Effects of nutrient cycling on contaminant fate in aquatic food webs

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There is ongoing global concern about increasing anthropogenic releases of mercury (Hg) because it is deposited into aquatic systems, transformed by bacteria to methyl Hg (MeHg), and biomagnified (increases in concentration from prey to predator) through aquatic food chains to concentrations that may be toxic for fish-eating consumers or the fish themselves. Stable nitrogen isotopes (delta15N) are commonly used to understand the relative trophic position of aquatic organisms and the rate at which contaminants such as Hg are transferred through food webs. In diverse ecosystems, mercury concentrations in aquatic biota are significantly and positively related to their delta15N, but the rate (slope) of this transfer varies across systems for reasons that remain unexplained. The fate of Hg in aquatic systems is linked to the abiotic and biotic cycling of sulphur. Methyl Hg binds to the S-containing amino acid cysteine for uptake into bacteria and for storage in proteins of fish and plants. Although little information exists for aquatic food webs, terrestrial organisms at high trophic levels contain the greatest amounts of cysteine in their proteins, and cysteine content in plants is affected by water chemistry. This suggests that cysteine content of organisms varies in aquatic food webs and across systems, perhaps explaining among-system differences in Hg biomagnification. The next five years of my NSERC Discovery program will continue to refine our understanding of the food web processes affecting contaminant transfer through aquatic ecosystems. Both within and across lakes and streams that vary in their chemical characteristics (high through low nutrients; acidic through near neutral pH), we will examine whether MeHg in primary consumers is predicted by their cysteine content, and whether cysteine is better predictor of the food web transfer of MeHg than delta15N. Results will be used to develop and refine models to better describe among-system variability in concentrations of this contaminant, and to contrast this process in systems that vary in their chemical, physical and biological characteristics.