EFFECTS OF HYPOLIMNETIC WATER AERATION AND IRON-PHOSPHATE PRECIPITATION ON THE TROPHIC LEVEL OF LAKE KRUPUNDER

Lake Krupunder is a small, glacial, stratified lake. Before restoration it was classified as eutrophic to polytrophic. Stepwise the lake was restored to a mesotrophic status: 1. In April 1990 a deep water aeration plant was installed. Although the entire hypolimnion was sufficiently supplied with oxygen the trophic level remained high because of substochiometric natural iron concentrations. 2. In September 1991 a technical iron-phosphate precipitation was carried out, dispensing 1.8 t of iron-chloride-sulfate ('Ferri floc') into the hypolimnion using the aerator. Until July 1992 the chlorophyll-a concentration decreased by 90%/77% (maximum/mean value).