in fish was 0.7‰ higher than the baseline, consistent with biophysical and chemical isotope fractionation in the aquatic environment. While THg concentrations in periphytophagous fish were higher in the gold-mining area, disturbed by inputs of suspended particles, than in TS, the ensemble of Hg isotope shifts in fish is affected by the difference of biotic (methylation/demethylation) and abiotic (photochemistry) processes between both areas and did therefore not allow to resolve the contribution of gold-mining-related liquid Hg(0) in fish tissues. Mercury isotopes of MeHg in fish and lower trophic level organisms can be complementary to light stable isotope tracers.

Contamination levels and habitat use influence Hg accumulation and stable isotope ratios in the European seabass Dicentrarchus labrax

Hg accumulation in marine organisms depends strongly on in situ water or sediment biogeochemistry and levels of Hg pollution. To predict the rates of Hg exposure in human communities, it is important to understand Hg assimilation and processing within commercially harvested marine fish, like the European seabass Dicentrarchus labrax. Previously, values of Δ¹⁹⁹Hg and δ²⁰²Hg in muscle tissue successfully discriminated between seven populations of European seabass. In the present study, a multi-tissue approach was developed to assess the underlying processes behind such discrimination. We determined total Hg content (THg), the proportion of monomethyl-Hg (%MeHg), and Hg isotopic composition (e.g. Δ¹⁹⁹Hg and δ²⁰²Hg) in seabass liver. We compared this to the previously published data on muscle tissue and local anthropogenic Hg inputs. The first important finding of this study showed an increase of both %MeHg and δ²⁰²Hg values in muscle compared to liver in all populations, suggesting the occurrence of internal MeHg demethylation in seabass. This is the first evidence of such a process occurring in this species. Values for mass-dependent (MDF, δ²⁰²Hg) and mass-independent (MIF, Δ¹⁹⁹Hg) isotopic fractionation in liver and muscle accorded with data observed in estuarine fish (MDF, 0-1‰ and MIF, 0-0.7‰). Black Sea seabass stood out from other regions, presenting higher MIF values (≈1.5‰) in muscle and very low MDF (≈-1‰) in liver. This second finding suggests that under low Hg bioaccumulation, Hg isotopic composition may allow the detection of a shift in the habitat use of juvenile fish, such as for first-year Black Sea seabass. Our study supports the multi-tissue approach as a valid tool for refining the analysis of Hg sourcing and metabolism in a marine fish. The study's major outcome indicates that Hg levels of pollution and fish foraging location are the main factors influencing Hg species accumulation and isotopic fractionation in the organisms.

The Twilight Zone as a Major Foraging Habitat and Mercury Source for the Great White Shark

The twilight zone contains the largest biomass of the world's ocean. Identifying its role in the trophic supply and contaminant exposure of marine megafauna constitutes a critical challenge in the context of global change. The white shark (Carcharodon carcharias) is a threatened species with some of the highest concentrations of neurotoxin methylmercury (MeHg) among marine top predators. Large white sharks migrate seasonally from coastal habitats, where they primarily forage on pinnipeds, to oceanic offshore habitats. Tagging studies suggest that while offshore, white sharks may forage at depth on mesopelagic species, yet no biochemical evidence exists. Here, we used mercury isotopic composition to assess the dietary origin of MeHg contamination in white sharks from the Northeast Pacific Ocean. We estimated that a minimum of 72% of the MeHg accumulated by white sharks originates from the consumption of mesopelagic prey, while a maximum of 25% derives from pinnipeds. In addition to highlighting the potential of mercury isotopes to decipher the complex ecological cycle of marine predators, our study provides evidence that the twilight zone constitutes a crucial foraging habitat for these large predators, which had been suspected for over a decade. Climate change is predicted to expand the production of mesopelagic MeHg and modify the mesopelagic biomass globally. Considering the pivotal role of the twilight zone is therefore essential to better predict both MeHg exposure and trophic supply to white sharks, and effectively protect these key vulnerable predators.

