Multiple techniques for lake restoration

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 annum1 in 1965. In 1977, the sewage plant was rebuilt to include chemical flocculation, reducing the total external phosphorus load to about 5 tons annum1. 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