The loss of aquatic mollusc species, particularly mussels, is a global phenomenon that is mainly attributed to human activities (Layzer, Gordon & Anderson 1993; Strayer & Ralley 1993; Vaughn & Hakenkamp 2001). The most destructive impacts are alterations of stream hydrology as a result of damming and river regulation. River–floodplain habitats in particular have been strongly affected by these measures, which have initiated long-term trends towards terrestrialization and fragmentation of the river–floodplain systems (Dynesius & Nilson 1994; Ward, Tockner & Schiemer 1999).
In order to re-establish (semi)natural conditions within the human-altered river–floodplain systems, rehabilitation programmes have become increasingly important (Ormerod 2003; Giller 2005). Fluvial dynamics have proved to be important driving forces for the development of long-term self-sustaining alluvial river landscapes that exhibit a high degree of biodiversity (Amoros & Roux 1988; Ward & Stanford 1995; Tockner et al. 1999; Ward et al. 2002; Palmer et al. 2005). Thus the key step in river–floodplain restoration schemes is the enhancement of the lateral integration between the river and its floodplain (Schiemer 1999; Schiemer, Baumgartner & Tockner 1999; Buijse et al. 2005), which can be done by side-arm reconnections. This approach, which is described as passive and slow (Giller 2005; Gillilan et al. 2005), allows natural hydraulic forces to reshape waterbodies and reinstall the natural heterogeneity. Reconnecting old river branches may increase fine sediment removal, foster geomorphic processes and thereby increase the turnover rate between aquatic and terrestrial habitats. Other goals achievable via side-arm reconnection include compensation for structural shortfalls in the main channel and reduction of the bed scour in the main channel (Reckendorfer et al. 2005).
For ecologically orientated planning of side-arm reconnections, it is essential to gain an understanding of the relationship between hydrological connectivity and the requirements and performance of the biological components of the floodplain ecosystem. Such an understanding, together with knowledge of community assembly, interactions and functions, is crucial for quantitatively assessing the ecological outcome of restoration projects (Pywell et al. 2003). The importance of such conceptual models has recently been emphasized by Jansson et al. (2005). During this process of describing the ecological mechanisms prior to adopting particular restoration strategies, potentially conflicting processes may be identified and strategies may even be reconsidered (Jansson et al. 2005).
The hydrological connectivity between the main course of a river and various waterbodies lying in the alluvial floodplain strongly influences biogeochemical fluxes, biodiversity and the food-web structure of these waterbodies. Several of these aspects [e.g. the effect of water retention time on carbon fluxes, phytoplankton and bacterial productivity (Aspetsberger et al. 2002; Hein et al. 2003) but also on zooplankton biomass and community structure (Baranyi et al. 2002) and zooplankton grazing effects (Keckeis et al. 2003)] have been analysed in detail in the Danube Restoration Program, the first major restoration programme executed in the free-flowing section of the Austrian Danube east of Vienna in the Donau-Auen National Park (DANP). It has also been demonstrated that hydrological connectivity is significant for the diversity patterns of various vertebrate and invertebrate groups, including molluscs (Tockner et al. 1999), where diversity was highest in backwaters with intermediate connectivity.
While the significance of hydrology in determining the distribution of molluscs has been suggested by many authors (Foeckler 1990; Strayer 1993; Foeckler et al. 1994; Mouthon 1998), detailed information on the effects of hydrology are lacking and, until now, only qualitative statements on the outcome of river–floodplain rehabilitation schemes are possible.
Bivalves and gastropods have an important impact on riverine ecosystems through filter feeding, deposit feeding, bioturbation and nutrient excretion (Brown, Alexander & Thorp 1998; Vaughn & Hakenkamp 2001). Together with the well-known threat of extinction for these taxa, it is crucial to gain an improved understanding of the factors determining their distribution, for both ecosystem restoration and conservation efforts.
The present study analysed the composition of mollusc associations in a river–floodplain system based on the performance of individual species with respect to hydrological connectivity. We relate a quantitative measure of connectivity to species’ traits such as body size, feeding habits and desiccation resistance, with the aim of generalizing on the mechanisms underlying community responses to hydrological change.
Specifically, we addressed the following questions. (i) What effect does varying hydrological connectivity have on different mollusc species? (ii) Can meaningful, quantitative predictions be based on an easily derived hydrological parameter? (iii) What implications do these predictions have for the planning of future restoration programmes?