chapter1.3. Transect studies (I, II)

Transect studies were carried out in Lake Ontojärvi and Lake Lentua in 1984–86 (Hellsten et al. 1989, I, II). On the basis of the distribution of different shore types and exposure, 52 transects were selected from Lake Ontojärvi and 53 from Lake Lentua. The entire material consisted of 6,303 quadrats (1 m²) situated at one or two-metre intervals, depending on the homogeneity of the bottom or the vegetation. The bottom substrate was visually classified into six classes: peat, stone, gravel, sand, muddy sand and mud. Macrophyte data were collected along transects by using quadrats (1 m²), but for deeper areas an underwater scope was used. Isoetid-dominated vegetation was analysed with the aid of a bucket equipped with a 3-m handle (Mäkirinta 1978). A rake was also used to determine the species composition of elodeids and water mosses (Maristo 1941). The number of quadrats was 2,717 in Lake Ontojärvi and 3,585 in Lake Lentua, of which 1884 and 1945 were without vegetation in Lake Ontojärvi and in Lake Lentua, respectively. In each quadrat, both species composition and coverage were determined. The coverage percentage data were transformed to the enhanced scale of Braun-Blanquet: + = 1, 1 % = 2, 2% = 3, 3–4 % = 4, 5–10% = 5, 15–25% = 6, 30–45% = 7, 50–75% = 8, 80‐100% = 9. In addition to macrophyte data, coverage by stones (K) was transformed to a 1–9 scale.

In this study the term “macrophyte” was used very broadly to apply to both aquatic species and species present in the shore vegetation (e.g. Carex vegetation), including freshwater sponge (Spongilla lacustris L.). Some plants difficult to identify were treated collectively (e.g. Sparganium sp.). The nomenclature of vascular plants was accordant with Hämet-Ahti et al. (1998) and that of bryophytes with Koponen et al. (1995).

Macrophyte data of separate plots were analysed by using Canonical Correspondence Analysis (CCA) (ter Braak 1986). Classification of the vegetation data was done by TWINSPAN (Hill 1979). The classification was partly concordant with the typology of Mäkirinta (1978, 1989). Discriminant analysis was used to predict the distribution of different vegetation types (Moss et al. 1987).