Bathymetric expansion of an invasive gammarid (Dikerogammarus villosus, Crustacea, Amphipoda) in Lake Léman

After the arrival of Dikerogammarus villosus SOVINSKIJ in Lake Léman (2002) and its rapid expansion, the bathymetric distribution of gammarids was investigated in 2004-2005 at one location in the lake. D. villosus is able to colonize a wide range of substratum types (cobbles, gravel, sand/clay with Dreissena aggregates) down to a depth of 10 meters. Maximum abundance reached 235 ind. m-2 in November 2004 between 0.5 and 1 m. The native gammarids (Gammarus pulex (LINNAEUS) and G. fossarum (KOCH) were found in low numbers almost exclusively at a depth of 10 meters. The replacement of G. pulex and G. fossarum by D. villosus is confirmed as no more native species have been found since 2004 between 0 and 5 meters in sites where they were previously abundant. Even though the decline of invertebrate richness was not observed in the littoral zone, our results suggest that the native species are threatened.