Cause–effect–response chains linking source identification of eroded sediments, loss of aquatic ecosystem integrity and management options in a steppe river catchment (Kharaa, Mongolia)

Although sparsely populated, the progressive degradation of Mongolia’s rivers, lakes and groundwater, driven by land-use changes, poses a key challenge for the future sustainable development of the country. This paper deciphers the cause–effect–response chain between river bank degradation, changes of the ecological status, declines of ecosystem functions and priority measures with the case of the Kharaa River in Northern Mongolia. The underlying research approach comprised: (1) hydromorphological characterisation of the Kharaa River, (2) water quality assessments, (3) determination of the riverbed composition including hyporheic zone properties, (4) the analysis of riverine biota (macroinvertebrates and primary producers) and (5) the identification of the sources of suspended and settled sediments. The assessment revealed a gradient of spatially heterogeneous river bank erosion due to the degradation of the riparian vegetation caused by overgrazing and wood utilization. As the most prominent ecological response, the biomass of benthic algae decreased and macrozoobenthic community metrics changed continuously along the pressure gradient, accompanied by shifts of habitat related functional traits. At the same time, the hyporheic zone dimensions and functioning were affected by suspended and infiltrated sediments in multiple ways (restricted spatial extent, lowered hydraulic connectivity, lower metabolism, ecologically critical quality of pore water). Geochemical and radionuclide fallout isotope fingerprinting has identified riverbank erosion as the main source of the suspended sediments in the Kharaa River, when compared to gully and land surface erosion. Erosion susceptibility calculations in combination with suspended sediment observations showed a strong seasonal and annual variability of sediment input and instream transport, and a strong connection of erosional behaviour with land-use. Amongst others, the protection of headwaters and the stabilization of the river bank erosion hotspots in the mid-stream sections of the Kharaa River are the priority measures to avoid further degradation of the aquatic ecosystem status and functions.