Changes of emergent aquatic macrophyte cover in seven large boreal lakes in Finland with special reference to water level regulation

Abstract

The latest studies on the development of aquatic vegetation in Finland have mainly focused on relatively small lakes. Research on emergent macrophytes in large lakes poses challenges as the habitats as well as the conditions of the lakes vary. Black and white aerial photographs from 1947–1963 and colour, mainly infrared aerial photographs from 1996–2000 were used to study quantitative vegetation changes in seven large (80–1116 km2) lakes in Finland: Kemijärvi, Unnukka, Kallavesi, Päijänne, Näsijärvi, Pyhäjärvi and Vanajavesi. Vegetation changes were evaluated by comparing the situation before large-scale water level regulation and eutrophication to the current situation. Quantitative changes in emergent vegetation were frequently noticeable. Vegetation had decreased by 31–93% in three study lakes due to the mean water level rise and increased by 49–73% in three study lakes due to the spring flood reduction and suitable surrounding soils. The cover changes were in general relatively uniform between different study sites in each of the study lakes. The regulated and unregulated situations differed statistically from each other. The results point out the crucial impact of changing water levels and the significance of the surrounding soils to the cover of emergent aquatic vegetation.