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Relationships between organic matter composition and methyl mercury content of offshore and carbon-rich littoral sediments in an oligotrophic lake

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Abstract:

Relationships between organic matter (OM) compounds and methyl mercury concentrations ([MeHg]) have been examined in littoral and offshore sediments of Lake Lusignan (Québec). The highest [MeHg] were generally found at the sediment–water interface with exceptionally high concentrations at littoral sites (5.8 ± 1.3 ng·g dry weight–1), which were four times more elevated than at offshore sites (1.6 ± 0.77 ng·g dry weight–1). Source-specific fatty acid (FA) biomarkers identified that littoral sediments contained more than twice as much terrestrial and bacterial OM compounds than offshore sediments, whereas the amount of labile algal OM was three times higher at littoral sites. Results indicate that [MeHg] were higher in the presence of labile OM substrates, and the amount of terrestrial OM compounds could not predict [MeHg]. Correlations between [MeHg] and FA of Desulfovibrio desulfuricans (a sulfate-reducing bacterium producing MeHg) could significantly account for 36% of [MeHg] at offshore sites; however, no significant relationships were found at littoral sites. This study illustrates that the microbial dynamics involved in producing and degrading MeHg in lacustrine sediments are complex and cannot be predicted solely by the quantification of FA biomarkers in D. desulfuricans or by biomarkers in the OM itself.

Nous avons examiné les relations entre la matière organique (MO) et les concentrations du méthylmercure (MeHg) dans les sédiments littoraux et pélagiques du lac Lusignan (Québec). En général, les concentrations du MeHg les plus élevées ont été trouvées à l'interface sédiment–eau dans les zones littorales (5,8 ± 1,3 ng·g poids sec–1). Les concentrations étaient quatre fois plus élévées que celles mesurées dans les zones pélagiques (1,6 ± 0,66 ng·g poids sec–1). Les analyses des acides gras (AG), utilisés comme biomarqueurs des sources de MO, ont permis d'établir qu'il y avait deux fois plus de MO terrigène et bactérienne et trois fois plus de MO labile provenant des algues dans les sédiments littoraux que dans les sédiments pélagiques. Nos résultats montrent que les concentrations en MeHg étaient plus élévées en présence de MO labile et que la quantité de MO terrigène ne pouvait pas prédire les concentrations du MeHg. Les corrélations entre les concentrations du MeHg et des AG de la bactérie Desulfovibrio desulfuricans, une bactérie sulfato-réductrice impliquée dans la dynamique du MeHg, pouvaient expliquer 36 % de la présence du MeHg dans les sites pélagiques. Par contre dans les sites littoraux, ces corrélations n'étaient pas significatives. Notre étude démontre que les dynamiques microbiennes impliquées dans la production et la dégradation du MeHg dans les sédiments lacustres sont complexes et la quantification des biomarqueurs des AG trouvées dans D. desulfuricans ou les AG de la MO ne peuvent pas prédire les teneurs en MeHg dans les sédiments.

Document Type: Research Article

Publication date: July 1, 2003

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  • Published continuously since 1901 (under various titles), this monthly journal is the primary publishing vehicle for the multidisciplinary field of aquatic sciences. It publishes perspectives (syntheses, critiques, and re-evaluations), discussions (comments and replies), articles, and rapid communications, relating to current research on cells, organisms, populations, ecosystems, or processes that affect aquatic systems. The journal seeks to amplify, modify, question, or redirect accumulated knowledge in the field of fisheries and aquatic science. Occasional supplements are dedicated to single topics or to proceedings of international symposia.
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