AQUATIC MACROPHYTES AND ECOLOGICAL GRADIENTS IN 57 SMALL LAKES IN SOUTHERN FINLAND

Records made in the late 1970s in 57 small (0.5-200 ha) lakes in south ern Finland (about 61 degrees 30'N) were used to examine ecological gr adients, species assemblages, and two a priori classifications of lake s. The a priori groupings were: (1) general limnological types (hypert rophic, eutrophic, clear-water meso-eutrophic, mesotrophic, clear-wate r oligotrophic and brown-water oligotrophic lakes); (2) the traditiona l Finnish botanical lake type classification. The material was classif ied with TWIN-SPAN and ordinated with detrended correspondence analysi s (DCA) and canonical correspondence analysis (CCA). The species richn ess and the occurrence of various growth forms vary greatly, being hig hest in the clear-water meso-eutrophic and eutrophic lakes, and lowest in brown-water oligotrophic waters. For helophytes, however, species richness was highest in eutrophic and hypertrophic lakes. The trophic state (indicated by the specific conductivity and the summer water pH) determines the occurrence of many helophyte species; for hydrophytes the principal determinants are trophic state and water transparency. T here was a relatively good correspondence between the results of multi variate analyses and a priori classifications of lakes, especially in most nutrient-poor and nutrient-rich, as well as in some clear-water l akes. The mesotrophic and eutrophic lake categories, do, however, over lap widely. The ordination diagrams indicate an ecological continuum, and only relative importance of lake classifications. Ninety-four macr ophyte species separate into a few main groups along the oligotrophy-e utrophy gradient.