From Saprotrophic to Clear Water Status: the Restoration Path of a Degraded Urban Lake

Abstract

The study was conducted on Długie Lake in Olsztyn, which for 20 years since the mid-1950s served as a domestic and storm wastewater receiver, which led to its complete degradation. The discontinuation of wastewater inflow in 1976 caused a change in the trophic state from saprotrophic to hypertrophic. Evident improvement of water quality was possible only after the implementation of proper restoration techniques. Długie Lake was subjected to artificial aeration with thermal destratification (1987)(1988)(1989)(1990)(1991)(1992)(1993)(1994)(1995)(1996)(1997)(1998)(1999)(2000). After all opportunities to improve water quality in the lake by artificial aeration (low phosphorus sorption capacity of sediment) had been exhausted, it was decided that a phosphorus inactivation method using the coagulant PAX 18 be used (2001)(2002)(2003). Before restoration, the nutrient concentration in the near-bottom water layer of Długie Lake was very high at 22.9 mg TN L −1 and 3.50 mg TP L −1 . The average amount of chlorophyll a was ca. 200 μg L −1 , and the Secchi disc visibility did not exceed 1 m. In 2017, 14 years after termination of the lake restoration process, the total phosphorus concentration at the bottom was 0.21 mg P L −1 on average, and the total nitrogen was 1.5 mg N L −1 . The mean transparency of the water oscillated around approximately 5 m, and the amount of chlorophyll a was 1.86 μg L −1 . Studies have shown that the most important step in reclamation is to prevent pollutants from entering the lake to the maximum extent possible and to use a combination of several reclamation methods as a matter of good practice. Stable environmental conditions have developed in the lake, and the values of chlorophyll a and the Secchi disk visibility indicate that the lake has reached a mesotrophic state.

Behavior of Aluminum Compounds in Soft-Water Lakes Subjected to Experimental Reclamation with Polyaluminum Chloride

“…Even if a reduction in fertilizer levels is achieved, the effectiveness of these practices is rarely sufficient for eliminating the external load to a level that is acceptable for lake ecosystems; additionally, improvements in water quality are generally slow (Kajak 2001). Therefore, to increase the efficiency of lake restoration, various treatments have been applied, including technical measures (hypolimnetic withdrawal, artificial aeration, dredging, flushing, and capping- Olszewski 1961;Dunalska et al 2007;Siwek et al 2018;Łopata et al 2020), chemical measures (phosphorus inactivation, bottom sediment biochemical oxidation, and the use of anti-cyanobacteria agents -Ripl 1986;Wiśniewski et al 2010;Grochowska et al 2019;Chróst 2017), and biological measures (ichthyofauna structure steering, bioseston removal, macrophyte planting, biofiltration mollusk introduction, and the promotion of beneficial microorganisms- Shapiro and Wright 1984;Gołdyn 2000;Jeppesen et al 2012).…”

Phosphorus in the shallow, urban lake subjected to restoration - case study of Lake Domowe Duże in Szczytno

“…In subsequent months or growing seasons after the end of dosing, lake response scenarios may be different. With a high degree of isolation of the lake from external sources of pollution and a large depth, the symptoms of recurrent eutrophication may appear only after over a dozen years (Grochowska et al 2019). In theory, the more the lake is exposed to the supply of biogenic matter from the catchment, the faster the return of excessive production of indigenous organic matter by the regenerating algae populations.…”

Sorption Properties of the Bottom Sediment of a Lake Restored by Phosphorus Inactivation Method 15 Years after the Termination of Lake Restoration Procedures

“…Długie Lake has been massively polluted by raw sewage input, which occurred in the second half of 20th century for twenty years . As the result of that pollution, the lake was transformed into a saprotrophic lake type [5,6,29]. The sewage inflow cut off (in 1976) did not result in the improvement of lake water quality, because of the internal loading phenomenon taking place in the lake.…”

“…Water bodies located in the direct vicinity of human settlements have very often been the victims of avalanche degradation resulting from the discharge of untreated municipal or industrial sewage. There have been many cases in the world of urban lakes transforming into hypertrophic and even saprotrophic water bodies [3][4][5][6][7].…”