Plenary Sessions

We review current knowledge of important processes for the global distribution of mercury (Hg), including recent research advances, current uncertainties, and best estimates of concentrations and pools sizes in the atmospheric, terrestrial, and aquatic environments. Recent advances include the availability of new global datasets with measurements that now cover areas of the world where previously environmental Hg data were missing. Integration of such data into global and regional models are continually improving estimates of global Hg cycling. Stable Hg isotopes techniques now provide valuable tools to quantify Hg sources, study transformation processes, and help constrain the relative importance of specific sources and transformation in the global context. Key uncertainties in the global cycling of mercury are related to exchange processes between the atmosphere, terrestrial, and aquatic ecosystems; atmospheric dry deposition including plant uptake, re-emissions, and oxidation processes; methylation and demethylation particularly in oceans; and residence and turnover times in terrestrial and aquatic environments.

A career public servant in both Democratic and Republican administrations, Gina McCarthy has been a leading advocate for common sense strategies to protect public health and the environment for more than thirty years. As the head of EPA under President Obama, she led historic progress to achieve the administration’s public health and environmental protection goals and Climate Action Plan. In 2015, McCarthy signed the Clean Power Plan, which set the first-ever national standards for reducing carbon emissions from existing power plants, underscoring the country’s commitment to domestic climate action and spurring international efforts that helped secure the Paris Climate Agreement. During Gina McCarthy’s tenure, EPA initiatives cut air pollution, protected water resources, reduced greenhouse gases and strengthened chemical safety to better protect more Americans, especially the most vulnerable, from negative-health impacts associated with environmental pollutants. Internationally, McCarthy worked with the UN and WHO on a variety of efforts and represented the U.S. on global initiatives to reduce high risk sources of pollution. Known for her pragmatic approaches and disarming, plain-speaking style, McCarthy has earned the respect of the environmental, public health and business communities with her thorough understanding of all sides of climate, air quality, chemical safety, environmental justice and health equity, and water, land and natural resource protection and restoration issues. A gifted communicator and strategist with a talent for making environmental issues nonpartisan, highly personal, and solidly backed by science and the law, McCarthy is consistently credited with finding common ground and forging sustainable, common sense solutions.

The environmental cycling of mercury can be affected by natural and anthropogenic perturbations. Of particular concern is how these perturbations increase mobilization of mercury from sites and alter the formation and bioaccumulation of methylmercury, the toxic form of mercury for humans and wildlife. The scientific community has made significant advances in recent years in understanding the processes contributing to the risk of methyl mercury in the environment. These discoveries include a better understanding of how landscape changes (e.g. forestry practices, urbanization) can influence mobilization of mercury. Moreover, recent research has yielded insights on the response times scales for ecosystems experiencing changes in mercury loadings to surface waters, as well as loadings of other relevant constituents that influence mercury biogeochemical cycling and methyl mercury accumulation in aquatic organisms. While fish consumption is a major path of methyl mercury exposure for humans, we now know that rice can be a major route of exposure for certain populations. However, the extent of exposure and factors influencing methyl mercury bioaccumulation in rice still needs to be realized.

The complex cycle of mercury and its bioavailable form, methylmercury, in the environment, coupled with its variable toxicokinetics and diverse manifestations of adverse effects, complicate the determination of ecological and human health risk because mercury often does not follow linear pathways of release-exposure-effects. Instead, both ecological and socioeconomic factors influence the direction and magnitude of mercury exposure and bioaccumulation through food webs (and to humans), whereas environmental stressors interact with mercury to shape the onset and extent of adverse effects associated with exposure. Human behavior and socioeconomic variables also affect exposure and outcomes on local, regional, and global scales.