Dynamics of Phosphorus in sediments of a naturally acidic lake

Abstract

The mechanisms which controls the fixation and/or release of P in sediment of an extremely acidic lake (pH = 2.0 to 3.0) and its response to the influence of eutrophic urban waste water were investigated. The results, in the chemical composition, in the mineralogy of the sediment and in the material as obtained from sediment traps, show that the lake sediments are mainly volcanic material reflecting volcanic features of the basin. The sedimentation rate calculated for the lake (2.510−2 mg m−2 day−1) was higher than that observed in other similar glacial lakes in both Andean Patagonia and also elsewhere in the world. The Total Phosphorus concentration in sediments was higher than figures reported by other authors for mining acid lakes, and the main fraction of P was found associated with organic matter. There was no control by Fe or Al on P, because both are in solution at pH < 3.0. It was concluded that changes in the natural input of nutrients (derivatives of Copahue volcano fluid, the discharge of sewage, or basin run-off) are responsible for a high concentration of SRP and N-NH4+ in the lake. Laboratory experiments showed that sediments have no ability to retain phosphorus and a continuous release of P from the sediments into the water column was observed. The assays where the pH was artificially increased showed that the P still remains in solution until at least pH 7.0. It was concluded that changes in the natural input of nutrients due to: 1) the volcanic fluids, 2) the increase in sewage charges, or 3) surface runoff upstream, maintain a high trophic state with high concentrations of dissolved P and N-NH4+, although the threshold of neutral pH in the lake is exceeded. This study will enable a better understanding about of the mechanism of release/fixation of phosphorus in acidic sediments in order to assist in making decisions regarding the conservation and management of this natural environment.