Multiple techniques for lake restoration

Abstract

Lake Finjasjön is a shallow, eutrophic lake (area 1100 ha, mean depth 3 m, maximum depth 13 m) in southern Sweden. In the 1920s, the lake was clear, with a summer Secci depth of about 2 m. During the first half of the 20th century, untreated sewage from the town polluted the lake. In the 1930s, the lake began to show eutrophic characteristics, and in the 1940s, the cyanobacterium Gloetrichia echinulata dominated in summer. In 1949, the first municipal sewage treatment plant was built. The treatment was, however, insufficient, since the lake continued to be the recipient of the effluent with the result that the occurrence of cyanobacteria became more frequent. Species such as Microcystis and Anabaena caused skin rash and allergic symptoms among swimmers. The phosphorus load on Lake Finjasjön increased as the population of Hässleholm grew and reached a peak value of 65 tons annum⁻¹ in 1965. In 1977, the sewage plant was rebuilt to include chemical flocculation, reducing the total external phosphorus load to about 5 tons annum⁻¹. Despite this improvement the lake did not recover from its chronic and toxic cyanobacterial blooms. Phosphorus-leaking black sediments were identified as the cause of the lake’s failure to recover. Some 60% of lakebed area is covered with sediments on average 3 m thick. Dredging the sediments was started on a large scale in 1987. Five years later, 25% of the sediment area had been removed but the dredging was stopped since phosphorus continued to be released into the water from these areas. In 1992, a new restoration policy, a combination of further reduced external nutrient loading and food-web manipulation was initiated. A constructed wetland (30 ha) to reduce phosphorus and nitrogen was created in connection to the effluent from the sewage treatment plant. Protection zones along the feeder streams into Lake Finjasjön were also established. A cyprinid reduction programme by trawling was carried out between 1992 and 1994. When it started, the fish community was composed of 90–95% bream and roach. After two years of trawling, the ratio between piscivorous and planktivorous fish was 1:1. In 1994 and 1995, the transparency increased due to a considerably reduced biomass of phytoplankton and a radically altered phytoplankton community. The monoculture of Microcystis was replaced by a diverse phytoplankton community. The increased transparency made possible the development of submerged macrophytes such as Elodea, Myriophyllum and Potamogeton. The internal loading of phosphorus decreased dramatically in 1994 and 1995, possibly as a result of reduced sedimentation of phytoplankton.