River restorations : Morphological effects on colonization and succession of aquatic and riparian organism groups
The European Water Framework Directive (2000/60/EC) claims a good ecological status of all ground and surface waters in the European Union according to chemical, hydromorphological and biological conditions. Unfortunately, rivers were strongly altered in the last centuries. Therefore, the number of restoration measures strongly increased in the last decade. Aquatic organism groups (e.g., benthic invertebrates and fish), which are used to assess the ecological status, show low or no responses to restoration, although a strong correlation of hydromorphological measures and biotic responses is expected. Some studies addressed the factor time as the major setback in the missing restoration effects. Currently, the knowledge on time spans required for successful recolonization of restored sections is poor. Effects of restoration measures on riparian organism groups are also less investigated, although single studies reveal strong responses to changing habitat conditions. Furthermore, studies which compare effects of restoration on both aquatic and riparian organism groups are missing. This thesis focused on hydromorphological river restoration measures in Germany and their effects on aquatic and riparian habitats and organism groups. The following questions were addressed: - How do riparian organism groups respond to hydromorphological restoration measures? - How do habitats and species assemblages of the river and the floodplain develop in the first years after restoration and over time? - How do aquatic and riparian organism groups differ in their responses to restoration measures? To answer the first question, the effects of hydromorphological restoration on riparian habitats and organism groups were analyzed by using the example of carabid beetles and floodplain vegetation in a dataset of 24 restoration measures. For the second and third question, aquatic and riparian habitats and species assemblages were investigated in two restored mid-sized mountain rivers at different time spans after restoration. Thus, pioneer colonization and successional processes were analyzed. Restoration effects differed between aquatic and riparian, but also between the individual organism groups. For these differences, the impact of multiple factors is suggested, e.g., the magnitude of habitat generation, dispersal abilities of organism groups, the presence of source populations and accessibility of restored sections for dispersing species. Morphological river restoration measures increased habitat diversity mainly in riparian areas with strong benefits for carabid beetles and floodplain vegetation. The strong responses of these species to habitat changes render them suitable indicators for morphological restoration measures. Missing or respectively minor responses of benthic invertebrates and fish reflected the minor enhancement of substrate diversity on the river bottom of restored sections combined with low dispersal ability and the lack of source populations in the direct surroundings. In contrast, aquatic macrophytes react fast and strong to restoration. For them, shallow bankside areas have a high importance as propagules can accumulate in these areas resulting in fast colonization of restored sections subject to the condition that source population upstream from restored sections are present. The results of this thesis highlight the decisive role of the factor time for the colonization of restored sections. Differing dispersal abilities of organism groups result in different time spans required for colonization of newly created habitats. Organism groups with high dispersal ability, e.g., riparian carabid beetles, are direct colonizers. In contrast, aquatic organism groups which have a lower dispersal ability and are currently suffering from multiple pressures need more time to reach restored sections. A high distance to source populations and deficits in longitudinal connectivity might retard or inhibit colonization of restored sections. In case of mountain rivers, the frequency and intensity of high discharges is another important factor influencing the development of habitats and colonization patterns of restored sections over time.
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