Qualitative and quantitative investigations of phytoplankton in Lake Ringsjon, Scania, Sweden

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.