MACROZOOBENTHOS ABUNDANCE AND COMMUNITY COMPOSITION IN 3 LOWER RHINE FLOODPLAIN LAKES WITH VARYING INUNDATION REGIMES

The relative importance of lake inundation regime, littoral substrate and season for macrozoobenthos abundance and community composition was studied in three Lower Rhine floodplain lakes situated along a flood duration gradient. In each lake the macrozoobenthos density, biomass a nd community composition were determined monthly over one year in two contrasting littoral substrates (sandy-mineral sediments and clayey-or ganic sediments). The influence of the lake inundation regime on the m acrozoobenthos abundance was tested against those of littoral substrat e and season with a three-way analysis of variance (ANOVA). Between-la ke variation in macrozoobenthos abundance was clear for Bivalvia, Ephe meroptera, Heteroptera and Trichoptera. Bivalvia (Pisidium sp.) were m ost abundant in the frequently flooded lake, Heteroptera [Micronecta s p., Sigara falleni (Fieber), S. striata (L.)] in the rarely flooded la ke and Trichoptera [Mystacides longicornis (L.), M. nigra (L.)] and Ep hemeroptera [Caenis horaria (L.), C. luctuosa Burmeister] in the isola ted, non-flooded lake, suggesting an impact of inundation regime on th e abundance of these dominant taxa in Lower Rhine floodplain lakes. In each lake the annual average abundance of macrozoobenthos was dominat ed by Gastropoda [Bithynia tentaculata (L.), Potamopyrgus antipodarum (Gray), Valvata piscinalis (Muller)]. Between-lake variations in the r elative densities of macrozoobenthos functional feeding groups suggest s an impact of the inundation regime on the macrozoobenthos communitie s via the availability of nutritional resources. Relative densities of filter-feeders (e.g. Pisidium sp.) were highest in the frequently flo oded lake with its high phytoplankton biomass and poorly developed veg etation. Shredders (e.g. Mystacides sp.) were most numerous in the non -flooded lake with its well developed aquatic vegetation and low phyto plankton biomass. Within-lake variations in macrozoobenthos abundance could be attributed to littoral substrate and season. The importance o f substrate on macrozoobenthos abundance was most clear for Oligochaet a and Trichoptera. The importance of season on macrozoobenthos abundan ce was clear for the total macrozoobenthos and for all taxonomic group s, except Oligochaeta. Correlations between the total macrozoobenthos abundance and seasonally fluctuating parameters, i.e. flood pulse dura tion, water level, water temperature and vegetational coverage in the lakes, suggest that hydrological parameters were most important in the frequently flooded lake, whereas vegetational coverage and water temp erature were most important in the isolated, non-flooded lake.