Comparing and evaluating the dragonfly fauna (Odonata) of regulated and rehabilitated stretches of the fourth order metarhithron Gurtenbach (Upper Austria)


Mitigation measures carried out at the regulated metarhithron Gurtenbach in Upper Austria were evaluated by a survey of the dragonfly fauna. The assessment method developed in this study was based on the longitudinal distribution of dragonflies along riverine biocoenotic regions (the “Rhithron-Potamon-Concept” explains changes in species composition along a river’s length). Numerically expressed habitat preferences led to the definition of a set of six reference species. According to the requirements of the EU Water Framework Directive the current situation of the odonate species was compared with this inventory of river type-specific reference species and assessed in a five-tiered classification system of the “ecologic status”. At the regulated stretch the record of five species (including one reference species) was classified as class IV (“poor ecological status”). At the three rehabilitated stretches a total of 23 species were found. Two stretches were classified as showing “good ecological status” (class II), due to the occurrence of two autochthonous reference species (Calopteryx virgo, Onychogomphus forcipatus). The sensitive method applied not only allowed the evaluation of the differences between regulated and rehabilitated stretches but also the assessment of potamalisation effects within the rehabilitated section due to river bed widening and backwater influences.

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