The hyporheic interstitial was recognized as an integral zone within the aquatic ecosystem bearing important functions for both adjacent compartments, surface and ground water, about 50 years ago. Since then, rather disciplinary works gained knowledge on the organismic community of this ecotone, its spatial extent, the role of distinct parameters such as hydrology and morphology, temporal characteristics, process dynamics, the role for stream or groundwater quality and restoration measures. However, a systematic study on the risks to the hyporheic functions was missing to date. This thesis combined existing methods in order to gather an integrated set of information allowing for the assessment of the ecotonal status. This approach was applied to investigate the functional behavior towards stressors like increasing nutrient and fine sediment input into a rather pristine environment. An interdisciplinary risk assessment and the establishment of adapted measures was called for as land-use scenarios for the studied catchment area indicated progressive onland erosion. Therefore firstly, an integrated monitoring scheme was drawn up and conducted at three sites along a river that underlay a stressor gradient such as mentioned before. Secondly, the data sets were analysed in order to evaluate the status of the hyporheic funtions at the riffles. Thirdly, a coupled surface-subsurface modelling approach was set up to further study the impact of the stressors on the ecotonal integrity. And fourthly, an interdisciplinary consideration combined with studies on the catchments sediment budget and the rivers ecological status was applied to identify measures for the restoration and protection of the aquatic ecosystem. The analysis of the data gathered with the help of the established monitoring scheme revealed that elevated nutrient or fine sediment input lead to biological or physical clogging, respectively, with consequences for the hyporheic zone functions. The surface - ground water connectivity was either lowered in summer months, when biofilm growth was highest, or permanently, as fine sediment particles infiltrated into the interstices of the riverbed sediment. Scouring did not seem to take place as high amounts of fine particles were found in the matrix after discharge events of snowmelt and summer precipitation. With respect to the biogeochemical regulation function, biofilm material appeared to provide an autochthonous carbon source boosting microbial substance turnover. The sediment underneath the physical clogged layer was cut off from carbon and oxygen rich surface water and thus was not reactive. However, the enhanced surface area provided by the fine sediment within the topmost sediment layer seemed to support microbial processing. The inclusion of the results of a study concerning the ecological status at the investigated reaches lead to the deduction that biological clogging at the present degree was not affecting habitat quality. Whereas the physical clogging had tremendeous and lasting effects on the macroinvertebrate community which carries to the conclusion that sediment management within the studied catchment is of uttermost importance. A scenario analysis reflecting distinct clogging degrees and types with a calibrated model of a studied riffle within a pristine reach proved the observed loss of hydrologic connectivity due to physical and biological clogging. Further, a treshold of oxygen consumption rates above which the reproduction of salmonid fish would be unsuccessful was identified for the settings of the middle reaches. In summer month with low discharge it seemed to be likely that this treshold might be reached. Following, a dynamic discharge may be decisive to protect the ecotonal integrity. The integration with the outcome of an investigation regarding the sediment sources within the catchment allowed for two suggestions. On the one hand, river bank restoration and protection within the middle reaches need to be prioritised, and on the other hand, the conservation of the natural vegetation at the steep slopes within the mountaineous areas need to be undertaken in order to secure the pristine aquatic environment of this area. Hyporheic zone research of the last decade was driven by testing hypotheses on the functional significance of distinct spatial and temporal configurations in the field and by new modelling approaches. However, data on the quantification of the ecological impact of clogging processes were lacking. The thesis contributed to the systemic understanding of the hyporheic zone being affected by physical and biological clogging and new field data within a degrading pristine environment were generated, accessible for further hyporheic research. The interdisciplinarity enabled comprehensive statements for the usage of an Integrated Water Resources Management plan.

Water resources in the semi-arid to arid areas of Central Asia are often limited by low precipitation, and hence vulnerable to impacts of global change, i.e. socio-economic development and climate change. Both, socio-economic development and climate change are very likely causing significant changes as water resources are affected by two main effects: Firstly, growing population and industrial activities in the region raise the pressure on water resources due to increasing water abstractions. Secondly, air temperature in the region has been rising in the past far above global average and it is expected to increase further, which will lead to changes in runoff generation and therefore water availability. Increasing temperature as well as increasing water abstractions will affect water quantity and consequently water quality as a result of higher pollution intake or reduction in dilution capacity. Thus, it is of crucial importance to analyse and assess the state of current and future water resources to implement sustainable water management as the above mentioned effects very likely causing significant changes of water resources. Within the last years, the number of scientific research studies using large-scale models to simulate water availability and water use has increased substantially. Several new datasets from earth observations and new or improved models have been published (Werth et al. 2009; Werth and Güntner 2010; van Beek et al. 2011). Nevertheless, those studies focussed on water quantity and did not take into account impacts on water quality induced by global change although changes in water quality affecting aquatic ecosystems and species. Furthermore, spatially explicit large-scale modelling studies have not been carried out for Mongolia and Central Asia to get a comprehensive overview and assessment. To address this research gap, the large-scale water resource modelling framework WaterGAP 3 was applied to Central Asia with a focus on Mongolia to simulate impacts on current and future water resources. WaterGAP 3 consists of hydrology, water use and water quality sub-models in order to simulate current and future water quantity and quality.

Since the 1990s water quality monitoring projects using aquatic insects or macroinvertebrates as bioindication in Mongolia has mostly occurred in rivers drainage to the Arctic Ocean. They have been conducted to identify different anthropogentic stressors and impacts upon these running water ecosystems. However, there are still knowledge gaps and uncertainties concerning the research of these macroinvertebrates, in particular, a life cycle study of representative species are one such section of information missing. The specific aim for the study was to determine their life cycles and secondary production of selected species in the Kharaa River Basin, Mongolia, where these animals are exposed to harsh environment conditions. The main challenges for the research project were selecting the most suitable methods for use in the field sampling campaigns as well as establishing biomonitoring criteria for the target species under the extreme harsh climatic conditions. The research also sorts to address the pre-existing taxonomical identification problems. Consequently, a multi-habitat quantitative sampling method, and emergence traps type ‘Model week’ were selected. Five specific traits were chosen as selection criteria from the literature, where the life cycles of numerous species were investigated under comparable conditions to this study. Based on those five distinct criteria, a total of 18 species from EPT group (Ephemeroptera, Plecoptera, and Trichoptera) were selected for deeper analysis. This thesis provided the first quantitative results on the life cycle, production, growth rate and emergence of aquatic insects from Mongolia, to allow comparisons with studies in other regions using the same methods. However, it still needs more quantitative research of population dynamics for a wider range of species including fecundity, accurate development rates, mortality losses (e.g., due to predation), and food availability across environmental gradients of hydraulic conditions and substrate types. In conclusion, last not least it is essential to obtain knowledge especially about life cycle strategies of macroinvertebrates to identify the indicator-properties of single species and to predict re-colonisation potential of disturbed habitats and to evaluate the efficiency of management measures.