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.