Aquatic communities across the Eurasian steppe belt are facing increased anthropogenic pressures that result from rapid population growth and catchment wide land-use changes. The particular variety, intensity, overlay and legacy of these impacts provide a unique setting to investigate ecological responses of multiple stressors. We studied macroinvertebrate communities along the Kharaa River in Mongolia, which displayed a distinct, downstream directed gradient of nutrient enrichment, disturbed bank morphology, reduced riparian vegetation, elevated turbidity and increased fine sediment intrusion into the hyporheic zone. