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 southern Finland (about 61 degrees 30'N) were used to examine ecological gradients, species assemblages, and two a priori classifications of lakes. The a priori groupings were: (1) general limnological types (hypertrophic, eutrophic, clear-water meso-eutrophic, mesotrophic, clear-water oligotrophic and brown-water oligotrophic lakes); (2) the traditional Finnish botanical lake type classification. The material was classified with TWIN-SPAN and ordinated with detrended correspondence analysis (DCA) and canonical correspondence analysis (CCA). The species richness and the occurrence of various growth forms vary greatly, being highest 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. There was a relatively good correspondence between the results of multivariate analyses and a priori classifications of lakes, especially in most nutrient-poor and nutrient-rich, as well as in some clear-water lakes. The mesotrophic and eutrophic lake categories, do, however, overlap widely. The ordination diagrams indicate an ecological continuum, and only relative importance of lake classifications. Ninety-four macrophyte species separate into a few main groups along the oligotrophy-eutrophy gradient.