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Forms of Mercury Mercury (Hg) can be found in three main forms in Arctic aquatic systems: elemental, methylated, and Hg (II). It can only be introduced in the environment through volcanism, geothermic processes, and anthropological sources, and unlike nutrient cycling, mercury does not follow cyclical distribution patterns. It can also be found in mineral form, as both cinnabar and metacinnabar .In its various forms mercury is indicated to be scavenged by fish and primary producers, which is supported by its vertical distribution in the water column. Elemental mercury is found in the atmosphere and surface waters. With a solubility similar to oxygen, it is capable of dissolution and can be found in supersaturated concentrations under ice, but undersaturated in open water. This is the main form of mercury input into the ocean.

Methylmercury is one of these three forms that stem from elemental mercury and is more common than its other methylated form, dimethylmercury. It is the most toxic form, acting as a neurotoxin in organisms due to its ability to combine with lipids and fats. Dimethylmercury, the lesser product of methylation, is created through the reduction of Hg (II). Inorganic methylation has been documented as a result of anaerobic and aerobic microbes, as well as reduction reactions. It is this form that unicellular organisms ingest and is slowly eliminated relative to its uptake rate.

Hg (II) enters the ocean as a gas through the atmosphere and undergoes methylation and demethylation reactions. Not commonly found in undersurface waters, Hg (II) reappears in ocean sediment. The most predominant route of Hg input into the Arctic Ocean is long-range transport, as opposed to point source introduction which can be acute.

Atmospheric Interactions Mercury is found in its elemental gas form. Due to this, the residence time for mercury is six to twelve months and can be oxidized into more reactive forms, such as reactive gaseous Hg and particulate elemental Hg. These two forms have a significantly shorter residence time, where they are thus deposited because of atmospheric mercury depletion events (AMDE). This is most noted in the spring when favorable conditions occur in the Arctic (e.g. Cold temperatures, adequate sunlight) and the presence of reactive, catalyzing halogens in sea ice (i.e. Bromine, Chlorine, Bromine Oxide, Chlorine Oxide). However, there are uncertainties revolving around which major atmospheric oxidants play more specific roles. Previous data indicates elemental mercury in the atmosphere is a result of Hg (II) reduction in snow and becomes its gaseous form.

Water Column Elemental mercury levels tend to be low in arctic ocean waters. Due to there being no strong distribution patterns in total mercury, there are also no strong patterns for Hg (II), since it is a product of mercury methylation. There appears to be a correlation between higher Hg (II) levels and distance from river inflows, further suggesting rivers to be a source. Euphotic zones in the Arctic show both biological and photochemical Hg (II) reduction as well as methylmercury photodegradation. Regarding euphotic zones, wind speed and ice cover are shown to dictate elemental mercury concentration. Low concentration has been observed with high Chlorophyll levels, while dissolved gaseous mercury concentration has been observed highest in ice covered areas. This relationship indicates mercury evasion caused by ship passages. The aphotic zones of the Arctic show signs of biotic reduction of Hg (II), which is considered to be the primary mechanism for elemental mercury production. Reductive demethylation is another possible mechanism, however not as prominent.

Methylmercury is shown to be in low concentrations in open surface waters, and this is thought to be the result of photodemethylation decreasing surface water concentrations. Mixing throughout the water column is thought to be the primary transport system for methylmercury and dimethylmercury, and also play a role in the deposition of methylmercury to land (dimethylmercury is evaded to the atmosphere, where it undergoes photodegradation and becomes methylmercury).

The sedimentary layer in coastal and estuarine systems hold a prominent reserve of mercury transported from rivers, and it is believed to be due to a combination of small particle size, burrowing macrofauna, and high organic matter concentrations. Mercury is then recycled, however, the rates of methylation are dependent on the physical composition, chemistry, and movement of the sediment, thus physical and biological aspects such as hydrodynamics, bioturbation, and organic matter content influence the biogeochemical potential for methylation. It is possible that mercury is transferred from the sedimentary layer into the water column through physical processes such as passive diffusion, advection, and particulate resuspension.