Can we restore badly degraded urban lakes?

•We examined the effect of long-term inflow of sewage on the TSI of urban lakes.•Catchments and sub-catchments of the individual lakes were delineated.•Phosphorus load was determined using Vollenweider’s hydrological model.•Actual load of phosphorus was 1000% higher than the critical one.•The value of TSITP was extremely high—103.•We recommend the innovative, comprehensive restoration actions to improve water quality.

We investigated the effect of long-term inflow of untreated municipal sewage on the trophic status of the four urban lakes (Mielenko, Karczemne, Klasztorne Małe, Klasztorne Duże) connected by the river into one system. We delineated the catchments and sub-catchments of the individual lakes, to determine their hydrological parameters as well as the physicochemical properties of water in the lakes, their surface inflow and outflow as well as water discharged to lakes from stormwater and drainage ditches. Results of the TSI demonstrated that Karczemne Lake and Klasztorne Małe Lake were the most polluted of the analyzed lakes. The value of TSITP was extremely high: 103 and 85, respectively. The biggest hazard for the quality of waters in the lakes was: angling in Mielenko Lake (44.8%); point sources (42.1%) and the inflow from Mielenko Lake (33.5%) to Karczemne Lake; the inflow from Karczemne Lake (60.4%) and point sources (35.9%) for Klasztorne Małe Lake; and the inflow from Klasztorne Małe Lake (95.2%) to Klasztorne Duże Lake. According to Vollenweider’s hydraulic model, the actual phosphorus load introduced to Mielenko Lake is double the critical load (7.8 kg y−1); in Karczemne and Klasztorne Małe Lakes, the critical load is exceeded by 700% (12.1 kg y−1 and 37.8 kg y−1, respectively) and in Klasztorne Duże Lake, the actual load is 1000% higher than the critical one (the critical load—96.0 kg y−1). With such high phosphorus concentrations, natural inhibition of the lake’s severe degradation is impossible. To achieve permanent water quality improvement we recommend the comprehensive protective and restoration actions: constructing pipelines and perimeter drainage ditches with waterproof walls sealed with PVC vinyl sheet piling along the entire lake system; removal of the bottom sediments from Karczemne Lake and some of the bottom sediments from Klasztorne Małe Lake; phosphorus inactivation in the lakes Mielenko, Karczemne and Klasztorne Małe, along with supportive actions in the form of biomanipulation.