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

摘要

The study was conducted on Dugie Lake in Olsztyn, which for 20years 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. Dugie Lake was subjected to artificial aeration with thermal destratification (1987-2000). 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 (2001-2003). Before restoration, the nutrient concentration in the near-bottom water layer of Dugie Lake was very high at 22.9mg TN L-1 and 3.50mg TP L-1. The average amount of chlorophyll a was ca. 200gL(-1), and the Secchi disc visibility did not exceed 1m. In 2017, 14years after termination of the lake restoration process, the total phosphorus concentration at the bottom was 0.21mg P L-1 on average, and the total nitrogen was 1.5mgNL(-1). The mean transparency of the water oscillated around approximately 5m, and the amount of chlorophyll a was 1.86gL(-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.