Half a century of oligochaete research in Estonian running waters

About 51500 specimens from 1542 samples, collected over the years 1954–1975 and 1986–1999 in different running water bodies throughout Estonia, were identified. Tubificidae prevailed in the material, with Limnodrilus hoffmeisteri forming about 40%. This species was followed by the tubificids Tubifex tubifex, Potamothrix hammoniensis, Psammoryctides barbatus, L. udekemianus and Spirosperma ferox, the naidid Stylaria lacustris, and the lumbriculid Stylodrilus heringianus. Two main ecological assemblages were distinguished: the pelophilous assemblage, dominated by L. hoffmeisteri, and the psammophilous one, where usually P. barbatus was dominant. The relationships between different species and the chemical parameters of water were usually weak but in contrast, correlated well with sediment preferences. In organically enriched reaches, L. hoffmeisteri usually dominated. The fauna of the streams of the islands was poorer in species due to their small size rather than geographical isolation. Some recent antropochorous Ponto-Caspian invaders have only reached the lowermost reaches of the two largest rivers. Some brackish water species were found in the mouth of the Pärnu River. No essential differences were found between the comparable sets of oligochaete samples collected in 1954–1975 and 1987–1997 in the Estonian running waters.