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 vege tation were studied in regulated Lake Ontojarvi and unregulated Lake L entua in northern Finland by using permanent plots. The study was carr ied out during 1984-1988 and the abundance of plant species was measur ed yearly. The annual changes were measured by comparing 1988 and the abundance of plant species was measured yearly. The annual changes wer e measured by comparing the mean dissimilarities (D-m) in species abun dance and the diversity (mean number of species, N-sm) between the dif ferent 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 i ncreased ecological stress was observed on the littoral zone; the numb er of species was lower in Lake Ontojarvi than in Lake Lentua. In Lake Ontojarvi, the area of almost permanent submersion (sublittoral) show ed higher dissimilarity (D-m) and lower diversity (N-sm) values compar ed to Lake Lentua. The vegetation in both lakes was well adapted to th e disturbance caused by waves and penetrating ice. The diversity and D -m values were lower on the exposed shores in both lakes compared to s heltered shores. The peaty bottoms were the most stable environment, w hereas the muddy bottoms were unstable in our research lakes. Generall y speaking the vegetation in Lake Ontojarvi is equally stable as in La ke Lentua. Both diversity and dissimilarity values were slightly highe r in Lake Lentua. The vegetation in regulated Lake Ontojarvi is well a dapted to the ecological disturbance caused by the fluctuating water l evel.