Internal dynamics of inorganic and methylmercury in a marine fish: Insights from mercury stable isotopes

Mercury isotope ratios in fish tissues have been used to infer sources and biogeochemical processes of mercury in aquatic ecosystems. More experimental studies are however needed to understand the internal dynamics of mercury isotopes and to further assess the feasibility of using fish mercury isotope ratios as a monitoring tool. We exposed Olive flounder (Paralichthys olivaceus) to food pellets spiked with varying concentrations (400, 1600 ng/g) of methylmercury (MeHg) and inorganic mercury (IHg) for 10 weeks. Total mercury (THg), MeHg concentrations, and mercury isotope ratios (δ²⁰²Hg, Δ¹⁹⁹Hg, Δ²⁰⁰Hg) were measured in the muscle, liver, kidney, and intestine of fish. Fish fed mercury unamended food pellets and MeHg amended food pellets showed absence of internal δ²⁰²Hg and Δ¹⁹⁹Hg fractionation in all tissue type. For fish fed IHg food pellets, the δ²⁰²Hg and Δ¹⁹⁹Hg values of intestine equilibrated to those of the IHg food pellets. Kidney, muscle, and liver exhibited varying degrees of isotopic mixing toward the IHg food pellets, consistent with the degree of IHg bioaccumulation. Liver showed additional positive δ²⁰²Hg shifts (∼0.63‰) from the binary mixing line between the unamended food pellets and IHg food pellets, which we attribute to redistribution or biliary excretion of liver IHg with a lower δ²⁰²Hg to other tissues. Significant δ²⁰²Hg fractionation in the liver and incomplete isotopic equilibration in the muscle indicate that these tissues may not be suitable for source monitoring at sites heavily polluted by IHg. Instead, fish intestine appears to be a more suitable proxy for identifying IHg sources. The results from our study are essential for determining the appropriate fish tissues for monitoring environmental sources of IHg and MeHg.

A "seabird-eye" on mercury stable isotopes and cycling in the Southern Ocean

Since mercury (Hg) biogeochemistry in the Southern Ocean is minimally documented, we investigated Hg stable isotopes in the blood of seabirds breeding at different latitudes in the Antarctic, Subantarctic and Subtropical zones. Hg isotopic composition was determined in adult penguins (5 species) and skua chicks (2 species) from Adélie Land (66°39'S, Antarctic) to Crozet (46°25'S, Subantarctic) and Amsterdam Island (37°47'S, Subtropical). Mass-dependent (MDF, δ²⁰²Hg) and mass-independent (MIF, Δ¹⁹⁹Hg) Hg isotopic values separated populations geographically. Antarctic seabirds exhibited lower δ²⁰²Hg values (-0.02 to 0.79 ‰, min-max) than Subantarctic (0.88 to 2.12 ‰) and Subtropical (1.44 to 2.37 ‰) seabirds. In contrast, Δ¹⁹⁹Hg values varied slightly from Antarctic (1.31 to 1.73 ‰) to Subtropical (1.69 to 2.04 ‰) waters. The extent of methylmercury (MeHg) photodemethylation extrapolated from Δ¹⁹⁹Hg values was not significantly different between locations, implying that most of the bioaccumulated MeHg was of mesopelagic origin. The larger increase of MDF between the three latitudes co-varies with MeHg concentrations. This supports an increasing effect of specific biogenic Hg pathways from Antarctic to Subtropical waters, such as Hg biological transformations and accumulations. This "biogenic effect" among different productive southern oceanic regions can also be related to different mixed layer depth dynamics and biological productivity turnover that specifically influence the vertical transport between the mesopelagic and the photic zones. This study shows the first Hg isotopic data of the Southern Ocean at large scale and reveals how regional Southern Ocean dynamics and productivity control marine MeHg biogeochemistry and the exposure of seabirds to Hg contamination.

Contrasting Spatial and Seasonal Trends of Methylmercury Exposure Pathways of Arctic Seabirds: Combination of Large-Scale Tracking and Stable Isotopic Approaches

Despite the limited direct anthropogenic mercury (Hg) inputs in the circumpolar Arctic, elevated concentrations of methylmercury (MeHg) are accumulated in Arctic marine biota. However, the MeHg production and bioaccumulation pathways in these ecosystems have not been completely unraveled. We measured Hg concentrations and stable isotope ratios of Hg, carbon, and nitrogen in the feathers and blood of geolocator-tracked little auk Alle alle from five Arctic breeding colonies. The wide-range spatial mobility and tissue-specific Hg integration times of this planktivorous seabird allowed the exploration of their spatial (wintering quarters/breeding grounds) and seasonal (nonbreeding/breeding periods) MeHg exposures. An east-to-west increase of head feather Hg concentrations (1.74-3.48 μg·g^-1) was accompanied by significant spatial trends of Hg isotope (particularly Δ¹⁹⁹Hg: 0.96-1.13‰) and carbon isotope (δ¹³C: -20.6 to -19.4‰) ratios. These trends suggest a distinct mixing/proportion of MeHg sources between western North Atlantic and eastern Arctic regions. Higher Δ¹⁹⁹Hg values (+0.4‰) in northern colonies indicate an accumulation of more photochemically impacted MeHg, supporting shallow MeHg production and bioaccumulation in high Arctic waters. The combination of seabird tissue isotopic analysis and spatial tracking helps in tracing the MeHg sources at various spatio-temporal scales.

