Can sustainable restoration be effective in reducing internal phosphorus loading? - A long-term study of a small, hypereutrophic lake.

A long-term study of internal phosphorus (P) loading was conducted in a small, dimictic and hypereutrophic lake subjected to sustainable restoration treatments. In order to improve the poor water quality, manifested by intensive cyanobacterial blooms and meta- and hypolimnetic oxygen deficits, P inactivation with an iron sulfate was initially applied and then an innovative 'continuous nitrate treatment' method was introduced. This second method consisted of the treatment of bottom sediments using water with a high concentration of nitrates, directed into the lake's hypolimnion from the lake's two small natural tributaries. Internal P loading was studied using undisturbed sediment cores collected from the profundal and littoral of the lake, both prior to and during the application of restoration over 15 years. A reduction in the internal P loading of more than 80% was observed due to the nitrate treatment. The method mainly affected profundal sediments and to a minor extent the littoral zone and preserved the ecosystem in equilibrium even during a system failure and limited N-NO3 supply, preventing a return to the pre-restoration state. This is a promising method that could be applied to other lakes, although with caution, because it requires verification with reference to other lakes that differ in terms of morphometry and catchment area.