The contribution of long‐term isolated water bodies to floodplain fish diversity

1. Floodplains and their water bodies are typical ecotopes of large lowland rivers. The lowland Oder River, Germany, provided a rare opportunity to study fish assemblages of comparable floodplain water bodies differing by >50 years of isolation history. We hypothesised that true floodplain specialists peak in rarely connected water bodies, while frequently flooded waters support tolerant generalists. 2. Three macrohabitats, main channel (MC), temporarily connected and isolated floodplain water bodies, were sampled by electro‐fishing and their fish assemblage characters recorded. 3. Long‐term isolation of floodplain water bodies had a significant effect on the fish assemblage by promoting species preferring still water. Limnophilic and floodplain specialist species significantly increased with isolation. 4. Fish densities, species richness and diversity clearly differed between MC sites and floodplain water bodies. Shannon’s species diversity index peaked in both the MC and isolated waters and was lowest in the temporarily flooded waters where eurytopic fish dominated. 5. The significant gain in abundance and numbers of limnophilic species in the isolated compared to the temporarily flooded water bodies underpinned the significant contribution of long‐term isolated waters to the gamma‐diversity of large floodplain rivers, which should be considered in floodplain rehabilitation.