Can we stop the degradation of lakes? Innovative approaches in lake restoration

We studied an urban, flow-through lake with a large total catchment area, in the context of implementing complex protection and restoration methods in the shoreline zone and within the lake’s basin. The study was found that the annual total load of nutrients was 4 688 kg P and 50 652 kg N, which – when converted to the lake’s unit area – corresponded to 8.2 g P m−2 y−1 and 88.4 g N m−2 y−1. Taking into account the surface inflows alone, the current actual phosphorus load of the lake is over 20-fold higher than the permissible one. An additional source of phosphorus is the lake’s bottom sediments, which is confirmed by the negative balance of this element (−2 882.5 kg P y−1). Despite the polymictic type of the lake, oxygen deficits may appear near its bottom, which intensify the internal enrichment process. At present, the primary production in the lake is extremely large, which is indicated by a very high oxygen content (the maximum value of 230%) coinciding with a very high pH value (around 9.7 at both sites) as well as a high chlorophyll a and seston content (158 mg m−3 and 30.6 mg dm−3 at sites S-1, 176 mg m−3 and 33.4 mg dm−3 at sites S-2, respectively), and a low visibility of Secchi disc (around 0.5 m at both sites). At such advanced trophy of the lake and high content of phosphorus (TSI(TP) = 100), it is impossible to stop the rapidly progressing degradation of this water body without man’s intervention. The only possible way to eliminate allochthonous sources of pollutants is by constructing a system of pipelines and waterproof walls for transporting pollutants outside the lake’s catchment. Selection of adequate technological solutions must be preceded by a detailed analysis of hydrological and morphometric properties of the water body, parameters of its catchment and the potential use of lake water in the context of economic calculations. An innovative approach may involve self-oxygenation of lake waters by supplying some of a lake’s surface inflow water to the lake’s bottom. This method can assist in improving oxygen conditions, which to a large extent will reduce the internal supply. It can also be a supporting method in the process of phosphorus inactivation with the use of safe ferrous coagulants, whose efficiency of phosphorus precipitation and immobilization depends on the oxygen content.