Floodplain restoration by reinforcing hydrological connectivity: expected effects on aquatic mollusc communities

1. One of the main targets of river-floodplain restoration is the reconnection of former side channels. While there is information about the overall impact of such measures, far less is known about specific species' response patterns to hydrological connectivity. 2. The present study examined the composition of aquatic mollusc communities based on the performance of individual species with respect to hydrological connectivity in the Donau-Auen National Park, Austria. Species' traits were used to make generalizations about community responses to hydrological changes. 3. We introduced a connectivity parameter (Cd, expressed in days year(-1)) that could easily be derived from the river hydrograph and altitude of the inflow areas. This parameter integrated several key features of floodplain waterbodies and reflected the cause-effect chain of restoration schemes, thus allowing the outcome of restoration measures to be quantitatively predicted. Endangered rheophilic mollusc species reached higher frequencies as connectivity increased; for most eurytopic molluscs, however, increased connectivity reduced abundance. 4. With respect to species' traits, the proportion of large gastropods declined continuously with increasing Cd, whereas the percentage of gastropods with globolose shells and the relative number of strongly calcified (thick-shelled) individuals both increased. Species dominating in isolated sites were characterized by a higher resistance to desiccation, a food preference for higher plants and a preference for less shaded habitats. They matured later and typically had more offspring than rheophilic species. 5. Species number peaked at connectivity (Cd) values of about 1 month year(-1). Isolated and lotic habitats hosted significantly fewer mollusc species. Species turnover was highest when Cd was between 0 days and 2 months. 6. Synthesis and applications. Empirical models based on a connectivity parameter can be used to predict the consequences of restoration measures on the aquatic mollusc community in the Donau-Auen National Park. The analysis of species' traits also allows general prognoses for other river-floodplain systems. Furthermore, initial results based on other taxa indicate a broad applicability of the parameter. The methodology thus provides both an urgently needed and a practical tool for predicting the impact and success of restoration schemes.