Mercury isotope signatures in sediments and marine organisms as tracers of historical industrial pollution

Isotopic composition of mercury (Hg) in marine organisms and sediment cores was used to identify sources and reconstruct historical trends of contamination in the coastal-marine area of Rosignano Solvay (Italy), affected by Hg pollution from a chlor-alkali plant on the near land. Sediments show a wide range of Hg concentration and Hg isotope signatures. Particularly, coupled Hg concentration and δ²⁰²Hg values trace inputs from different sources. The two depth-profiles clearly indicate three distinct periods: "pre-industrial" (before 1941), "industrial" (between 1941 and 2007) and "post-industrial" (after 2007) ages. This is also corroborated by sediment chronology, using ²¹⁰Pb dating method, validated through ¹³⁷Cs. Marine organisms are characterized by Hg isotope signatures comparable to "post-industrial" surface sediments. Notably, specimens of Mullus spp. evidence isotope composition comparable to the "industrial" sediments, thus suggesting a still active role of those sediments as source of Hg for the benthic fish compartment. The small amount of MIF and the Δ¹⁹⁹Hg/Δ²⁰¹Hg ratio recorded in organisms are reasonably consistent with limited processes of MMHg demethylation in the water column.

Mercury isotopes as tracers of ecology and metabolism in two sympatric shark species

In coastal ecosystems, top predators are exposed to a wide variety of nutrient and contaminant sources due to the diversity of trophic webs within inshore marine habitats. Mercury contamination could represent an additional threat to shark populations that are declining worldwide. Here we measured total mercury, carbon and nitrogen isotopes, as well as mercury isotopes, in two co-occurring shark species (the bull shark Carcharhinus leucas and the tiger shark Galeocerdo cuvier) and their potential prey from a coastal ecosystem of the western Indian Ocean (La Réunion Island). Our primary goals were to (i) determine the main trophic Hg sources for sharks and (ii) better characterize their diet composition and foraging habitat. Hg isotope signatures (Δ¹⁹⁹Hg and δ²⁰²Hg) of shark prey suggested that bull sharks were exposed to methylmercury (MeHg) produced in offshore epipelagic waters, while tiger sharks were exposed to offshore mesopelagic MeHg with additional microbial transformation in slope sediments. Δ¹⁹⁹Hg values efficiently traced the ecology of the two predators, demonstrating that bull sharks targeted coastal prey in shallow waters while tiger sharks were mainly foraging on mesopelagic species in the deeper waters of the island slope. Unexpectedly, we found a positive shift in δ²⁰²Hg (>1‰) between sharks and their prey, leading to high δ²⁰²Hg values in the two shark species (e.g. 1.91 ± 0.52‰ in bull sharks). This large shift in δ²⁰²Hg indicates that sharks may display strong MeHg demethylation abilities, possibly reflecting evolutionary pathways for mitigating their MeHg contamination.

Kinetics and metabolism of mercury in rats fed with mercury contaminated rice using mass balance and mercury isotope approach

Consumption of mercury (Hg) contaminated rice can be a major environmental health issue but the toxicokinetics is not well known. Hg isotopes have been shown to be good tracers in studying Hg exposure and metabolic processes. We established a Hg mass balance and Hg isotope model in rats fed with Hg contaminated rice (THg 51.3 ng/g; MeHg 25 ng/g) for 90 days to investigate Hg toxicokinetics. Overall 80% of feeding THg was recovered in rat body and excrement, while the excrement accounted for 55% of total observed THg in rats. Feces were the main route of Hg elimination in rats, while urinary excretion was negligible. However, only 32% of utilized MeHg was recovered in rats, indicating significant demethylation of MeHg in rat body. Positive net fractionations of δ²⁰²Hg (relative to the feeding rice) were observed in hair and blood samples (1.21‰ and 1.25‰, respectively), which have similar trend with the results obtained in human hair study, exhibiting higher δ²⁰²Hg values (2‰- 3‰) than consumed fish and rice. Most importantly, we observed negative net fractionations in feces (-0.44‰), which confirmed the missed Hg with negative δ²⁰²Hg signal. We concluded that mass balance and Hg isotope are useful tools for quantifying toxicokinetics of Hg. Demethylation of MeHg in the intestine were the important detoxification process in rat body characterizing with negative net Hg fractionations in feces.

