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Summary
With phosphorus concentrations of 300 mg P · m−3 and primary production rates of about 400g C · m2 · year−1 the Baldeggersee (Switzerland) is highly eutrophic. The lake is meromictic and shows also during winter time below 40 to 66 m anoxic conditions. In 1976, 11.3 tons phosphorus entered the lake, while the critical P-loading is 2 tons per year. Since the P-loading can be decreased by external measure only to about 6 tons per year, the lake was restored by means of destratification and oxygenation of the hypolimnion.
In winter 1982 the lake was destratified by bubbling compressed air in a depth of 60 m (200 n m3 · h−1). Within two weeks the whole lake circulated and for a first time since 1958 4 mg O2 · l−1 could be measured down to 66 m. In summer 1982 the technical device for oxygenation of the hypolimnion with small bubbles of pure oxygen gas was tested. Starting from May 1983, 4.5 tons O2 per day were successfully brought into the hypolimnion. The reduced substances (NH4+, S2-, CH4, Fe2+) disappeared and the anoxic conditions changed to aerobic ones. The six oxygen diffusore were placed at depths of 40 to 60 m and no extremely high oxygen concentrations were observed, indicating a good horizontal and vertical transport of oxygen. By means of artificial destratification in winter and oxygenation of the hypolimnion in summer the deep Baldeggersee exhibited again aerobic conditions. Any apparent toxicity for fish could not be detected either through chemical measurements (O2 oversaturation, N2, NO2−2) or by observation of the biota.