Extreme summertime flood events are expected to become more frequent in European rivers due to climate change. In temperate areas, where winter floods are common, extreme floods occurring in summer, a period of high physiological activity, may seriously impact floodplain ecosystems. Here we report on the effects of the 2002 extreme summer flood on flora and fauna of the riverine grasslands of the Middle Elbe (Germany), comparing pre- and post-flooding data collected by identical methods. Plants, mollusks, and carabid beetles differed considerably in their response in terms of abundance and diversity. Plants and mollusks, displaying morphological and behavioral adaptations to flooding, showed higher survival rates than the carabid beetles, the adaptation strategies of which were mainly linked to life history. Our results illustrate the complexity of responses of floodplain organisms to extreme flood events. They demonstrate that the efficiency of resistance and resilience strategies is widely dependent on the mode of adaptation.

Floods are fundamental for the maintenance of floodplain biodiversity. As a result, wellfunctioning floodplains are characterized by a high spatio-temporal heterogeneity. Most floodplainorganisms need this shifting landscape mosaic to fulfil their environmental requirements and display a range of adaptations to survive floods. However, in temperate areas, where winter floods are common, extraordinary floods occurring in summer, a period of high physiological activity, may seriously impact the floodplain fauna. This is especially true for guilds characterized by low mobility, such as molluscs. Here we examined the immediate and longer-term response of Elbe grassland molluscs to the extreme 2002 Elbe summer flood in terms of abundance, diversity, and community composition by comparing pre-and post-flood data collected with identical methods. The flood favoured the colonization of aquatic species and led to a shift of the community towards a more hydrophilic composition. Both diversity and abundance increased significantly in the first year following the flood but decreased later gradually to the pre-flood levels. The high spatiotemporal habitat heterogeneity played an important part in the maintenance of mollusc diversity by increasing refuge opportunities and favouring the maintenance of various mollusc communities with different environmental requirements within the floodplain.

The goals of the subproject “molluscs” within the inter-disciplinary research project “Indicator systems for the characterisation and prediction of ecological changes in floodplain systems” were: – to develop further existing mollusc-based indicator systems of site quality and to test their transferability, – to characterise grassland sites within the recent floodplains of three study areas along the Elbe River, – to analyse the relationships between indicator species-/groups and abiotic parameters, – to compile and use selected species traits in the analytical process. The results clearly show several characteristic species groups related to the hydrology of the sites (i.e. inundation and desiccation regime) and on to the degree of agricultural use. These dependencies can be interpreted by the simultaneous analysis of the species traits. “Models” are proposed, that are applicable to nature protection measures at the landscape scale

The lower Salzach River lost its natural river dynamics as a result of channelization, which caused its bed to deepen, its groundwater table to be lowered, and its floodplains to gradually dry out. These changes of the floodplain have lead to a loss of natural biotopes with their biocoenoses. By using water mollusc communities as bioindicators these losses are detectable at a landscape level. A numerical analysis of a water mollusc survey in 1989 defined six water mollusc communities identifying six water-body types: three flowing (streams, ditches, and rivers) and three standing (young former meanders, large deep groundwater-influenced backwaters and small shallow groundwater-influenced backwaters) within the lower Salzach valley. These communities and water-body types are interpreted using results from water mollusc communities of other floodplain systems and by comparison with earlier water mollusc surveys in the lower Salzach valley. Then predictions are made on the future fate of the investigated floodplain water. Finally the investigated water bodies are evaluated for nature conservation in the face of plans to impound the lower Salzach River. KEY WORDS Salzach Alpine river Water molluscs Ecological diagnosis Conservation assessment Falkner (1990) gives information on ecological demands (cJ Lozek, 1964), distribution and degree of threat of each species. Diepolder (1990) andFoeckler (1989) give information on the environment, sampling methods, measured variables, and data process by numerical statistics using Wildi and Orloci (1983). RESULTS Water molluscs, their communities and characterization of water bodiesIn the lower Salzach and its floodplains 36 aquatic mollusc species were found, one of which had occurred only in the dead state Aplexa hypnorum. Seidel(1971-73) had recorded 45 species, of which 10 of these were missing in 1989: Viviparus contectus, Acroloxus lacustris, Radix ampla, Gyraulus acronicus, Gyraulus crista, Segmentina nitida, Unio crassus, Anodonta anatina, Pisidium pseudosphaerium, and Pisidium tenuilineatum. One species, Pisidium casertanum f. ponderosa is new since Seidel's survey. 93.3 per cent of the species found belong to the holarctic, paleartic, european, cosmopolitan faunal elements generally distributed in Central Europe. There is only one species from the pontic (Dreissena polymorpha), east-alpine-karpatic (Bythinella austriaca) and sibiric-boreo-alpine (Gyraulus acronicus) elements-the last recorded only by Seidel.After reducing commonly distributed and rare species within the study area, cluster analysis elucidated six aquatic mollusc communities: three flowing (Figure 2) and three standing water (Figure 3) types within the three main regions of the area (present and former floodplains and streams above the valley floor) partly merging from one to another (Table I). Characteristic species-combinations define the different mollusc communities. The site-factors of the water body groups, defined by comparison with results from comparable studies, differ from each other and are ...