Methylmercury produced in upper oceans accumulates in deep Mariana Trench fauna

Monomethylmercury (MMHg) is a potent toxin that bioaccumulates and magnifies in marine food webs. Recent studies show abundant methylated Hg in deep oceans (>1000 m), yet its origin remains uncertain. Here we measured Hg isotope compositions in fauna and surface sediments from the Mariana Trench. The trench fauna at 7000-11000 m depth all have substantially positive mass-independent fractionation of odd Hg isotopes (odd-MIF), which can be generated only in the photic zone via MMHg photo-degradation. Given the identical odd-MIF in trench fauna and North Pacific upper ocean (<1000 m) biota MMHg, we suggest that the accumulated Hg in trench fauna originates exclusively from MMHg produced in upper oceans, which penetrates to depth by sorption to sinking particles. Our findings reveal little in-situ MMHg production in deep oceans and imply that anthropogenic Hg released at the Earth's surface is much more pervasive across deep oceans than was previously thought.

Review of stable mercury isotopes in ecology and biogeochemistry

Due to the advent of cold vapor-multicollector-inductively coupled plasma mass spectrometry (CV-MC-ICP-MS) in the past two decades, many research groups studying mercury (Hg) biogeochemistry have integrated stable Hg isotopes into their research. Currently, >200 studies using this technique have been published and this has greatly enhanced our understanding of the Hg biogeochemical cycle beyond what Hg concentration and speciation analyses alone can provide. These studies are largely divided into two groups: (i) controlled experiments investigating fractionation of Hg isotopes and refining tools of isotopic analyses, and (ii) studies of natural variations of Hg isotopes. It is now known that Hg isotopes undergo both mass dependent fractionation (MDF; reported as the ratio of mass ²⁰²Hg to ¹⁹⁸Hg) and mass independent fractionation (MIF), with MIF occurring at odd masses (¹⁹⁹Hg, ²⁰¹Hg) to a larger magnitude and at even masses (²⁰⁰Hg, ²⁰⁴Hg) to a much smaller magnitude. The two types of MIF are controlled by different photochemical processes. The range of isotopic variations of MDF, odd-MIF, and even-MIF are now well documented in a diverse set of environmental samples, and researchers are continuing to explore how the field of Hg isotope biogeochemistry can be further developed and taken to the next level of understanding. One application that has received considerable attention is the use of Hg isotopes to examine the environmental controls on the production and degradation of methylmercury (MeHg), the most toxic and bioaccumulative form of Hg. Since MeHg is efficiently assimilated and biomagnified along food chains, MeHg has the potential to be a robust ecological tracer. In this review, we give an updated overview of the field of Hg isotopes and focus on how Hg isotopes of MeHg can be used to address fundamental ecological questions, including energy transfer across ecosystem interfaces and as a tracer for animal movements.

Selenium and stable mercury isotopes provide new insights into mercury toxicokinetics in pilot whales

High exposures of mammalian species to inorganic mercury (Hg^II) and methylmercury (MeHg) have been associated with adverse effects on behavior and reproduction. Different mammalian species exhibit varying responses to similar external exposure levels, reflecting potential differences in Hg toxicokinetics. Here, we use Hg stable isotopes, total Hg, MeHg and selenium (Se) concentrations measured in multiple tissues of North Atlantic pilot whales (Globicephala melas) to investigate processes affecting the distribution and accumulation of Hg^II and MeHg. We find that simple mixing of two distinct isotopic end-members: MeHg (1.4‰) and Hg^II (-1.6‰) can explain the observed variability of δ²⁰²Hg in brain tissue. A similar isotopic composition for the MeHg end-member in the brain, muscle, heart, and kidney suggests efficient exchange of MeHg in blood throughout the body. By contrast, the Hg isotopic composition of the liver of adult whales is different from younger whales and other tissues that follow the two-end member mixing model. Measured Se:Hg ratios are lowest in adult whales with the highest levels of MeHg exposure. In these individuals, Se availability is likely reduced by complexation with demethylated Hg^II. We speculate that this results in a higher fraction of labile Hg^II eliminated from the liver of adult whales compared to young whales and subsequent redistribution to other tissues, potentially affecting toxicity.

