The hyporheic zone bears key hydro-ecological functions such as hydrological connectivity of surface and groundwater ecosystems and biogeochemical regulation of substance dynamics. These functions are controlled by the sediment permeability that in turn is affected by biological and physical clogging. A number of conceptual models have been developed on the interactions and feedbacks between these functionalities; surprisingly, comprehensive field data are scarce. Thus, the aim of this study was to assess these functions and their vulnerability along a stressor gradient of nutrient and fine-sediment input at riffle scale. Three sampling sites along the Kharaa River (Northern Mongolia) were selected that represented conditions from being unaffected, affected by biological clogging and impaired by physical clogging. A significant impact on the hydrological connectivity was detected, as the vertical downward flux and the solute penetration depth decreased along the reaches. Simultaneously, the heterogeneity in biogeochemical patterns and the vertical extent of the oxygen gradients declined. Whilst biological clogging was apparently unstable and the biofilm supported the hyporheic organic carbon pool, it revealed to affect the two functions to a lesser extent when compared with physical clogging. In contrast, physical clogging seemed to be more permanent and restricted microbial metabolism to the uppermost centimetres, thus decreasing the active sediment depth. Moreover, fine-sediment particles enhanced the sediment surface area, thereby creating a high respiration potential that resulted in high values of community respiration and subsequent oxygen depletion. Concluding, the control of fine-sediment emissions has to be a priority issue in river restoration and catchment management. measured for O 2 (WTW 350i, CellOx 325; WTW MultiLine 34x0, FDO 925, WTW GmbH and Co., 965 ALTERATION OF HYPORHEIC FUNCTIONS THROUGH CLOGGING

A previous study investigating the ecological status of the Kharaa River in Northern Mongolia reported fine-grained sediments as being a major stress factor causing adverse impacts on the benthic ecology. However, the source of these sediments within the catchment as well as the specific impact on hyporheic zone functions in the Kharaa River remained unclear. Therefore, the objective of the current study was to investigate the underlying sourcereceptor system and implement an integrated monitoring approach. Suspended sediment sources within the Kharaa catchment were identified by using extensive spatially distributed sediment sampling and geochemical and isotope fingerprinting methods. On the receptor side, the ecological implications across a gradient of fine-grained sediment influx were analyzed using a distinct hyporheic zone monitoring scheme at three representative river reaches along the Kharaa River. Results of suspended sediment source monitoring show that during snowmelt runoff, riverbank and gully erosion were the dominant sources. During the summer period, upland erosion contributed a substantial share of suspended sediment. Finegrained sediment influx proved to be the cause of habitat loss in the hyporheic zone and benthic oxygen production limitation. This combined catchment and in-stream monitoring approach will allow for a better understanding and spatially explicit analysis of the interactions of suspended sediment transport and hyporheic zone functioning. This information has built the basis for a coupled modeling framework that will help to develop efficient management measures within the Kharaa River basin with special emphasis on rapidly changing land-use and climatic conditions.

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