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Methyl Mercury (MeHg) Bioaccumulation/Biomagnification in the Arctic

           Of all the different mercury compounds that are transported into the Arctic ecosystem from both organic and anthropogenic sources, methylated mercury (MeHg) is a high-profile metal of great concern. It is considered to be a potent neurotoxin that bioaccumulates as it travels through each increasing trophic level of the Arctic marine food web resulting in adverse biological effects. Unlike other toxins or contaminants that enter the biosphere, MeHg is not processed through the body and released back into the ecosystem. It biomagnifies and increases in concentration over time, commonly by ingestion through predator-prey relationships, where top apex predators face the highest risk of exposure.

           Species in higher trophic levels tend have greater dietary and energetic requirements as well as longer lifespans therefore, facilitating an increase in MeHg trophic transfer. Once an organism is exposed, the body stores MeHg at a faster rate than it can be processed or excreted where it is then absorbed into the gastrointestinal tract, liver, tissues, and crosses the blood-brain barrier. MeHg is so readily taken up and stored, in orders of magnitude, because it binds to the sulfhydryl groups of amino acids, which are key components of protein formation. While fauna in upper trophic levels exhibit the highest concentrations of MeHg, the initial exposure starts with its uptake by phytoplankton, algae, and bacteria in the water column, usually through diffusion or active transport, to which it can then be transmitted across marine food webs.  

Biomagnification in Arctic Marine Biota

           The Arctic marine food web is composed of seven functional groups spanning through five distinct trophic levels including primary producers, invertebrates, bony fishes, sea birds, marine mammals, and humans. To understand how MeHg biomagnifies as it transfers up the food web, it is important to take into account seasonal variation, environmental and physical factors, as well as ecological constraints such as trophic dynamics, community structure, planktonic growth rates, feeding behavior, and organismal life-history characteristics. Similarly, biogeochemical factors will strongly influence the MeHg reservoir available for both passive and active uptake in lower trophic levels. For example, dissolved inorganic carbon (DOC) will strongly influence transmission such that when DOC levels are low, there is an increase in the bioavailability of MeHg which will allow for easier diffusion and transport through the cell membrane of grazing phytoplankton.

           While phytoplankton introduce and initiate transfer into the marine food web, trophic models evaluating MeHg transfer from both primary consumers, including smaller herbivorous zooplankton and secondary consumers such as larger carnivorous zooplankton, have suggested that biomagnification is most prominent with the latter of the two size categories. Trophic MeHg dilution occurs with small herbivorous zooplankton given thar their low grazing fluxes and higher elimination rates inhibit them from accumulating more MeHg than they are receiving from their prey. Conversely, larger zooplankton show elevated magnification rates due to increased grazing which leads to increased lifespan and greater predation potential.

           The introduction of MeHg at the lowest levels of the food web creates a gateway for biological uptake into higher trophic species. Research pertaining to foraging behavior of Arctic beluga whale (Delphinapterus leucas) populations has further outlined the distribution of MeHg among different species of bony fishes as direct impact of dietary exposure.  During the summer the Beaufort Sea beluga whales sexually segregate into three distinct habitat groups, each with different ranges and variation in foraging preferences, which yields different energetic requirements per group. Belugas that reside in shallow estuarine habitats feeding on species of coastal fishes such as Pacific herring, cisco, Saffron cod, and Arctic cod exhibited the lowest concentration of MeHg , followed by belugas who foraged in offshore habitats near the ice edge. Lastly, belugas foraging on benthic amphipods, flounder, and sculpin in deep water surrounded by heavy ice cover exhibited the highest concentration.

           Through the sampling of eggs and muscle tissue biomagnification of methyl mercury has also been exhibited in multiple species of marine seabirds including kittiwakes, murres, eiders, fulmars, and gulls where the concentration varies between species as a result of migration patterns, dietary preference, and habitat distribution. Seabird eggs can often display high enough concentrations of MeHg that results in declining reproductive success as well as reduced hatchability and clutch size. Furthermore, numerous studies analyzing both current and archived historic tissues (hair, teeth, and feathers) of Artic species and humans, some of which date back as far as 800 years, indicate there has been a ten-fold increase in mercury concentration since the mid-to-late 19^th century; which suggests an average rate of increase of approximately 1% to 4% per year.

           While increasing MeHg concentration in Arctic marine biota has been established to be occurring over long time scales, approximately 90% of current contamination in higher trophic Arctic animals is of anthropogenic origin. Current data shows that multiple species of seabirds and marine mammals such as polar bears (Ursus maritimus), pilot whales (Globicephala melas), hooded seals (Cystophora cristata), ringed seals (Pusa hispida), and belugas have concentrations of MeHg in their tissues that exceed the toxicological threshold which contribute to neurochemical effects, kidney, and liver damage. Conversely, one of the only Arctic species to not exhibit significant increasing levels of MeHg compared to historic trends are walruses (Odobenus rosmarus) given the fact their diet primarily consists of clams and bivalves, which tend to exhibit low Hg levels.

Measuring Toxin Transport and Why It’s Important

           Evaluating the concentration of MeHg as it biomagnifies through higher trophic levels in the Arctic has proven to be difficult due to remoteness, accessibility, sample sizes, and lack of resources. However, research has identified relationships between stable isotope ratios and signatures in relation to Hg concentration in animal tissues. An animal’s relative trophic status and feeding patterns over short timescales can be defined by measuring the abundance of stable isotopes of elements found in the food web, especially carbon (δ¹³C) and nitrogen (δ¹³N), which can infer contaminant transfer from predator to prey. Since trophic interactions among species are a key defining factor influencing the degree of bioaccumulation and biomagnification of MeHg, stable isotope measurements are considered an integrative way to interpret contaminant trends of marine food webs.

           Another way mercury is identified across the food chain is through measuring its molar ratio to corresponding selenium (Se) concentrations among seabirds and mammals. Se is a homeostatically regulated trace element that exhibits degrees of variability and is also considered highly toxic. While not conclusive and no exact protective molar ratio between the two elements has been established, it is proposed that Se counteracts the toxic effects of Hg by synthesizing metal binding proteins where Hg is then bound as an insoluble Se compound.  In order to be used as a viable risk management and communication tool for the general public, more research is needed to identify if a protective linear relationship of Se:Hg exists as well as how this ratio reaches different thresholds across multiple species and populations. Identifying global sources and sinks of MeHg is not only an environmental issue, but more importantly it is a human health issue. Adverse effects of MeHg biomagnification among higher tropic mammals include changes to central nervous, reproductive, cardiovascular, and immune system functions.  This is especially important in regards to Northern Peoples that make up Arctic communities. Their native reliance on fish as well as large terrestrial and marine mammals as part of a cultural subsistence lifestyle adds a complex human health dimension such that these food sources are vectors for human MeHg exposure.