engleski

Seasonal dynamics of molybdenum in a permanently anoxic lake

Mo is an essential enzyme cofactor in nitrogenase and nitrate reductase, necessary to nitrogen assimilation by phytoplankton (cyanobacteria and microalgae respectively). Mo is also considered in sediment records as a proxy of anoxic paleo-environments where sulphide builds up and precipitate molybdenum. In order to understand dissolved Mo removal mechanisms in freshwater bodies we investigated the lake Pavin water column (Puy-de-Dome, France). Lake Pavin is meromictic and at the bottom, about 30 meters out of 91 meters remain permanently anoxic and constitutes a natural “ test tube” . The anoxic bottom layer overlaps a large density gradient layer (mesolimnion) where maximum reduced sulphur species concentrations are observed, and a more homogeneous layer where reduced compounds accumulate (monimolimnion). The Mo size fraction was studied in the meso- and monimolimnion and compared to dissolved organic carbon (DOC) behaviour. Previous studies showed clearly Mo removal within the monimolimnion and chemical association of dissolved Mo with DOC. In this work, we demonstrate that this association is effective with large reactive organic polymers (> 1kD). Monthly sampling of rivers input, water column solutions and solid samples (sediment traps) allowed to quantify Mo and nitrogen lake annual budget. Seasonal evolution of vertical profiles for dissolved Mo, nitrate, ammonium, sulphate and sulfide are compared and related to nitrate assimilation in the epilimnion and iron sulphide formation in the mesolimnion.

molybdenum; anixic lake

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