Mercury isotope variations within the marine food web of Chinese Bohai Sea: Implications for mercury sources and biogeochemical cycling

Mercury (Hg) speciation and isotopic compositions in a large-scale food web and seawater from Chinese Bohai Sea were analyzed to investigate methylmercury (MeHg) sources and Hg cycling. The biota showed ∼5‰ variation in mass dependent fractionation (MDF, -4.57 to 0.53‰ in δ²⁰²Hg) and mostly positive odd-isotope mass independent fractionation (odd-MIF, -0.01 to 1.21‰ in Δ¹⁹⁹Hg). Both MDF and odd-MIF in coastal biota showed significant correlations with their trophic levels and MeHg fractions, likely reflecting a preferential trophic transfer of MeHg with higher δ²⁰²Hg and Δ¹⁹⁹Hg than inorganic Hg. The MDF and odd-MIF of biota were largely affected by their feeding habits and living territories, and MeHg in pelagic food web was more photodegraded than in coastal food web (21-31% vs. 9-11%). From the Hg isotope signatures of pelagic biota and extrapolated coastal MeHg, we suggest that MeHg in the food webs was likely derived from sediments. Interestingly, we observed complementary even-MIF (mainly negative Δ²⁰⁰Hg of -0.36 to 0.08‰ and positive Δ²⁰⁴Hg of -0.05 to 0.82‰) in the biota and a significant linear slope of -0.5 for Δ²⁰⁰Hg/Δ²⁰⁴Hg. This leads us to speculate that atmospheric Hg⁰ is an important source to bioaccumulated MeHg, although the exact source-receptor relationships need further investigation.

Assessment of Hg contamination by a Chlor-Alkali Plant in riverine and coastal sites combining Hg speciation and isotopic signature (Sagua la Grande River, Cuba)

Chlor-alkali plants (CAP) are recognized as major sources of mercury (Hg) in the environment. In this work, Hg concentration, speciation and isotopic signature were determined in sediments and biota (fish and oyster) from Sagua La Grande River (SG River) and the adjacent coastal zone in the vicinity of a CAP (Cuba). High Hg concentrations in surface sediments (up to 5072 ng g^-1), mainly occurring as inorganic Hg, decrease with the distance from the CAP along the SG River and seaward. Meanwhile, Hg concentration and speciation in riverine catfish (Claria gariepinus) muscle (1093 ± 319 ng g^-1, ˜70% as MeHg) and coastal oysters (Crassostrea rizophorae) (596 ± 233 ng g^-1, ˜50% as MeHg) indicate a direct impact from CAP. Hg isotopic signature in sediments, following both mass dependent (MDF) and mass independent fractionation (MIF), exhibits a clear binary mixing between CAP pollution (+0.42‰, δ²⁰²Hg; -0.18‰, Δ²⁰¹Hg) and regional background end-member (˜ -0.49‰, δ²⁰²Hg; +0.01‰, Δ²⁰¹Hg). The combination of speciation and isotopic information in biota and sediments allows to trace Hg contamination pathways from contaminated sediments to the biota, establishing the importance of both methylation and demethylation extent in both river and coastal sites before Hg species bioaccumulation.

Mercury bioaccumulation in tilefish from the northeastern Gulf of Mexico 2 years after the Deepwater Horizon oil spill: Insights from Hg, C, N and S stable isotopes

Mercury (Hg) concentration in fish of the Gulf of the Mexico (GoM) is a major concern due to the importance of the GoM for U.S. fisheries. The Deepwater Horizon (DWH) oil spill in April 2010 in the northern GoM resulted in large amounts of oil and dispersant released to the water column, which potentially modified Hg bioaccumulation patterns in affected areas. We measured Hg species (methylmercury (MMHg) and inorganic Hg (IHg)) concentrations, and light (C, N and S) and Hg stable isotopes in muscle and liver tissues from tilefish (Lopholatilus chamaleonticeps) sampled in 2012 and 2013 along the shelf break of the northeastern GoM. Fish located close to the mouth of the Mississippi River (MR) and northwest of the DWH well-head (47 km) showed significantly lower Hg levels in muscle and liver than fish located further northeast of the DWH (>109 km), where 98% of tilefish had Hg levels in the muscle above US consumption advisory thresholds (50% for tilefish close to the DWH). Differences in light and Hg stable isotopes signatures were observed between these two areas, showing higher δ¹⁵N, and lower δ²⁰²Hg, Δ¹⁹⁹Hg and δ³⁴S in fish close to the DWH/MR. This suggests that suspended particles from the MR reduces Hg bioavailability at the base of the GoM food chains. This phenomenon can be locally enhanced by the DWH that resulted in increased particles in the water column as evidenced by the marine snow layer in the sediments. On the other hand, freshly deposited Hg associated with organic matter in more oligotrophic marine waters enhanced Hg bioaccumulation in local food webs. Comparing Hg isotopic composition in liver and muscle of fish indicates specific metabolic response in fish having accumulated high levels of MMHg.

