Mercury in Fish: A Pictorial Description of the Mercury Cycle in Lakes



Summary: Toxic methylmercury becomes a concern for human health as it bioaccumulates in fish. In fact, methylmercury is sometimes present in predatory fish at the top of the food chain at concentrations 1,000,000 times that of the background level. When these fish travel long distances, bioaccumulation then contributes to the global transport of mercury.  As human populations consume fish, they become exposed to methylmercury, with the highest exposure correlating to the consumption of fish at the highest trophic levels.

The figure above depicts mercury cycling in a lake and its watershed. With regard to anthropogenic sources, this starts as emission and transport and ends with the bioaccumulation of methylmercury (here MeHg). These mercury emissions are transported across large distances in the form of elemental mercury [Hg (0)]. Elemental mercury is oxidized in the atmosphere to become a highly reactive gaseous form of mercury [Hg (II)]. Hg (II) is then deposited through precipitation (wet deposition)              and by surface contact (dry deposition) into bodies of water, such as this lake. Anaerobic bacteria then convert some of this deposited Hg (II) to methylmercury which is bioaccumulated and then bioconcentrated. How much Hg (II) is converted to methylmercury depends on the convergence of various biotic and abiotic reactions such as photodegradation and redox (oxidation-reduction) reactions that result in some of the mercury being buried, some of it being released back to the atmosphere, and some of it being bioaccumulated.

A recent study by Harris et al. studied the response of lakes to changing inputs of atmospheric mercury. It found that mercury levels in fish respond directly and quickly to changing rates of mercurial atmospheric deposition. Furthermore, other studies have shown observed decreases in the amount of mercury in fish in areas where mercury deposition has recently declined. Mercury as a pollutant is widespread, and at on oceanic level, it will take much longer for inventories to respond to changes in atmospheric levels. However, evidence that lakes will respond to these changes support the idea that emissions reductions will be effective.

Note: This is particularly good evidence to support the idea that a global effort to reduce mercury emissions, through the Minamata Convention, will have some direct and quickly observable results.

Engstrom, Daniel R. 2007. “Fish respond when the mercury rises.” Proceedings of the National Academy of Sciences 104, no. 42: 16394-16395.


Mercury Science in the Ocean: The Marine Biogeochemical Cycle of Mercury

The form of mercury of main concern to human health is methylmercury, also known as monomethylmercury (MMHg). Under anaerobic conditions (conditions where oxygen is unavailable) both sulfate and iron-reducing bacteria can convert mercury to MMHg, thus making it available for bioaccumulation in organisms. Mercury is converted to methylmercury in aquatic ecosystems, and this mechanism is particularly prevalent in freshwater ecosystems such as wetlands. However, when it comes to marine ecosystems there is much that remains unknown about the exact methylation mechanisms and the rates of methylation.

It has been established that human, or anthropogenic, interferences in the global mercury cycle have had a significant impact. Society is mostly concerned with the toxicity of MMHg which is found in aquatic ecosystems where both natural and anthropogenic mercury is often deposited, and where there are large populations of active Hg methylating bacteria. As mercury is deposited in these ecosystems, the production of MMHg, followed by its bioaccumulation (uptake by marine organisms) and biomagnification (increase in levels of MMHg in marine organisms higher up the food chain as they eat one another) often results in fish with levels of MMHg that are much higher than those deemed safe by the US Environmental Protection Agency.

Human exposure to MMHg occurs most commonly through consumption of fish. Furthermore, most fish that are consumed by humans come from marine environments, particularly those in coastal regions. Since this is widespread knowledge, it would be logical to assume that much of the marine biogeochemical cycle of mercury has been well-investigated and is well-understood. However, this is unfortunately not true. Fitzgerald, Lamborg, and Hammerschmidt characterize the marine biogeochemical cycle of mercury as “undersampled and understudied.” In fact, knowledge of trace metals in the oceans was less than 40 years old at the time of this paper’s publication (2006). Efforts before this time were hindered by analytical deficiencies. The techniques now used to produce high quality Hg measurements were actually not even developed until the 1980s.



