Changes in the flora and vegetation of 113 Finnish lakes during 40 years

The 113 Finnish lakes studied by Lauri Maristo in the 1930's were reinvestigated in 1980—1985. The pH values of those lakes had not clearly altered, but transparency had, as a rule, diminished. The number of aquatics had generally increased, but some clearwater aquatics, such as Potamogetón praelongus Wulfen and Myriophyllum alterniflorum DC. had become rarer. Two species had disappeared, 17 species declined, and 57 species had become more frequent. Six species were found as newcomers. These floristic changes were probably caused by eutrophication due to leaching of agricultural and forest fertilizers or effluents of industry. The greatest vegetation changes were the decline of Schoenoplectus lacustris (L.) Palla stands and increase of Phragmites australis (Cav.) Trin. ex Steudel belts. In some lakes aquatic mosses currently form dense carpets. The succession caused by eutrophication can be seen as a development from oligotrophic lake types towards eutrophic ones. The most remarkable changes had taken place in Potamogeton-type, Potamogeton filiformis-Chara-type and Typha-Alisma-type lakes. Presently six lakes can be assigned to different botanical lake types than in Maristo's classification.