Modifications in the trophic state of an urban lake, restored by different methods

The rapid acceleration of eutrophication calls for effective methods to reverse or at least decelerate this process and its consequences, such as phytoplankton blooms, deterioration of water transparency, loss of oxygen balance as well as nitrogen and phosphorus internal loading from bottom sediments. Many different restoration methods have already implemented in order to improve the quality of lake water in Poland and around the world. Science reports on lake restoration mostly present short-term results achieved with one method, sometimes with a combination of few technologies, methods. Noteworthy is the scarcity of longitudinal observations and descriptions of persisting changes in environmental conditions and trophic status after a lake restoration process. The aim of the study was to present long-term changes in the trophic status of an urban lake which were been achieved through artificial aeration with thermal destratification and phosphorus inactivation, based on water quality parameters and an analysis of biotic and abiotic conditions prevailing in this lake. The study was conducted on Długie Lake, situated in the town of Olsztyn. This lake had acted as a sink of raw domestic sewage (400 m3 day-1) for 20 years. After preliminary conservation operations, the lake was restored by the artificial circulation and phosphorus inactivation methods. Prior to the restoration effort, the lake had been classified as hypertrophic, with an average TSI index value of 79. The average chlorophyll a concentration was 63 μg l-1 and water transparency did not exceed 0.3 m. The implementation of two restoration methods improved the trophic level and the lake became mesotrophic. The improvement seems permanent, as the mean TSI index is 50 ten years since the termination of the restoration procedures.