| Abstract: | Redevelopment and Restoration Measures for Lake Elsterstausee (Leipzig) Lake Elsterstausee is an artificial lake in the south of the town Leipzig. It was built in 1933, has an actual area of 50 ha, and an average depth of 2 m. Between 1970 and 1991, the lake was used for intensive fishery. Because of fish feeding and supplying the lake with highly nutrient loaded water from river Elster the waterbody was in a polytrophic state during this time. Secchi depth seldom exceeded 0.4 m, but pHvalues often exceeded 9. Therefore and because of ammonia concentrations between 1.5 and 4.0 mg/L fish kills often occured. Lake Elsterstausee had no usability as bath waterbody although it was orginally built for this purpose. In 1991, a complete programme for redevelopment and res-toration of the lake started. The main measures were the total exchange of trophic loaded water and the only supply with treated groundwater, the drastic decrease of fish populations – in 1991 140 tons of fish (90% carps) were removed from the lake, – the development of healthy wild fish populations and of a eutrophication diminishing food chain, and the development of a reed zone almost around the lake. The groundwater treatment was steadily improved between 1992 and 1996. Actually, the following optimized system works: Drained groundwater from the surface lignite mine Zwenkau (pH = 6.95) and percolating water from the heaps (pH = 3.01) are mixed in the proportion 1:1 after intensive aeration. The water flows through a ditch that is 1 600 m long and has a performance up to 300 m3/h. In the well-diversified ditch, up to 90% of the iron is removed by precipitation and filtration effect of submerged macrophytes and filamentous algae. In the same time, deacidification occurs primarily by sulfate reduction in the sediment. Meanwhile, the water has a pH of 4.64 immediately after mixing, this value enhances up to the end of the flowing distance to 7.3. Internal restoration of the lake started in November 1991 with the total removal of loaded water and of almost all carps and white fishes. In 1993, 4 000 carnivore fishes (zanders, pikes, welses) were given into the lake to keep white fish popu-lations on a low level. Additionally, every year between 5 and 7 kg of water fleas (Daphnia sp.) were added. By means of this initial biomanipulation, food chain in lake Elsterstausee was stabilized. As results of the above mentioned measures, nutrient content in the waterbody decreased and secchi depth increased steadily. Between 1991 and 1997, trophy degree according to Klapper developed from 5 (hypertrophic) to 3 (eutrophic). |