Above is a simplification of what is known of the marine mercury cycle. The top figure represents the preindustrial cycle, which occurred around 200 years ago. The bottom figure is how the cycle currently operates.  The fluxes are in Mmol per year and the reservoirs (inside of the boxes) are in Mmol with estimates of rates of change for each reservoir in the bottom figure. As can be seen from these figures, the distribution of Hg in the oceans is not well understood. Attempting to better understand these cycles will help to answer questions of how predicted anthropogenic changes are constrained, how and if Hg is currently increasing on the Earth’s surface (as these figures suggest), and how these changes will influence the concentration of MMHg in biota.

Fitzgerald, William F., Carl H. Lamborg, and Chad R. Hammerschmidt. 2007. “Marine biogeochemical cycling of mercury.” Chemical Reviews 107, no. 2: 641-662.

The Mercury Cycle

The Mercury Cycle

The Mercury Cycle: Mercury is released from both natural and anthropogenic sources. Volcanoes, land emissions, and geothermal vents are all sources of natural mercury emissions. The budget of these combined sources is estimated to be about 500 megagrams per year, but this number is uncertain. In order to determine the actual historical budget, it is necessary to distinguish between primary emissions and preindustrial secondary fluxes. Primary emissions refers to sources resulting from the lithosphere. Preindustrial secondary fluxes of mercury from the land result from soil mercury that originated from the atmosphere. Distinguishing between these is important to understanding how anthropogenic emissions will disturb mercury’s natural cycle. Anthropogenic emissions originate from mining and industrial processes, biomass combustion, and fossil fuel combustion. Mercury can also be found in commercial and consumer products and processes and further mercury emissions occur when these products are incinerated. In many developing nations, a large source of mercury emissions is artisanal and small-scale gold mining which employs mercury in the mining process. These anthropogenic contributions have altered the biogeochemical cycle of mercury. Because of these activities, more mercury is circulating through the cycle, and will continue to do so for hundreds and thousands of years. As mercury cycles through the atmosphere, ocean, and land, it has implications for both human and environmental health. This historical mercury has heightened the natural fluxes between the atmosphere and the ocean and between the atmosphere and the land. In the figure above, preindustrial fluxes and inventories are in black while anthropogenic contributions are in red.

Selin NE (2009) Global Biogeochemical Cycling of Mercury: A Review. Annual Review of Environment and Resources 34:43-63.

The Credibility, Salience, and Legitimacy of Science in the Stockholm Convention on Persistent Organic Pollutants

Summary: In 2001, delegates from 100 countries signed the Stockholm Convention on Persistent Organic Pollutants (POPs). Noelle Selin analyzes how scientific information influenced the policy discourse leading up to this treaty and how the credibility, salience, and legitimacy of this information influenced which a most influential.

The Stockholm Convention built off the precedent of the 1998 Århus protocol on POPs to the Convention on Long-Range Transboundary Air Pollution (LRTAP). This protocol was preceded by an assessment process that was subsequently used in the negotiating process for the Stockholm Convention. Selin looks at the elements of the negotiating process where these assessments were used to show that the LRTAP assessments were quite influential. She focuses the most on the issues of the legitimacy and salience of these assessments since these criteria were found to be more important and controversial than credibility.

Many participants in the POPs negotiation process considered LRTAP to be an important precedent. First of all, LRTAP’s assessment process defined the POPs problem and pushed it onto a global agenda. Even the term POP is evidence of science influencing negotiators and was coined jointly between scientists and policymakers in an effort to frame the scope of the policy. The assessments also served to mobilize actors around the issue and create a knowledge base that was shared by all these actors. Other assessment processes achieved this as well, for example a series of subregional workshops on POPs which encouraged parties to assess whether POPs were being domestically regulated and to conduct national scientific assessments of these pollutants. Finally, LRTAP assessments provided bargaining focal points and road maps during the negotiations. Because the scientific data was largely considered to be credible and legitimate, assessment-based conclusions, such as selecting which substances to regulate, proceeded fairly quickly and without controversy.  The fact that these substances could be agreed to on a political level and that this mandate was not questioned throughout the process, points to the idea that the legitimacy of the science was not brought into question very much during the process.

It was found that the LRTAP assessments were particularly salient for developed countries but less so for developing nations as the concerns they addressed did not apply for many tropical countries and countries in the Southern Hemisphere. For example, the LRTAP assessments framed POPs as a global issue because of long-range transport and accumulation of POPs in remote areas. However, many developing countries were actually more concerned about POPs as local pollutants and were primarily motivated to deal with contamination problems by obtaining technical and financial assistance.

