Effects of lake water level regulation on the dynamics of littoral vegetation in northern Finland

The effects of water level regulation on the dynamics of littoral vegetation were studied in regulated Lake Ontojarvi and unregulated Lake Lentua in northern Finland by using permanent plots. The study was carried out during 1984-1988 and the abundance of plant species was measured yearly. The annual changes were measured by comparing 1988 and the abundance of plant species was measured yearly. The annual changes were measured by comparing the mean dissimilarities (D-m) in species abundance and the diversity (mean number of species, N-sm) between the different research years in the same square. The mean dissimilarities in Lake Ontojarvi and Lake Lentua were 0.238 and 0.297, respectively. The difference between the lakes was not significant. The effect of the increased ecological stress was observed on the littoral zone; the number of species was lower in Lake Ontojarvi than in Lake Lentua. In Lake Ontojarvi, the area of almost permanent submersion (sublittoral) showed higher dissimilarity (D-m) and lower diversity (N-sm) values compared to Lake Lentua. The vegetation in both lakes was well adapted to the disturbance caused by waves and penetrating ice. The diversity and D-m values were lower on the exposed shores in both lakes compared to sheltered shores. The peaty bottoms were the most stable environment, whereas the muddy bottoms were unstable in our research lakes. Generally speaking the vegetation in Lake Ontojarvi is equally stable as in Lake Lentua. Both diversity and dissimilarity values were slightly higher in Lake Lentua. The vegetation in regulated Lake Ontojarvi is well adapted to the ecological disturbance caused by the fluctuating water level.