Restoration of flow-through lakes - Theory and practice

The attempts to restore Lake Wolsztynskie, by direct dosing to the water iron sulphate (PIX 113) and chloride polyaluminium (PAX 18) in 2005-2006, and then iron chloride to the lake's bottom sediments in 2012-2013, resulted in only short-lasting improvement of water quality . In 2016, the physicochemical and biological parameters have shown that the lake should be classified into eutrophic and even hypertrophic, primarily because of the high concentrations of phosphorus and chlorophyll a as well as low visibility. Although in theory, the methods of restoration were selected correctly, in practice, the improvement of water quality in a flow-through lake without eliminating the influx of water from allochthonous sources of pollution was impossible. The total load of nutrients arriving in Lake Wolsztynskie from the direct catchment, that is point and atmospheric sources, was 1877 kg P and 17 137 kg N , respectively. At the moment, the actual load of phosphorus in Lake Wolsztynskie is about 12-fold higher than the one deemed as tolerable by the ecosystem. About 86% of the phosphorus load is delivered from the main river (Dojca River), thus, elimination of this source of pollution is the necessary condition for attaining lasting improvement of the water quality in this lake. Analyzing the hydrological conditions, landscape relief as well as economic and environmental conditions, a possible solution of cutting the Dojca River waters from Lake Wolsztynskie was given. (C) 2018 European Regional Centre for Ecohydrology of the Polish Academy of Sciences. Published by Elsevier B.V. All rights reserved.