Note: This is an excellent example of the basic structure for my thesis since I am seeking to do a similar analysis with the Mercury Convention. In particular I liked the opening section that described exactly which assessments were used and which were most influential before delving into why they were particularly influential.

Selin, Noelle E. 2006. “From Regional To Global Information: Assessment of Persistent Organic Pollutants.” Global Environmental Assessments: Information and Influence. Cambridge, MA: MIT. 175-200.

“Pez- Peste”

Pez Peste

Summary: A sculpture named “Pez-Peste” was presented to the intergovernmental negotiating committee and its parties by Ms. Lillian Corra of the International Society of Doctors for the Environment. The sculpture was meant to symbolize the irreversible consequences of mercury contamination in order to impress upon Governments the importance of reaching an effective agreement to eliminate mercury emissions in the environment. During her presentation, Ms. Corra reminded the committee that reducing mercury pollution is the responsibility of all different sectors. Mercury has a number of adverse effects on both humans and wildlife and is particularly prevalent in fish.

The sculpture was created and donated by an Argentinean artist named Nicolas Garcia Uriburu. The sculpture was created by Mr. Fernando Lugris of Uruguay, the Chair of the intergovernmental negotiating committee. “Pez-Peste” subsequently became the symbol for the Convention and followed the negotiation committee throughout the entire process.

Discussion: The sculpture itself is an interesting representation of a fish covered in tiny balls. These are undoubtedly meant to symbolize the small, silvery mercury balls that people are familiar with from their use in thermometers. However, this is a clear case of artistic license since, from a science viewpoint, fish do not come into contact with mercury in this form. What actually happens is that mercury is released, evaporates into the air, and eventually makes its way to a body of water. Once in the water, microorganisms convert mercury into the even more toxic form of methylmercury. This is the form that makes its way into fish when the fish eat smaller aquatic organisms. As larger fish eat these fish and the food chain continues, the amount of mercury in the fish builds up. Fish are also relatively slow at eliminating methylmercury, so the concentration of methylmercury inside fish is often much higher than that of the surrounding water. Thus, not only is the mercury internalized in fish, but it is done so in a form that is very different from the familiar silvery balls. However, “Pez-Peste” serves as a way to communicate a scientific problem to people in a way that is understandable, despite sacrificing science while doing so. The sculpture is clearly recognizable as an animal that has been horribly transformed because of the mercury balls. It consequently brings to mind the concerns over human health when people consume fish that have been contaminated by mercury. Thus, although the science being portrayed is inaccurate, “Pez-Peste” effectively communicates the danger of mercury and the importance of taking action. This is a case of scientific credibility and legitimacy being sacrificed for public salience.

OEHHA. 2013. “OEHHA Methyl Mercury in Fish.” OEHHA: Office of Environmental Health Hazard Assessment., Web. 25 Sept. 2013.

UNEP. n.d. “Sculpture “Pez-Peste”.” UNEP: United Nations Environment Programme. UNEP, Web. 25 Sept. 2013.

Shaping the Minamata Convention: History, Politics, and Science

The Minamata Convention on Mercury focused on the life-cycle of mercury and how that relates to a number of environmental and human health risks. The final text of the treaty was informed by a variety of historical, political, and scientific factors.

Historically, international agreements on hazardous substances have been made up of several legally free-standing but politically related agreements. The Minamata Convention continues this strategy by linking a variety of different issues covering waste management, capacity building, and technology transfer to mercury. Legal experts also used text from earlier conventions on hazardous chemicals to draft the mercury treaty language. Specifically, the treaty text incorporates existing strategies from the Stockholm Convention to phase-out and restrict mercury usage. The treaty also uses a Stockholm Convention strategy to allow countries to register for a five-year exemption from this requirement.

