Effects of environmental flows in a restored floodplain system on the community composition of fish, macroinvertebrates and macrophytes

Floodplains were extensively altered by anthropogenic activities, resulting in modified flow dynamics essential for maintaining diverse riverine communities. There is growing interest in restoring environmental flows by artificially modulating discharge as a potential management option in regulated rivers. In the context of a large floodplain restoration project along the upper Danube, the effects of different magnitudes of discharge on fish, macroinvertebrates and macrophytes in different habitat types of the floodplain system were studied. Community composition of fishes and macroinvertebrates changed after peak-flows of up to a 20-fold increase of mean discharge, whereas species richness and coverage of macrophytes were hardly affected. Over all habitat types, similar community response patterns were evident for macroinvertebrates, whereas in fishes, the most pronounced changes occurred in floodplain ponds that only become connected to the main channel at high flows. Community changes for fish and macroinvertebrates were mostly attributed to higher numbers of individuals (3-fold increase for fishes) rather than to changes of species richness (maximum fish species richness change < 10%), however, rheophilic specialists such as Barbus barbus were much more abundant after e-flows. These findings suggest that environmental flows can have meaningful and desired conservation effects on the aquatic community composition of fishes and macroinvertebrates, depending on the magnitude of the flow and on the specific habitat type. They also stress the importance of managing dynamic flow regimes that must comprise both minimum low flows as well as flood extremes to sustain freshwater biodiversity of high conservation value.