The environmental cycling of mercury (Hg) can be affected by natural and anthropogenic perturbations. Of particular concern is how these disruptions increase mobilization of Hg from sites and alter the formation of monomethylmercury (MeHg), a bioaccumulative form of Hg for humans and wildlife. The scientific community has made significant advances in recent years in understanding the processes contributing to the risk of MeHg in the environment. The objective of this paper is to synthesize the scientific understanding of how Hg cycling in the aquatic environment is influenced by landscape perturbations at the local scale, perturbations that include watershed loadings, deforestation, reservoir and wetland creation, rice production, urbanization, mining and industrial point source pollution, and remediation. We focus on the major challenges associated with each type of alteration, as well as management opportunities that could lessen both MeHg levels in biota and exposure to humans. For example, our understanding of approximate response times to changes in Hg inputs from various sources or landscape alterations could lead to policies that prioritize the avoidance of certain activities in the most vulnerable systems and sequestration of Hg in deep soil and sediment pools. The remediation of Hg pollution from historical mining and other industries is shifting towards in situ technologies that could be less disruptive and less costly than conventional approaches. Contemporary artisanal gold mining has well-documented impacts with respect to Hg; however, significant social and political challenges remain in implementing effective policies to minimize Hg use. Much remains to be learned as we strive towards the meaningful application of our understanding for stakeholders, including communities living near Hg-polluted sites, environmental policy makers, and scientists and engineers tasked with developing watershed management solutions. Site-specific assessments of MeHg exposure risk will require new methods to predict the impacts of anthropogenic perturbations and an understanding of the complexity of Hg cycling at the local scale.

Mercury is a global pollutant that has been emitted and released into the environment from numerous human activities including coal combustion, artisanal gold mining, mining of other metals, and chemical production, resulting in the elevated exposure of humans and wildlife throughout the world. Mercury contamination has been recognized by the international science and policy communities. In summer of 2017, the Minamata Convention on Mercury came into force. The treaty aims to protect human health and the environment from anthropogenic emissions and releases of mercury throughout its life cycle. During this same time, the 13th International Conference of Mercury as a Global Pollutant (ICMGP-13) was held in Providence, Rhode Island USA. As part of ICMGP-13, cutting edge research was distilled to address questions relating to global and regional sources and cycling of mercury contamination, the effects of mercury exposure on humans and wildlife, and the science needed for successful implementation of the Minamata Convention. This synthesis effort concluded that many of the most important factors influencing the fate and effects of mercury stem from environmental changes that are much broader than mercury releases alone. Alterations of mercury cycling, bioavailability and trophic transfer due to climate and land use changes remain critical uncertainties in the ability of the Minamata Convention to reduce mercury pollution. In the face of these uncertainties at global, regional and local scales, there are important policy and management actions that are needed over the short-term to support the reduction of mercury emissions and releases, including adequate monitoring and communication on risk from occupational exposures and fish consumption. Successful management of global and local mercury pollution will require integration of mercury research and policy in a changing world.

Atmospheric mercury (Hg) measurements from across Canada were compiled and analysed as part of a national Hg science assessment. Here we update long-term trends of Hg in air and precipitation, and present more extensive measurements on patterns and trends in speciated Hg species (gaseous elemental mercury—GEM, reactive gaseous mercury—RGM, and total particulate mercury on particles <2.5 μm—TPM_2.5) at several sites. A spatial analysis across Canada revealed higher air concentrations and wet deposition of Hg in the vicinity of local and regional emission sources, and lower air concentrations of Hg at mid-latitude maritime sites compared to continental sites. Diel and seasonal patterns in atmospheric GEM, RGM and TPM_2.5 concentrations reflected differences in patterns of anthropogenic emissions, photo-induced surface emissions, chemistry, deposition and mixing. Concentrations of GEM decreased at rates ranging from −0.9% to −3.3% per year at all sites where measurements began in the 1990s. Concentrations of total Hg in precipitation declined up to 3.7% yr⁻¹. Trends in RGM and TPM_2.5 were less clear due to shorter measurement periods and low concentrations, however, in spring at the high Arctic site (Alert) when RGM and TPM_2.5 concentrations were high, concentrations of both increased by 7%–10% per year.