Dominance patterns of planktonic algae in Swedish forest lakes

Dominance patterns of phytoplankton during a late phase ( August) in the seasonal sequence of species are presented from 15 Swedish forest lakes with little or no local anthropogenic impact. The main question to elucidate is if predominance of a small number of species (1 - 3) occurs during a mature phase of the annual succession i.e. if there is an evident competitive exclusion of species in favour of a few dominants. Ten August months per lake are used to illustrate interannual variations caused mainly by differences in weather conditions. In general, 1 - 3 dominant taxa do not reach more than 60% of the total phytoplankton volume. Only lakes exposed to some stress factors exhibit a more pronounced dominance pattern with 1 - 3 species occupying > 80% of the phytoplankton biomass. Stress factors are harsh climate ( arctic lake), light deficiency ( very brown water), acidification and occurrence of the invasive raphidophycean species Gonyostomum semen. The variation in phytoplankton assemblages in relation to environmental variables and years was tested by classification and ordination methods (TWINSPAN, CCA). The consistency of the species/lake groupings and the set of explanatory environmental variables was checked in a discriminant function analysis. Species associations during investigated years and environmental variables show a very good consistency and 75% of the lakes was classified in the same species group irrespective of year, indicating stable species assemblages from summer to summer.