The river runs through it: The Athabasca River delivers mercury to aquatic birds breeding far downstream

This study examined factors contributing to temporal variability (2009–2017) in total mercury (THg) concentrations in aquatic bird eggs collected in the Peace-Athabasca Delta and Lake Athabasca in northern Alberta. Factors examined included year of egg collection, site of collection, bird species, bird diets, annual surface-mineable oil sands production, forest fires, and flow of the Athabasca River. Surface mining activities associated with Alberta’s Athabasca oil sands are situated north of Fort McMurray, Alberta, adjacent to the northward-flowing Athabasca River. Previous studies have found that oil sands industrial operations release mercury into the local (within ~50 km) environment. An information-theoretic approach revealed that the best model for explaining egg THg levels included Athabasca River flow, bird food source, and bird species. Variability in egg THg levels was partly a reflection of differences in food sources, e.g. proportions of aquatic versus terrestrial food in bird diets. Annual fluctuations in maximal flow of the Athabasca River were also important with eggs collected following years of high maximal flow exhibiting higher THg concentrations. Furthermore, eggs collected in years of high versus low flow differed in their stable Hg isotope composition with less mass-independent fraction of ¹⁹⁹Hg and ²⁰¹Hg in years of high flow. Riverine processes associated with suspended sediment were likely critical in regulating Hg availability to nesting birds. This study highlights the importance of the Athabasca River as a conduit for Hg transport to ecologically-sensitive downstream ecosystems such as the Peace-Athabasca Delta and Wood Buffalo National Park (a UNESCO World Heritage Site). Human activities that increase atmospheric Hg deposition to the Athabasca River watershed, or that enhance Hg releases to the river through erosion of Hg-bearing soils, will likely increase the availability of Hg to organisms inhabiting downstream areas.

Tracing Mercury Pollution along the Norwegian Coast via Elemental, Speciation, and Isotopic Analysis of Liver and Muscle Tissue of Deep-Water Marine Fish ( Brosme brosme)

Liver and muscle tissue of tusks ( Brosme brosme) have been analyzed for their THg and MeHg concentrations and Hg isotopic signatures for tracing Hg pollution along the Norwegian coast. Clear differences between tissue types and locations were established. At five of the eight locations, the Hg concentration in muscle exceeded the maximum allowable level of 0.5 mg kg^-1 wet weight. δ²⁰²Hg values in both tissue types indicated that Hg speciation affects the bulk Hg isotopic signature. Tusk liver seems to be more sensitive to immediate changes and to anthropogenic inorganic Hg, while the muscle rather reflects the Hg accumulated over a longer period of exposure. The δ²⁰²Hg values of liver and muscle also enabled different sources of Hg and exposure pathways to be distinguished. δ²⁰²Hg_muscle-δ²⁰²Hg_liver showed a clear correlation with the % MeHg in tusk liver for the coastal waters, but not for the fjords. The absence of significant differences in Δ¹⁹⁹Hg values between both tissues of tusk from the same location suggests that in vivo metabolic processes are the underlying reason for the differences in Hg speciation and in δ²⁰²Hg values. This work highlights the importance of selecting different tissues of marine fish in future Hg monitoring programs.

Historical records, distributions and sources of mercury and zinc in sediments of East China sea: Implication from stable isotopic compositions

The distribution and source of mercury (Hg) and zinc (Zn) in surface sediments from East China Sea (ECS) were studied using stable isotopes. Hg concentrations in surface sediments varied from 12.6 to 133.2 ng g^-1. Zn concentrations ranged between 45.9 and 133.6 ng g^-1. Hydrodynamic circulation and sedimentary process within the ECS were the dominant factors controlling the distribution and fate of heavy metals, referring from relationships of Hg and Zn concentrations and total organic carbon (TOC) and clay contents. Spatial variations in Hg isotopic compositions (δ²⁰²Hg) were observed in the ECS, with δ²⁰²Hg ranging from -1.86 to -0.83‰ and Δ¹⁹⁹Hg ranging from -0.07 to +0.26‰. The high δ²⁰²Hg values and relatively negative Δ¹⁹⁹Hg values in estuarine and inner shelf sites indicated that the riverine inputs from Yangtze River played an important role in delivering Hg to ECS. In contrast, Hg isotopic signatures in outer shelf sediments had relatively low δ²⁰²Hg and positive Δ¹⁹⁹Hg, implying that atmospheric Hg deposition could be relatively dominant source for Hg. Moreover, application of zinc isotopes to trace anthropogenic sources has been tested, with δ⁶⁶Zn ranging from 0.20 to 0.67‰. The results showed significant variations of δ⁶⁶Zn in the ECS, implying that Zn isotope compositions can be tracers of anthropogenic sources. Using these Hg isotope compositions, the source attributions of sediments in ECS were estimated based on a mixing model, suggesting that river-dominated inputs, discharging pollutants from industrial sources, and atmospheric deposition could dominate in the occurrence and fate of Hg in sediments of ECS.

