Abstract
Potamothrix vejdovskyi is one of several alien Ponto-Caspian species presently invading Europe and the New World. Its invasive success has been suggested to be dependent upon the trophic degree of the water, and this species is considered to be particularly indicative of mesotrophy (Lang & Reymond 1996). Bottom fauna surveys performed by the Swedish Fisheries Board in 1915 and 1916 and in 1933–1935 did not reveal the existence of P. vejdovskyi in Lake Mälaren, whereas extensive bottom fauna programs in the lake in 1967–1976 demonstrated a wide distribution of the species in the central, least polluted basins. Strict quantifications of the distribution of oligochaetes in those basins in 1974 showed that P. vejdovskyi then made up on average 28 % of the total abundance of oligochaetes. However, in later bottom fauna surveys in Lake Mälaren, performed by the Swedish Environment Protection Agency in 1981 and in 2009–2011), P. vejdovskyi could not be found at all. Sewage treatment around Lake Mälaren accelerated in the late 60’s and the phosphorus load is nowadays considerably lower than before treatment (about 30 % less in terms of total-P concentrations in the water). The question of why this species seems to have more or less disappeared from the lake remains unsolved.
