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Abstract Title: Suspended Particle Aggregates in Lakes as a Conducive Environment for Methylmercury Production
Presenter Name: Jean-Luc Loizeau
Company/Institution: University of Geneva
Session: Mercury in Freshwater Ecosystems
Co-Authors: Jean-Luc Loizeau,Andrea Gallorini,Natacha Toefield-Pasche

Abstract Information :

In recent years, evidence of methylmercury production in the oxic water column of the oceans has become increasingly clear. This production is likely associated with suspended aggregates collectively referred to as marine snow. While marine snow has been well studied, its freshwater equivalent, lake snow, has been getting less attention, although a few works suggested that it also provides a favorable micro environment for Hg methylation in freshwater systems. To evaluate the contribution of suspended particulate matter to MeHg production in in oxic waters of a deep lake, we sampled and analyzed particles from the water column of the peri-alpine Lake Geneva (Switzerland-France), and from its main tributaries using continuous flow filtration and continuous flow centrifugation, respectively. MeHg concentrations ranged from 0.48 ñ 0.09 ng/g to 9.61 ñ 0.67 ng/g in the lake particles, and from 0.30 ñ 0.08 ng/g to 2.41 ñ 0.14 ng/g in the tributary particles. In addition, micro measurements of dissolved oxygen inside large aggregates, collected in a polyacrylamide gel in sediment traps, revealed the presence of sub- to near anoxic conditions. Our results strongly suggest that lake snow provides favorable conditions for microbial Hg methylation, and that the high concentrations measured are the result. We developed a conceptual model with the lake snow as the main source of MeHg along Lake Geneva water column; a mass balance model of MeHg concentration showed that the steady-state is reached after 37 days, a duration consistent with particles residence times determined in previous works on Lake Geneva. These particles, enriched in MeHg, could be essential in the Hg cycling in the lake and constitute a major entry point into the lake food chain.

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