Comprehensive approach to restoring urban recreational reservoirs. Part 1 – Reduction of nutrient loading through low-cost and highly effective ecohydrological measures

Abstract

The study presented in this paper aimed to improve the water quality of a small, stormwater-fed urban river and a cascade of ponds and reservoirs (Łódź, Poland) that are intensively used for recreation. We tested a combination of conventional restoration methods (bottom sediment removal and biomanipulation) and comprehensive ecohydrological restoration methods (hybrid systems, sequential sedimentation-biofiltration systems (SSBS), floating islands, landform-adjusted shoreline vegetation and plant harvesting). As a result of sediment removal (9795 m3, cost: €80761), almost 12.5 tons of nitrogen and 197 kg of phosphorous were eliminated from the reservoirs. The hybrid systems and SSBSs reduced the nutrient transport and suspended solid levels by 49%–98.5% and 89.6%–98.6%, respectively, from the upstream catchments to the reservoirs (investment cost: €63929). Generally, restoration improved most of the water quality indicators, although an increase in the phosphate concentration was observed from 0.06 to 0.17 mg L−1 before restoration to 0.13–0.28 mg L−1 immediately after restoration as a result of disturbances resulting from bottom sediment removal. Four years after restoration, the concentrations of the total forms of nutrients and ammonium were still considerably lower than before restoration. The number of summer days with microcystins and with chlorophyll a concentrations above the WHO limits for recreational waters decreased. However, there was a gradual decline in the physicochemical parameters of the water due to the local impacts of human use (swimming and duck and fish feeding), rather than from other external or internal loads, which were effectively controlled.