chapter1.4. Permanent plots (III)

The study was done in Lake Ontojärvi and Lake Lentua in 1984–88. The field data covered the species frequency and the abundance of aquatic macrophytes and bryophytes, which were sampled yearly from August till the beginning of October in 1984 – 88 (see paper III for details). The sampling quadrats (0.5 by 0.5 m) were marked permanently by steel rods in 16 different shore areas selected on the basis of the results of the transects studies (I). The number of analysed quadrats varied yearly as follows:

The observations were made by setting a 0.25 m² steel frame divided into twenty-five 1 dm² wire mesh squares on the area bordered by rods. The abundance values were calculated from the presence of each species in the different sub-squares (0, 4, 8,...,100 %) and the frequency as a mean frequency of all the quadrats observed. Only the quadrats with continuous time series were included in the analysis.

The annual changes in vegetation were measured by comparing the dissimilarities in species composition between the different research years within each square (Nilsson & Keddy 1988). The mean values of all the observations on a given quadrat were used as an indicator of dynamics (D_o or D_l). Dissimilarity values were also calculated between the lakes (D_o/l). The total number (N_sto or N_stl) of observed species in the lake and the mean number (N_sm) of observed species on a given quadrat were used as an indicator of diversity.