Zooplankton composition in a gravel pit lake invaded by the Ponto-Caspian mysid Hemimysis anomala G.O. Sars 1907

During a zooplankton monitoring program in the Rhine catchment (Southern Germany) during the 2006 sampling year, we recorded the mysid Hemimysis anomala in an oligotrophic gravel pit lake. The crustacean and rotifer abundance in the lake was low; maximum density was 28 and 145 ind. L -1 , respectively. Crustaceans were dominated by copepods, which constituted 90- 100% of the zooplankton community. Cladocerans were represented only by Diaphanosoma brachyurum. Rotifers were dominated by Polyarthra spp. for most part of the year. We suggest that the reason for the lack of cladocerans (except Diaphanosmoma) could be competition and/or predation by H. anomala. That D. brachyurum could persist in the presence of Hemimysis may be a result of spatial segregation. Possibly, due to differences in thermal preferences, H. anomala avoids the warm epilimnion of the lake in summer while Diaphanosoma may prefer it.