How durable is the improvement of environmental conditions in a lake after the termination of restoration treatments

The study was conducted on Lake Długie,a water body lying in Olsztyn, a town situated in northeastern Poland. From 1956 to 1976raw wastewater was discharged into Lake Długie and therefore heavy pollution was the reason for the total degradation of this lake. The concentrations of nutrients were very high in the bottom water layer (TN 22.89 mg L−1, TP 3.50 mg L−1). The mean concentration of chlorophyll an exceeded 60 μg L−1 on average and Secchi disc visibility was less than 1 m. The discontinuation of sewage discharge into the lake caused the saprotrophic state to transform into hypertrophy. Further improvement of the environmental conditions in the lake was possible only after the implementation of an appropriate restoration method. The restoration procedure of this lake began in 1987. The first stage involved an artificial aeration method, and the second stage consisted of a phosphorus inactivation method. The applied restoration methods led to the improvement of Lake Długie's water quality. Even ten years after the restoration treatments ended, TP concentrations did not exceed 0.350 mg L−1, and TN - 3.0 mg L−1. Chlorophyll a contents oscillated in the range of 5-10 μg L−1, and Secchi disc visibility amounted to c.a. 4.5 m.