The Objectives of the Original Proposal Were: I) to Verify Whether Hg Methylation Occurs

The objectives of the original proposal were: i) to verify whether Hg methylation occurs within the littoral biofilm of lakes, and ii) to determine the relative importance of littoral vs. pelagic communities in Hg methylation

The project contributes to the understanding of the importance of aquatic littoral biofilms in the biogeochemical cycling of mercury and their significance as a direct or indirect path, as food resource, to mercury contamination of fish and wildlife, and hence to humans. The littoral biofilms are directly exposed to pollutants from the atmosphere, but also from the surface and underground watershed inputs. The biota of biofilm (periphyton) rapidly accumulate the dissolved substances, including lipophilic pollutants, in their matrix and therefore it constitutes a good indicator of the health risks for a given ecosystems. In the case of mercury (Hg), the littoral biofilms may not only accumulate Hg but it may also contribute to itsmethylation and to methylmercury (MeHg) accumulation. Therefore, littoral biofilms may represent an important contamination path, via grazers and detritivorous organisms, to fish and aquatic wildlife. Recent data on eastern boreal lakes and elsewhere have showed that the littoral communities are more important than the pelagic communities as the main food source for almost all fishes, in one stage or another of their cycle. It is also known that the macrophyte beds in some rivers, such as the St. Lawrence River, are refuge and feeding habitats for many of the commercial fish larvae (sturgeon, yellow perch, etc). The importance of the littoral communities in the transfer of contaminants to aquatic food webs is still very often ignored in Hg aquatic cycling studies and models. This project will in part fill this gap.

The aim of our research has not being modified since the original proposal but it has been enlarged due to our networking contribution to different multidisciplinary studies. Presently, our laboratory is involved in the following objectives: 1st. To verify the importance of aquatic periphyton biofilms on Hg methylation in different ecosystems (lakes and rivers); 2nd. To determine MeHg concentrations in periphyton communities in the St. Lawrence River, Bay of Fundy coastal areas and in boreal forest lakes; 3rd. To identify the transfer of Hg from the littoral to fish and wildlife through biofilm grazers and detritivors. The specific reasons for the objectives expansion were i) Our previous demonstration of the importance of the littoral biofilms on mercury and methyl mercury accumulation in several Quebec boreal lakes; ii) The in vitro revelation of methylation on periphyton biofilms growing on Canadian Shield lakes: iii) The gap between mercury decrease in the water, downstream of coastal brooks and streams, without apparent decrease in mercury concentrations in fish and coastal marine organisms. One hypothesis was advanced on the decrease of Hg from the water: the dissolvedstream Hg might be absorbed by periphyton biofilms. iv) The lack of agreement in the St. Lawrence River between Hg concentrations in fish species and the Hg levels of their potential prey (sediment macroinvertebrates) also raised new questions: Are the Hg concentrations in the St Lawrence fish more related to communities feeding on or from aquatic plants than to communities living on sediment? Is the MeHg produced in plants periphyton biofilms? If yes, is this MeHg a source of Hg for fish and wildlife via the periphyton biofilms consumers? To validate some of these hypotheses, our laboratory enlarged its objectives with the studies of: i)in situ netrates ofmercury methylation (methylation–demethylation) by periphyton biofilms growing on submerged and emergent aquatic plants. These plants are very abundant on wetlands and macrophytes beds (where fishing is intense) of the St Lawrence River; ii) MeHg accumulation on the periphyton biofilms and macroinvertebrates feeding on periphyton.