Libellenkundliche Bewertung von Restrukturierungsmaßnahmen an einem Fließgewässer in Österreich durch Prae- und Post-Monitoring (Trattnach, Oberösterreich) (Odonatological assessment of river rehabilitation by pre- and post-monitoring (River Trattnach, Upper-Austria)

In 2019, a 500 m section of the River Trattnach, which is situated in the municipality Schlüßlberg in the Austrian province Upper Austria, has been ecologically enhanced by restructuring the riverbed. The investigation of the Odonata carried out in 2016 (pre-monitoring) and 2021 (post-monitoring) aimed at the evaluation of the success of these measures. Five field trips at three 100 m stretches situated in this section were performed in both years. The regeneration measures at the river caused an increase of the total species number from five to ten and an increase of the number of the certainly, probably and possibly autochthonous species from four to eight. In 2021, four of the five rivertype-specific target reference species were detected: Calopteryx splendens, Calopteryx virgo, Gomphus vulgatissimus, and Onychogomphus forcipatus. The autochthonous occurrence of these rheophilous species and their abundances confirm the river-type specific biocoenotic region of this river section (transition between hyporhithron and epipotamon) and indicate the existence the existence of riverbank vegetation as well as of heterogeneous morphological features causing the formation of a mosaic of different current and sediment conditions. The riverbed restructuration had only little influence on the general course of the river channel, which remained mainly straightened. Therefore, limnophilous accompanying reference species benefited only to a small extent from the regeneration measures. The detection of freshly emerged G. vulgatissimus and O. forcipatus two years after finishing the measures documented the early colonisation of the modified river section by these species. The assessment of the dragonfly-based ecological status of the whole section subjected to rehabilitation measures and of the individual stretches was based on comparing the current dragonfly fauna with a river-type-specific reference community. The Rhithron- Potamon Concept represented the methodological framework. Possible differences between the status quo of the dragonfly fauna and the reference were assessed by the Odonata-River-Zonation-Index and by a scheme of five classes of ecological status stipulated by the EU Water Framework Directive: “high ecological status”, which corresponds to the reference state, “good ecological status”, “moderate ecological status”, “poor ecological status” and “bad ecological status”. The rehabilitation measures caused an improvement of the dragonfly-based ecological status: According to the results gained in the pre-monitoring, the dragonfly-based ecological status of the whole section was classified as “moderate”. The three individual stretches investigated were ranked as “poor” and “moderate”. The results of the post-monitoring revealed the “good dragonfly-based ecological status” for both, the whole section and the stretches. Even the ecological status of a regulated control-stretch, which was not subjected to rehabilitation measures, changed from “poor” to “moderate”. “Spread effects” from the enhanced section to the still regulated section are assumed to be the reason for this improvement.