Changes in food‐web pathways and ecosystem functions after sidearm re‐connection in a large river (Danube, Austria)

Introduction River regulation schemes generally led to changes in riverine assemblage composition and their trophic status, which often is reflected in a general loss or decrease of characteristic species and a concomitant increase or dominance of generalist species. Objectives This study examines changes of the hydrodynamic conditions, plankton biomass as well as changes in feeding guilds and the assemblages of macrozoobenthos and of fishes before and after the re‐connection of a sidearm of the main stem in a free‐flowing section of a large river (Danube in Austria). Methods In order to analyze the effects in the sidearm, a before–after control‐impact design was applied; a river section in the main stem served as a reference site. Results The higher and increased connectivity had sizable effects on hydraulic parameters, with the expected shift of prevailing lentic to year‐round lotic conditions in the sidearm. The composition of suspended solids changed significantly. Likewise, chlorophyll‐ a concentrations and zooplankton abundance decreased markedly. The new conditions in turn modulated the pathways and the structure of the food web. Conclusions The permanent flow‐through in the sidearm decreased the importance of detritus and forced the one of periphyton and biofilm as an energy and nutrient source for insect larvae which also affected higher trophic levels, that is, fishes.