Treatment of Overgrown Shallow Lakes Through Macrophyte Control: The Case Study of Lake Hornborga, Sweden

Abstract

A great number of lowered shallow lakes have become overgrown by perennial macrophytes. Because their ornithological and other environmental values have become lost, efforts are made to restore them. The most serious damage on the ecosystems is caused by the development of a rhizome and root felt and the accumulation of huge masses of coarse detritus. Removal of macrophyte detritus and biomass (including the root-felt) necessitates access to a variety of technical equipment in order to clean the bottom before the water level is raised in a lowered, overgrown lake. One such example is Lake Hornborga (30 km(2)), before degradation following water-level lowering and drainage, once ranked as one of the most valuable waterfowl lakes in northwestern Europe. After drainage the lake had become almost completely overgrown by reed and sedge. After the Swedish Government had decided to investigate the possibilities to restore the lake, limno-logical, ornithological and other field studies started in 1967. Ecotechnical restoration methods and prognoses for a restored lake were presented in 1972. The project goal was to transform the reed areas to open water (about 11 km(2)) and to keep the sedge-covered part (about 18 km(2)) for emergent vegetation. In 1982, the Parliament unanimously accepted the restoration plan including a raise of the water level by 1.4 m. The implementation of the plan was, however, delayed by drawn-out discussions about dikes and water depths most suitable for birds. Not until 1995 was the water level finally raised by 0.8 m. The long-term development of the ecosystem is being documented.