The river flow regime is one of the key parameters in river ecosystems as it controls physical habitat conditions, biological and ecological processes and river ecosystem functioning. River flow conditions have been substantially altered globally due to water regulation and climatic changes resulting in detrimental impacts on the functioning and health of river ecosystems. Given its importance, several investigations have been conducted to increase the knowledge and improve the understanding of ecological effects of flow alterations. Species of stream macroinvertebrates are a major organism group in river ecosystems that are highly sensitive to environmental changes. Current knowledge regarding the flow preferences of stream macroinvertebrates is mostly based on species’ qualitative ecological traits stemming from expert knowledge or literature analyses. These established qualitative data are difficult to be linked to e.g. quantitative discharge data that could be used in predictive modelling of species diversity in space and over time. This research deficit, make it difficult to quantitatively predict the effects of climate-induced flow changes on river biota. To fill this research gap, it is crucial to better understand the quantitative changes in e.g. species’ abundance to environmental stressors such as flow alterations. Empirically-driven predictive relationships might be established for individual species by linking their abundance along wide ranges of environmental gradients to any environmental variables, e.g. different flow conditions. Any changes in flow can be described by indicators of hydrologic alterations (IHA metrics) that provide information on duration, frequency, magnitude, rate and timing of flow events. These predictive relationships can be used to assess species responses to climate-change-induced flow alterations. In this thesis, potential changes in the abundance of stream macroinvertebrates due to the effects from climate-change-induced flow alterations are analysed. The thesis is divided into three parts: Firstly, a non-linear modelling approach is applied for a German-wide dataset which enabled to link the abundance of species to river flow to quantify flow preferences of stream macroinvertebrates along the range of a variety of flow conditions, i.e. various IHA metrics. Secondly, this approach is used in two contrasting river catchments in the lowland and lower mountainous region of Germany to quantitatively assess potential changes in species’ abundance due to projected changes in flow conditions under the climate scenario RCP 8.5. Thirdly, potential variability in projected abundance of individual species under 16 climate models derived from various combinations of global and regional climate models are examined. The effects of variability in climate model predictions on species’ abundance and functional trait composition are tested. Based on these results, the ecological effects of changes in species’ abundance of sampling sites are assessed. The response relationships derived from the German-wide dataset showed that on average one-third (18-40% of 120 taxa depending on the IHA metric) of stream macroinvertebrates can be considered as ubiquitous with a broad hydrological tolerance, while about two-thirds of the taxa (35-53% of 120 taxa depending on the IHA metric) respond to either specific ranges of flow conditions with detectable optima for their occurrence or show monotone increasing/decreasing trends (23-41% of 120 taxa depending on the IHA metric). The habitat suitability for the taxa that showed preferences to specific ranges of flow conditions may be potentially affected by climate-change-induced flow alterations. The results from the catchment-scale study revealed that climate change would most strongly affect the low-flow conditions, which can lead to decreasing abundance of individual species as far as 42%. However, due to strong increasing abundance of generalist taxa, the average response of all species over all metrics indicated increasing overall species assemblage abundance in 98% of the studied river reaches. The predictions of climate models showed more increasing trends in flow conditions within the lowland area (11 of 16 climate models) compared to the lower mountainous region (6 of 16 climate models). Furthermore, the predicted species’ abundance differed significantly depending on the climate model used, especially in the lower mountainous region. This high variability lead to less significant changes in the overall abundance of species and functional groups in the lower mountainous region compared to the lowland area. The projected changes in species’ abundance showed more significant ecological alterations in the lower mountainous region compared to the lowlands. We argue that the causes lie, on the one hand, with stronger climate-change impacts on rivers with higher flows, which leads to homogenisation of physical habitat conditions. On the other hand, it is due to the higher number of specialists in the lower mountainous region (26 of 134 species) compared to the lowland area (5 of 60 species). The results provide empirical evidence that the functional trait compositions will be affected by flow alterations, but the effects would be regionally different. For example, flow alterations lead to increasing abundance of rheophilic and tolerant rhithral species in the lowland area, which is referred to as “rhithralisation effect”. The rather large number of stream macroinvertebrates with clear flow preferences in both the German-wide (35-53% of 120 taxa depending on the IHA metric, Chapter 2) and the catchment-scale studies (75-91% of 134 taxa in the lower mountainous region, and 85-98% of 60 taxa in the lowland area depending on the IHA metric, Chapter 3 and 4) reveal the potentially strong influence of climate-change-induced flow alterations on these species. However, among a variety of causes such as inherent uncertainties in ecological models induced by e.g. data availability, the ability to predict these changes is also limited by the uncertainty in predicting climate change itself. These results go one step further than the qualitative assessment of species responses to environmental changes and support the current knowledge that flow alterations and their effects on species’ abundance might be a global phenomenon. The main findings of this thesis underline the high susceptibility of stream macroinvertebrates to ongoing climate-change-induced flow alterations. Concerning the methodology, a clear recommendation for future predictions is to reduce uncertainty inherent in climate change models and thus to improve future predictability of e.g. species’ abundance. The analyses applied in this thesis are applicable to forecast climate change impacts at different spatial and temporal scales as well as for different stressors or species.
