ORGANIC-MATTER MINERALIZATION IN THE PORE-WATER OF A EUTROPHIC LAKE (AYDAT LAKE, PUY-DE-DOME, FRANCE)

The chemical composition of the pore water from the sediment of a eutr ophic lake is dominated by high concentrations of total dissolved CO2 (up to 12 mM), reduced soluble iron (up to 2 mM) and dissolved silica (up to 1 mM). The pH lies within the range of 6.70 +/- 0.02; this refl ects that the pore water is efficiently buffered by the CO2 acid/base system. This composition is directly related to the main diagenetic re actions which drive the organic matter mineralization i.e. methanogene sis and ferric oxides reduction. Other geochemical processes are of mi nor importance. A stoichiometric model based on these main reactions a llow us: (i) to define a general formula for the organic matter which is close to Redfield's one for the C:N ratio, while the C:P ratio is m uch higher owing to a probable adsorption of phosphorous onto reactive surfaces of the solid and due to heterotrophic bacterial uptake; (ii) to calculate a global first order kinetic constant which drives the o rgano-polymers breakdown. Due to the strong influence on the trophic s tatus of the lake caused by an excess of phosphate, special attention is devoted to this species. We show that the sediment-water interface is a source of dissolved phosphate when the hypolimnion is anoxic betw een May and November. This contribution represents about 17% of the ri ver input and should be taken into account in any attempt toward lake restoration.