To conquer and persist : colonization and population development of the Ponto-Caspian amphipods Dikerogammarus villosus and Chelicorophium curvispinum on bare stone substrate in the main channel of the River Rhine

Macroinvertebrate communities on the stones in the Rhine are dominated by the Ponto-Caspian amphipods Chelicorophium curvispinum (since 1987) and Dikerogammarus villosus (since 1995), which have invaded the Rhine through canals connecting the large rivers of Europe. Colonization of bare stones suspended in the water of the Rhine main channel was studied. At the same time the macroinvertebrates drifting in the water layer were sampled. Macroinvertebrate populations on the newly colonized stones were followed for two months (June - August 2002). Bare stones were colonized from the water layer, with D. villosus and C. curvispinum most numerous from the start. Species richness was highest after one month. D. villosus and C. curvispinum continued to dominate the macroinvertebrate community on the stones throughout the experiment, representing 70-95 % of the total number of macroinvertebrates. In the first period week of colonization, especially juveniles of both amphipod species settled on the bare stones. After one week, the number of adults of D. villosus increased. After one month, ovigerous females of D. villosus became abundant on the newly colonized substrate. The numbers of adult C. curvispinum increased after one month and ovigerous females were present after two months. Newly settled populations resembled the amphipod populations present in the water layer, but started to deviate as colonization time increased, indicating that development of populations on stones became increasingly autonomous and less dependent on new colonization by amphipods from the water layer. Ovigerous females of both amphipods were much more abundant on the stones than in the water layer. Juvenile C. curvispinum were smaller on the stones than in the water layer, indicating that the stone substrate is important for reproduction of at least C. curvispinum. Most juveniles of this species first grow to a certain body length on the stones before they start drifting off and swimming in the water layer. C. curvispinum and D. villosus densities were positively correlated in the early stages of colonization, but showed an inverse relationship after longer colonisation time. As both dominant amphipods colonize new stone substrate from the water layer where they drift or swim, they may not depend on extra vectors, such as shipping, for dispersal through the connecting canals and within the river.