Sedimentological and geochemical records of past trophic state and hypolimnetic anoxia in large, hard-water Lake Bourget, French Alps

Abstract : Sedimentological, geochemical and particle- size analyses were used to reconstruct the evolution of both trophic state and hypolimnetic anoxia in Lake Bourget (French Alps) during the last century. Radionuclide dating (210Pb, 137Cs and 241Am) confirmed the annual rhythm of laminations in the upper sediment profile. In Lake Bourget, biochemical varves are triplets composed of a diatomlayer (spring lamina), a bio-precipitated calcite-rich layer (spring/summer lamina), and a layer rich in organic matter and detrital particles (winter lamina). The onset of eutrophication and the first appearance of an anoxic facies occurred simultaneously and were dated by laminae counting to AD 1943±1 year. Persistent anoxic conditions began in AD 1960. Eutrophication is characterised by drastic increases in the flux of biogenic silica (mostly diatoms), lacustrine organic matter, and larger calcite crystals (15-30 lm). The increase of organic matter also represents a marker of the onset of anoxic conditions in the hypolimnion. Our results show that eutrophication was the main factor controlling anoxia in the hypolimnion. This eutrophication was caused mostly by the inflow of untreated sewage effluents, and to a lesser extent, by input of fertilizer-derived phosphorus during floods of the Rhone River and run-off from the lake catchment. The Rhone River, however, can also be a source of re-oxygenation via underflows that originate during flood events. Oxygenation of the hypolimnion is also controlled by low winter temperatures, which enable turnover of the lake. Thus, global warming, associated with a forecasted reduction in precipitation, might reduce the efficiency of hypolimnetic re-oxygenation in Lake Bourget.
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Contributeur : Pascale Talour <>
Soumis le : mercredi 26 août 2009 - 10:55:34
Dernière modification le : mercredi 19 septembre 2018 - 01:25:59

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Charline Giguet-Covex, Fabien Arnaud, Jérôme Poulenard, Dirk Enters, Jean-Louis Reyss, et al.. Sedimentological and geochemical records of past trophic state and hypolimnetic anoxia in large, hard-water Lake Bourget, French Alps. Journal of Paleolimnology, Springer Verlag, 2010, 43, pp.171-190. 〈10.1007/s10933-009-9324-9〉. 〈insu-00411113〉

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