How to achieve the “good ecological potential” according to the EU Water Framework Directive? A case study from the Inn impoundment Egglfing-Obernberg

According to the EU water framework directive, the water bodies of the Lower Inn at the Bavarian-Austrian border between the hydropower plant Passau-Ingling and the Salzach river mouth are classified as “heavily modified water bodies” (HWMB). Therefore, not a “good ecological status” but a “good ecological potential” has to be achieved.In accordance with the Austrian “heavily modified water body guideline”, a defined amount of measures of the “maximum ecological potential” has to be implemented to achieve the “good ecological potential”. The “maximum ecological potential” includes all potential mitigation measures that do not have significant adverse effects on the HWMB specified water use. Mitigation measures have been developed for the Inn impoundment Egglfing-Obernberg and most of them have been implemented as part of the project “Durchgangigkeit und Lebensraum” (“Connectivity and habitat”) between 2017 and 2019. The project contained the construction of a new side branch and an oxbow, the de-sedimentation and activation of existing oxbows and the construction of a nature-like bypass channel serving as fishpass as well as valuable fluvial habitat.Preceding fish ecological surveys have confirmed the hydromorphological deficits of the Inn impoundment Egglfing-Obernberg. In particular, a lack of spawning and juvenile habitats for rheophilic fish species and a strongly limited lateral connectivity can be emphasized. These deficits result in a low fish stock and the absence of species and age classes. Nevertheless, the endangered stone gudgeon has been rediscovered for the Inn-Salzach river system. First results of the monitoring of the fish migration in the nature-like bypass during last autumn indicated an unusually high number of migrating fish, especially subadult Cyprinids. Within the ongoing monitoring over the next couple of years, the impact of the mitigation measures in response to the objectives of the EU water framework directive will be examined. The authors expect comparably positive effects, based on existing fish ecological studies from the Danube river, which have indicated significant improvements of riverine fish populations.