Identification of sources and bioaccumulation pathways of MeHg in subantarctic penguins: a stable isotopic investigation

Seabirds are widely used as bioindicators of mercury (Hg) contamination in marine ecosystems and the investigation of their foraging strategies is of key importance to better understand methylmercury (MeHg) exposure pathways and environmental sources within the different ecosystems. Here we report stable isotopic composition for both Hg mass-dependent (e.g. δ²⁰²Hg) and mass-independent (e.g. Δ¹⁹⁹Hg) fractionation (proxies of Hg sources and transformations), carbon (δ¹³C, proxy of foraging habitat) and nitrogen (δ¹⁵N, proxy of trophic position) in blood of four species of sympatric penguins breeding at the subantarctic Crozet Islands (Southern Indian Ocean). Penguins have species-specific foraging strategies, from coastal to oceanic waters and from benthic to pelagic dives, and feed on different prey. A progressive increase to heavier Hg isotopic composition (δ²⁰²Hg and Δ¹⁹⁹Hg, respectively) was observed from benthic (1.45 ± 0.12 and 1.41 ± 0.06‰) to epipelagic (1.93 ± 0.18 and 1.77 ± 0.13‰) penguins, indicating a benthic-pelagic gradient of MeHg sources close to Crozet Islands. The relative variations of MeHg concentration, δ²⁰²Hg and Δ¹⁹⁹Hg with pelagic penguins feeding in Polar Front circumpolar waters (1.66 ± 0.11 and 1.54 ± 0.06‰) support that different MeHg sources occur at large scales in Southern Ocean deep waters.

Mercury Stable Isotopes Reveal Influence of Foraging Depth on Mercury Concentrations and Growth in Pacific Bluefin Tuna

Pelagic ecosystems are changing due to environmental and anthropogenic forces, with uncertain consequences for the ocean's top predators. Epipelagic and mesopelagic prey resources differ in quality and quantity, but their relative contribution to predator diets has been difficult to track. We measured mercury (Hg) stable isotopes in young (<2 years old) Pacific bluefin tuna (PBFT) and their prey species to explore the influence of foraging depth on growth and methylmercury (MeHg) exposure. PBFT total Hg (THg) in muscle ranged from 0.61 to 1.93 μg g^-1 dw (1.31 μg g^-1 dw ±0.37 SD; 99% ± 6% MeHg) and prey ranged from 0.01 to 1.76 μg g^-1 dw (0.13 μg g^-1 dw ±0.19 SD; 85% ± 18% MeHg). A systematic decrease in prey δ²⁰²Hg and Δ¹⁹⁹Hg with increasing depth of occurrence and discrete isotopic signatures of epipelagic prey (δ²⁰²Hg: 0.74 to 1.49‰; Δ¹⁹⁹Hg: 1.76-2.96‰) and mesopelagic prey (δ²⁰²Hg: 0.09 to 0.90‰; Δ¹⁹⁹Hg: 0.62-1.95‰) allowed the use of Hg isotopes to track PBFT foraging depth. An isotopic mixing model was used to estimate the dietary proportion of mesopelagic prey in PBFT, which ranged from 17% to 55%. Increased mesopelagic foraging was significantly correlated with slower growth and higher MeHg concentrations in PBFT. The slower observed growth rates suggest that prey availability and quality could reduce the production of PBFT biomass.