The treaty was further influenced by long-standing political disagreements that arose in other negotiations. This complicated the mercury negotiations since countries knew that compromising their interests in the mercury convention could have ramifications for how their interests would be met in other negotiations. For example, developing countries that were frustrated at the lack of funding in other agreements carried this frustration into the mercury negotiations. Differences in countries’ emissions further complicated the politics. According to UNEP, Asia (mostly China and India) emitted the most mercury at 47.6 percent of global emissions, followed by Africa at 16.8 percent and South America at 12.5 percent. Even though North America and the European Union have historically been large emitters, they now only contribute marginally to global emissions with 3.1 and 4.5 percent of global emissions, respectively. This disparity led to persistent North-South issues over how to assign the burden of reducing emissions. Developing countries insisted that financial and technical assistance should be “a condition for” implementation while developed countries argued that it should only be recognized as “essential for” implementation. In general, the resulting compromises ending up pleasing developed countries more than developing ones.

Science was also an important factor in the development of the Convention. The 2013 UNEP mercury assessment estimated that in 2010, 1,960 tonnes of mercury were released into the atmosphere as a result of human activity, and at least another 1,000 tonnes were released into the water. The four largest sources were artisanal and small-scale gold mining, coal-burning, primary production of non-ferrous metals, and cement production. Scientific uncertainty came into play in a number of instances, particularly with regard to mercury-containing thimerosal vaccine. This was a contentious issue as the WHO supported continued use of the vaccine, claiming that there was no scientific data to prove its use was a health issue and that restricting access would lead to restrictions on who could benefit from vaccines. Thimerosal is used as a preservative for vaccines so that they do not need to be refrigerated. However, arguments that mercury-free vaccines need to become more readily available eventually won out and thimerosal was explicitly excluded from use.

Selin, Henrik. (Forthcoming.) “Global Environmental Law and Treaty-Making on Hazardous Substances: The Minamata Convention and Mercury Abatement.” Global Environmental Politics.

Details of the Mercury Treaty

Summary: David Lennett, a member of the National Resources Defense Council, attended the Mercury negotiations in Geneva and blogged about some of the important aspects of the finalized treaty. He begins by summarizing his view of the treaty; essentially that it is strong in some respects but weak in others. In Mr. Lennett’s view, the provisions for phasing out mercury-containing products are strong, but the air emission control requirements for existing facilities delay action for too long. Nonetheless, overall he views the treaty as quite an accomplishment given the context of gridlock when it comes to other global environmental issues. Mr. Lennett then summarizes the key provisions of the treaty:

  • As soon as the Convention enters force, new mercury mines are prohibited and old ones must be phased out over the next 15 years.
  • Factories that use mercury to make chlorine and caustic soda must be phased out by 2025. Mercury that results from de-commissioning these plants cannot be sold or re-used in any other sector.
  • A country that wishes to import mercury must provide written consent. Without this consent, the trading of mercury is illegal.
  • Certain mercury products must be phased out by 2020. These products include certain batteries, many switches and relays, skin lightening creams and soaps, pesticides, biocides (except for vaccines), topical antiseptics, barometers, hygrometers, manometers, thermometers, and blood pressure cuffs.
  • Phase out dates can be extended if a country specifically requests an exemption. Extensions will last for five years and will become progressively harder to obtain, starting with the second requested exemption. From the second exemption on, all Parties to the Convention are required for approval.
  • Mercury in dental amalgams will be phased out, as will mercury in polyurethane and vinyl chloride monomer.
  • The treaty covers air emissions from industrial boilers such as lead, copper, zinc, and gold roasting and smelting processes. It also covers emissions from coal-fired power plants, cement plants, and waste incinerators. New or newly renovated sources must use the Best Available Techniques or Best Environmental Practices when it comes to mercury emissions. However existing sources, defined as sources in existence up to one year after the treaty comes into force, are subject to a range of different regulations. Compliance with these regulations is not required until 10 years after the treaty comes into force for that particular government.
  • Governments must develop and implement national action plans to address mercury use in small-scale gold mining.

Questions: How was scientific information used in the development of these provisions? Why is the provision for mercury emissions so weak? Note that, for existing plants, mercury emissions regulations do not come into effect until ten years after the treaty comes into force for that particular country. This would seem to provide a perverse incentive for countries with lots of existing industry. They could wait to ratify the treaty (since only 15 countries are needed for the treaty to actually go into force) and thus extend the air emissions requirement for years.

Lennett, David. “Mercury Treaty Finalized.” 2013. Web blog post. Switchboard: National Resources Defense Council Staff Blog. National Resources Defense Council, 22 Jan. Web. 18 Sept. 2013