Multiple techniques for lake restoration

Lake Finjasjon 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, w ith a summer Secci depth of about 2 m. During the first half of the 20th ce ntury, untreated sewage from the town polluted the lake. In the 1930s, the lake began to show eutrophic characteristics, and in the 1940s, the cyanoba cterium Gloetrichia echinulata dominated in summer. In 1949, the first muni cipal sewage treatment plant was built. The treatment was, however, insuffi cient, since the lake continued to be the recipient of the effluent with th e result that the occurrence of cyanobacteria became more frequent. Species such as Microcystis and Anabaena caused skin rash and allergic symptoms am ong swimmers. The phosphorus load on Lake Finjasjon increased as the popula tion of Hassleholm grew and reached a peak value of 65 tons annum(-)1 in 19 65. In 1977, the sewage plant was rebuilt to include chemical flocculation, reducing the total external phosphorus load to about 5 tons annum(-)1. Des pite this improvement the lake did not recover from its chronic and toxic c yanobacterial blooms. Phosphorus-leaking black sediments were identified as the cause of the lake's failure to recover. Some 60% of lakebed area is co vered with sediments on average 3 m thick. Dredging the sediments was start ed 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 polic y, a combination of further reduced external nutrient loading and food-web manipulation was initiated. A constructed wetland (30 ha) to reduce phospho rus and nitrogen was created in connection to the effluent from the sewage treatment plant. Protection zones along the feeder streams into Lake Finjas jon were also established. A cyprinid reduction programme by trawling was c arried out between 1992 and 1994. When it started, the fish community was c omposed of 90-95% bream and roach. After two years of trawling, the ratio b etween piscivorous and planktivorous fish was 1:1. In 1994 and 1995, the tr ansparency increased due to a considerably reduced biomass of phytoplankton and a radically altered phytoplankton community. The monoculture of Microc ystis was replaced by a diverse phytoplankton community. The increased tran sparency made possible the development of submerged macrophytes such as Elo dea, Myriophyllum and Potamogeton. The internal loading of phosphorus decre ased dramatically in 1994 and 1995, possibly as a result of reduced sedimen tation of phytoplankton.