Use of Mercury Isotopes to Quantify Mercury Exposure Sources in Inland Populations, China

Mercury (Hg) isotopic compositions in hair and dietary sources from Wanshan (WS) Hg mining area, Guiyang (GY) urban area, and Changshun (CS) rural area were determined to identify the major Hg exposure sources of local residents. Rice and vegetables displayed low δ²⁰²Hg and small negative to zero Δ¹⁹⁹Hg, and are isotopically distinguishable from fish which showed relatively higher δ²⁰²Hg and positive Δ¹⁹⁹Hg. Distinct isotopic signatures were also observed for human hair from the three areas. Shifts of 2 to 3‰ in δ²⁰²Hg between hair and dietary sources confirmed mass dependent fractionation of Hg isotopes occurs during metabolic processes. Near zero Δ¹⁹⁹Hg of hair from WS and CS suggested rice is the major exposure source. Positive Δ¹⁹⁹Hg of hair from GY was likely caused by consumption of fish. A binary mixing model based on Δ¹⁹⁹Hg showed that rice and fish consumption accounted for 59% and 41% of dietary Hg source for GY residents, respectively, whereas rice is the major source for WS and CS residents. The model output was validated by calculation of probable daily intake of Hg. Our study suggests that Hg isotopes can be a useful tracer for quantifying exposure sources and understanding metabolic processes of Hg in humans.

Tracing aquatic bioavailable Hg in three different regions of China using fish Hg isotopes

To trace the most concerned bioavailable mercury (Hg) in aquatic environment, fish samples were collected from three typical regions in China, including 3 rivers and 1 lake in the Tibetan Plateau (TP, a high altitude background region with strong solar radiation), the Three Gorges Reservoir (TGR, the largest artificial freshwater reservoir in China), and the Chinese Bohai Sea (CBS, a heavily human-impacted semi-enclosed sea). The Hg isotopic compositions in fish muscles were analyzed. The results showed that anthropogenic emissions were the main sources of Hg in fish from TGR and CBS because of the observed negative δ²⁰²Hg and positive Δ¹⁹⁹Hg in these two regions (TGR, δ²⁰²Hg: - 0.72 to - 0.29‰, Δ¹⁹⁹Hg: 0.15 - 0.52‰; CBS, δ²⁰²Hg: - 2.09 to - 0.86‰, Δ¹⁹⁹Hg: 0.07 - 0.52‰). The relatively higher δ²⁰²Hg and Δ¹⁹⁹Hg (δ²⁰²Hg: - 0.37 - 0.08‰, Δ¹⁹⁹Hg: 0.50 - 1.89‰) in fish from TP suggested the insignificant disturbance from local anthropogenic activities. The larger slopes of Δ¹⁹⁹Hg/Δ²⁰¹Hg in fish from TGR (1.29 ± 0.14, 1SD) and TP (1.25 ± 0.06, 1SD) indicated methylmercury (MeHg) was produced and photo-reduced in the water column before incorporation into the fish. In contrast, the photoreduction of Hg²⁺ was the main process in CBS (slope of Δ¹⁹⁹Hg/Δ²⁰¹Hg: 1.06 ± 0.06, 1SD). According to the fingerprint data of Hg isotopes, the most important source for aquatic bioavailable Hg in TP should be the long-range transported Hg, contrasting to the anthropogenic originated MeHg from surface sediments and runoffs in TGR and inorganic Hg from continental inputs in CBS. Therefore, the isotopic signatures of Hg in fish can provide novel clues in tracing sources and behaviors of bioavailable Hg in aquatic systems, which are critical for further understanding the biogeochemical cycling of Hg.

Seabird Tissues As Efficient Biomonitoring Tools for Hg Isotopic Investigations: Implications of Using Blood and Feathers from Chicks and Adults

Blood and feathers are the two most targeted avian tissues for environmental biomonitoring studies, with mercury (Hg) concentration in blood and body feathers reflecting short and long-term Hg exposure, respectively. In this work, we investigated how Hg isotopic composition (e.g., δ²⁰²Hg and Δ¹⁹⁹Hg) of blood and feathers from either seabird chicks (skuas, n = 40) or adults (penguins, n = 62) can accurately provide information on exposure to Hg in marine ecosystems. Our results indicate a strong correlation between blood and feather Hg isotopic values for skua chicks, with similar δ²⁰²Hg and Δ¹⁹⁹Hg values in the two tissues (mean difference: -0.01 ± 0.25 ‰ and -0.05 ± 0.12 ‰, respectively). Since blood and body feathers of chicks integrate the same temporal window of Hg exposure, this suggests that δ²⁰²Hg and Δ¹⁹⁹Hg values can be directly compared without any correction factors within and between avian groups. Conversely, penguin adults show higher δ²⁰²Hg and Δ¹⁹⁹Hg values in feathers than in blood (mean differences: 0.28 ± 0.19‰ and 0.25 ± 0.13‰), most likely due to tissue-specific Hg temporal integration. Since feathers integrate long-term (i.e., the intermoult period) Hg accumulation, whereas blood reflects short-term (i.e., seasonal) Hg exposure in adult birds, the two tissues provide complementary information on trophic ecology at different time scales.