Redox chemistry of mercury in natural waters. The redox state of Hg dictates its transport at the air/water interface, with Hg(0) being more volatile than Hg(II) species. The formation of Hg(0) and its evasion from lakes and ocean surfaces significantly influence regional and global Hg budgets. Through the elaboration of innovative field-based experimental approaches using ultra-trace techniques, we initially described the role of solar radiation on Hg(II) reduction in the field, both in freshwaters and in coastal marine waters. My team established the existence of photo-oxidation of Hg(0) under environmental conditions. Since then, my lab has published a number of papers on factors affecting both redox processes, such as levels of dissolved organic carbon, pH, salinity, the presence of a liquid Hg phase and the quality and quantity of solar radiation. Our results have been used to model Hg fluxes on the Great Lakes and St. Lawrence River and are considered in the assessment of global budgets and fluxes.
Linking mercury biogeochemistry, food chain dynamics, human health and education. During the past few years, we have participated in a major case study on the St. Lawrence River. This case study is a cornerstone of the COMERN network, since it provides a framework for the integration of knowledge across sciences, for the benefits of a targeted community (community of Lake St. Pierre). Mercury projects on atmospheric deposition, aquatic transformations, environmental microbiology, bioaccumulation and biomagnification across food webs were fully integrated to simultaneous studies at the health and environmental education levels (led by other team leaders). Within this initiative, my lab specifically worked on the role of macrophyte beds on the redox chemistry of Hg and showed that: 1) Hg(II) reduction was stimulated by light, temperature, iron and DOC; 2) Hg(0) oxidation was stimulated by chloride; 3) macrophytes and associated epiphytes were sites of adsorption/absorption of Hg(II) and Hg(0); 4) bacterial degradation of macrophytes led to anoxia and Hg(0) production. An intensive study linking aquatic redox chemistry to atmospheric evasion was conducted with Environment Canada and led to an improvement of current evasion models.
Mercury Experiment To Assess Atmospheric Loadings in Canada and the United States (METAALICUS). METAALICUS is a binational research initiative that aims at establishing a direct link between atmospheric deposition of Hg and the levels of Hg found in our fish resource. This initiative is a key component in the development of a scientifically-sound environmental policy towards atmospheric emissions of Hg in Canada and the US. It consists of artificially modifying the atmospheric loading to a lake by adding small amounts of three isotopically-labelled Hg solutions to a whole ecosystem (one solution being added to the lake, another to its wetland and the third to its upland), in order to follow its behavior and its resulting impact on Hg levels in fish. The main contributions of my team have been: 1) the discovery of the very rapid chemical reduction of Hg in snow (see below contribution IV); 2) the report of a role of mixotrophic algae in Hg cycling ; 3) the evidence for a very significant loss by volatilization of newly-deposited Hg in pilot studies conducted in enclosures and during the whole-lake ecosystem experiment.
Mercury dynamics in snow and its implication on regional / global Hg modelling. Deposition of mercury by snow Deposition of Hg by snow is an important flux of Hg in Northern regions, particularly during so-called Mercury Depletion Events (Schroeder et al. 1998 Nature). Models assume that, once deposited, Hg stays in the snow pack until snowmelt, when it is mobilized and transferred to terrestrial and aquatic systems. My lab established that most of the Hg deposited by snow is lost from the snow pack within 24 to 48 hours. We also demonstrated that photoreduction may play a pivotal role in this Hg depletion by favoring the formation of Hg0, which can be returned to the atmosphere. Before this work, reduction processes in snow were not well recognized. We worked with Environment Canada and McGill to insure the transfer of our results into current regional and global models, and we therefore helped to establish that the Arctic was a sink for Hg and was receiving approximately 325 tons of mercury annually.
Fish as biodelivery system for nutrients and contaminants. The biomagnification of bioaccumulative pollutants in food chains leads to high levels in predatory fish. Indeed fish constitute the main pool of methylHg (MeHg) in many aquatic systems. Through their movement, these fish can act as biodelivery system for contaminants. We established the role of fish migration in the transport of Hg, by studying streams receiving Pacific Salmon runs. We have paid particular attention to the stream-to-land transport of Hg from decaying fish. We also assessed the role of the settling of fish carcasses in lakes as a potential flux of contaminants in lakes, and found a rapid recycling of carcasses and transfer to necrophagous animals. This was done through the use of stable isotopes of Hg in the field, a novel approach.
The main techniques currently in use include the following:
Quantification of mercury and methylmercury by atomic fluorescence sprectrometry, in water, snow, and biological tissues
Quantification of mercury by combustion and atomic absorption spectrometry in solids
Quantification of biogenic thiols in water and algae by HPLC-fluorescence