Restoration of Lakes Through Sediment Removal, with Case Studies from Lakes Trummen, Sweden and Vajgar, Czech Republic

Abstract

For removal of nutrient-rich sediment from polluted and irreversibly damaged lakes, suction dredgers have been constructed to meet the demands formulated by limnologists. This limnological-technological cooperation has aimed at the precision dredging of sediment strata responsible for the internal nutrient loading of lake ecosystems. Suction dredging is mainly restricted to small lakes of high environmental value. Prior to in-lake measures, the external loading of an ecosystem has to be brought under control. Whenever possible pumped sediment ought to be used as fertiliser. The heavily polluted Lake Trummen, dredged in 1970-1971, was turned into an environmental asset. The cyanobacterial blooms (summer transparency 15-20 cm) disappeared and were replaced by a plankton community rich in species. The ecosystem as a whole recovered to a functional unit, characterised by a balance between production and decomposition. In the urbanised area of Vaxjo, Lake Trummen became available for a variety of recreation activities. The ecosystem conditions are continuously monitored. As in Lake Trummen, Lake Vajgar was in a degraded condition characterised by heavy blooms of cyanobacteria caused by intensive internal phosphorus loading. After unsatisfactory experiments with a suction dredger available on the market, an automatically-controlled, precision dredger was designed, constructed and used to remove the defined top sediment layer in 1991-1992. The Vajgar ecosystem immediately changed from being a source to functioning as a trap for phosphorus. However, because the external loading from the catchment area remained high, Lake Vajgar again gradually developed conditions favourable for cyanobacteria.