Water mollusc communities and bioindication of lower salzach floodplain waters

Abstract

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 ...

Connectivity and biocomplexity in waterbodies of riverine floodplains

“…Incision of the river bed leads to a lowering of the river level and subsequent lowering of the alluvial aquifer's water table. Consequently, increasing elevation differences between alluvial plugs forming at the ends of floodplain waterbodies and the mean river level, lead to a decline in surficial connectivity (Foeckler et al ., 1991; Babinski, 1992; Bravard, Amoros & Pautou, 1997). Lowering of the alluvial aquifer's water table also leads to a decrease in vertical connectivity with regards to seepage inflow, but where the waterbodies are supplied by hillslope groundwater, the river bed incision may actually increase the connectivity with such hillslope groundwater, leading to oligotrophication of the ecosystem (Bornette & Heiler, 1994; Bornette, Amoros & Rostan, 1996).…”

Vegetation as a Tool in the Interpretation of Fluvial Geomorphic Processes and Landforms in Humid Temperate Areas

“…Deposition of Wnes may progressively isolate aquatic ecosystems from shallow aquifers by clogging substrate interstices (Foeckler et al 1991), and consequently eliminate plant species that require water Figure 10.7 Percent of sites (>100) present for selected species during channel evolution following channelization: (A) erosional sites; (B) equilibrium sites, (C) aggradational sites. From Simon and Hupp's (1992) data renewal (Amoros et al 2000).…”

“…Presumably, it is the degree to which individual species have adapted to anoxia-related stresses that has led to the distinct and drastic changes in vegetation composition over very short (meters) (Jones 1956, van der Valk and Bliss 1971, Bilby 1977, and in some cases, may completely remove Wne-sediment deposits that had accumulated since the last scouring event. Natural successional processes that occur in riverine ecosystems can be slowed or stopped, depending on the intensity and frequency of Xood scouring (Sparks et al 1990, Foeckler et al 1991, Mu È ller 1995. In accordance with the intermediate disturbance hypothesis (Connell 1978, Sousa 1984, intermediate frequency and intensity of Xood scouring sustains a dynamic equilibrium in such ecosystems.…”

Extreme flood events favour floodplain mollusc diversity

“…In the past, several studies dealt with mollusc communities in floodplains (Foeckler, 1991;Foeckler et al, 1991;Obrdlík et al, 1995). However, most of them focused on aquatic species, describing community composition in relation with hydrological connectivity of floodplain water bodies (Reckendorfer et al, 2006) or along a longitudinal stream gradient (Mouthon, 1999).…”