SEDIMENT PHOSPHORUS AND INTERNAL PHOSPHATE FLUX IN THE HYDROELECTRIC RESERVOIR OF BORT-LES-ORGUES, FRANCE

The hydroelectric reservoir of Bort-les-Orgues (France) plays an impor tant role in regional tourism. However this function is under threat b ecause the lake is likely to become eutrophic. External phosphate load is large (about 250 t y(-1) of P are discharged to the lake) but inte rnal flux also has to be taken into account. The potential of P releas e at the sediment-water interface was studied. Not all forms of phosph ate are likely to be released, to know which part of the pool can be m obilized we used sequential extraction. In Bort-les-Orgues about 80% o f P, mostly iron-bound, was mobile. The percentage of calcium-bound P did not exceed 20%. Phosphate and iron fluxes were measured in a labor atory experiment. As long as oxygen concentration remained above 0.5 m g l(-1), no release was observed. Phosphate and iron were released tog ether when oxygen dropped to 0.5 mg l(-1) and redox potential reached - 350 mV at the sediment-water interface. Such conditions were common during summer, and the corresponding P flux was 18 mg m(-2) d(-1) PO4- P. This internal phosphate source should be taken into account in any project of lake restoration. Sequential extraction of phosphate and fl ux measurement are thus necessary tools for a better management of eut rophic water bodies.