Spatio-temporal patterns of macrobenthic invertebrates in a free-flowing section of the River Danube in Austria
Macrobenthic invertebrates were sampled quantitatively in the main channel of the Austrian Danube on 6 dates between April 1993 and February 1995. A modified air lift sampler (Pehofer 1998; Bretschko & Schönbauer 1998) was used. Due to flood control structures built during the last century, the stretch of the Danube investigated is severely regulated and characterized by straightening, steep and heavily stabilized banks, high current speeds up to 3 m s-1, and a water depth of up to 6 m with gravel sediments. Biologically important structures are restricted to a periodically inundated gravel bar and permanently inundated groyne fields. Three transects of the main channel were sampled in the centre, near the banks and near the groyne fields. Additionally, samples were also taken from areas between the groynes. Out of a total number of 181 reported taxa, only 6 species (Jaera istri, Stylodrilus heringianus, Canthocamptus staphylinus, Corophium curvispinum, Rheotanytarsus rhenanus and Rheotanytarsus sp.) were quantitatively important. Although insects were the most diverse group, with 109 out of 181 identified species so far, the most abundant groups with regard to their mean densities were Crustacea (Isopoda, Harpacticoida and Amphipoda) and "worms" (Oligochaeta and Nematoda) representing over 86 % of the sampled fauna. The dominant taxonomic units occurred over the entire cross section. Macroinvertebrate densities were surprisingly high, even in the shipping channel with the highest current speeds and greatest water depths, but stream fauna was more abundant and diverse in areas protected from the current, such as the groyne fields. The preference of the fauna for protected areas and the distinct temporal pattern, with highest densities in summer and lowest densities in winter, was independent of the discharge regime and is attributed to the life histories of the most frequent taxa.