Did zebra mussel fill the type habitat of a worldwide-rare freshwater brown macroalga?

1. The epilithon of steep sandstone and shale rock-walls (Molasse) was investigated in a part of Lake Constance (Überlinger See; southern Germany) between 2017 and 2019 and compared to the findings of Lauterborn and Zimmermann from 100 years ago. 2. The main objective was to find evidence to support the hypothesis that the brown alga Bodanella lauterbornii might have been displaced from its type habitat by invasive mussels due to competition for space (hard substrata). 3. Striking changes in phytobenthos species composition had occurred. Only scant remains of the rich and extensive algal vegetation were found. The once pronounced depth zonation with its characteristic indicator species no longer exists. Likewise, the worldwide-rare Bodanella lauterbornii, formerly a major component of the epilithon, was not found. 4. The most reasonable explanation for the strong decline in benthic colonisation by algae is that the zebra mussel had invaded the sandstone and shale rock-walls from the late 1960s onwards. Presently, this species forms extensive colonies on the walls up to a depth of 30 m, lately together with the new invader, the quagga mussel.