The algal record from Lake Ringsjon covers a period of more than 100 years.
Towards the end of the 19th century, the lake had a well-functioning comme
rcial fishery, when a brownish mess began to appear in the water, clogging-
up the fishing nets and making fishing difficult. This was the first record
of algal problems in the lake. Following complaints from the fishermen, th
e algal flora was investigated and the mess was stated to be a mass develop
ment of diatoms belonging to the genus Melosira ( Aulacoseira). Diatom maxi
ma then appeared regularly in spring and autumn, whereas blue-green algae o
nly occurred occasionally and in low abundances. Between 1900 and 1950, nut
rient concentrations slowly increased in the lake. Algal blooms of Anabaena
lemmermannii and Gloeotrichia echinulata began to appear in summer. Betwee
n 1960 and 1980, the lake developed into a hypertrophic status with extensi
ve blooms of blue-green algae from May to October, including high biomasses
of mainly Microcystis spp. This severe pollution started with increased us
age of the lake, increased tourism and recreation, intensified farming with
the introduction of artificial fertilization, and also the diversion of se
wage water from a sewage treatment plant. In 1968, cattle death was reporte
d and in 1981, algal toxicity was verified by mouse bioassay. After a perio
d of extremely poor water quality, the lake began to show signs of improvem
ent as a result of the nutrient and fish reduction programmes. The very hea
vy blooms of blue-green algae decreased and the duration of blooms became s
horter. The Microcystis species were to a certain extent replaced by Anabae
na and Aphanizomenon species, and biodiversity of algae increased. With res
pect to the algal community, the lake has improved considerably, but is sti
ll eutrophic and suffers summer blooms of toxic blue-green algae.