Occurrence of Limnothrix redekei Van Goor in Estonian lakes

Abstract

The occurrence of Limnothrix redekei in Estonian lakes was documented for the first time relatively recently, at the end of the 1930s. The distribution area expanded rapidly. Nowadays this species is discovered in 112 Estonian lakes. It is assumed that L. redekei has spread already into approximately 1/3 of all Estonian lakes. It is common in shallow unstratified or weakly stratified lakes, but also vice versa, in smaller (< 15 ha), deeper (avg. 10 m). strongly stratified lakes. Modern statistical technique (CCA) allows to study ordination patterns depending on several abiotic conditions, but it is hard to discover one single discriminating parameter. Comparing the lake groups with and without L. redekei, the 'redekei lakes' were characterized by significantly higher total phosphorus content, pH, and total alkalinity, and significantly lower water transparency. The same tendency of differences occurred within 'redekei lakes' if they were arranged by the biomass of L. redekei. The decrease of water transparency and increase of the other mentioned parameters is common to eutrophication. L. redekei seems to prefer slightly coloured hard-water eutrophic, mixotrophic (dyseutrophic) lakes. It is very abundant also in originally soft-water small and deep lakes, which due to pollution have become hypertrophic. The double-facedness of the species will be illustrated by the ecological niche that the species occupies in a large shallow lake (L. Vortsjarv, 270 km 2 , maximum depth 6 m) and in a strongly stratified lake (L. Verevi, 12.6 ha, maximum depth 11 m). Mild winters, earlier springs and prolonged vegetation period seem to be favourable to Limnothrix redekei or at least we can say, that this is an accompanying process to the expansion.