Annual budget of chemical elements in a eutrophic lake, Aydat lake (Puy-de-Dome), France

An annual survey of the chemical composition both in dissolved and particulate phases of a eutrophic lake was performed from September 1995 to October 1996. All major elements and many trace elements were analyzed in the tributary and in the water column. Element fluxes were determined via sediment traps. Several sediment cores and interstitial water samples were also analyzed. From these data, a method to calculate element budgets was derived. Chemical elements can be classified into five main groups: (1) Low solubility elements are at or below detection limits in dissolved fractions and exhibit a nearly constant ratio to Al in all solid phases: Al, Ti, REE. Be, Th, Zr. (2) High solubility elements occur essentially in the dissolved phase: alkali and alkaline earth, Cl, B. Within this group, the cations exhibit some reactivity: they are removed from solution in the surface layer of the lake and released at the water-sediment interface or within the sediment. (3) biogenic elements are efficiently trapped within the lake, with about 70-75% of Si and P supplied to the lake in the dissolved form and only 25 to 30% exported as dissolved species. (4) Elements with different oxidation states (Fe, Mn, Co) show an extensive recycling within the lake. Inputs represent only 20-50% of the amount released annually into solution. (5) C and N inputs exceed outputs, suggesting that a significant amount of nitrogen escapes from the lake as N-2 and some C may escape as CO2.