Effects of river restoration on ecosystem metabolism and trophic relationships
Following a long history of human pressures on riverine ecosystems, the European Water Framework Directive (WFD, Directive 2000/60/EC) came into force in 2000 and initiated a new period of river management in Europe. The directive aims at improving the ecological and chemical status of rivers in order to achieve the ‘good status’ of all surface waters until the year 2027. In Europe, degraded hydromorphology has been highlighted as a central impact to the ecological status of the rivers. For instance, in countries such as Germany, the hydromorphology of almost all river sections is affected to an extent that they fail to meet the WFD goals. In response, river hydromorphology is nowadays being restored at an increasing rate. Restoration of river hydromorphology has the potential to affect not only structural ecosystem features, including species composition and diversity, but also functional aspects, such as key ecosystem processes and trophic transfers of energy and nutrients. Despite this, the most-widely used parameters for assessing the success or failure of restoration projects are almost exclusively based on changes in community composition of different biological groups (e.g., fish, benthic invertebrates, and macrophytes). Functional metrics, even though increasingly recognized as a valuable addition to classical assessments, are rarely used to study restoration effects. Consequently, the outcomes of river restoration for key ecosystem processes (e.g., river metabolism) and trophic relationships (e.g., trophic structure of benthic invertebrate communities and trophic connectivity between river and land) remain poorly understood. Against this background, the present thesis focused on the following objectives: (i) The application of stable isotope analysis (δ13C, δ15N) together with quantitative community metrics to characterize changes in the trophic structure of benthic invertebrate communities following restoration. (ii) The characterization of the isotopic composition (δ13C, δ15N) of consumers in aquatic, riparian, and terrestrial habitats to assess restoration effects on the trophic connectivity between river and land. (iii) The assessment of reach-scale restoration effects on hydromorphology, habitat composition and hydrodynamics and the estimation of the corresponding responses of river ecosystem metabolism.