The effect of low flow and stream drying on the distribution and relative abundance of the alien amphipod, Echinogammarus berilloni (Catta, 1878) in a karstic stream system (Westphalia, Germany)

We found the alien amphipod, Echinogammarus berilloni (Catta, 1878) in a temporary karstic stream system (Westphalia, Germany). We investigated the effect of the seasonal low flow and desiccation on the distribution and the relative abundance of E. berilloni and on the native amphipods, Gammarus fossarum Koch, 1835 and G. pulex (Linnaeus, 1758). E. berilloni does not colonize the headwaters of the study area. Due to the seasonal low flow situation or even stream drying, this longitudinal limit of distribution can already occur in a further downstream section. After total drying the recolonization of E. berilloni in the temporary stream section mainly happens by drift from the permanent upstream sections, if a population has been established there. The relative abundance of E. berilloni and of the native amphipods, G. pulex and G. fossarum within the temporary stream sections is highly correlated with the temporary discharge regime. While in the permanent downstream sections E. berilloni successfully competes with the native gammarids and almost completely replaces them, Gammarus spp. dominate in the temporary reaches.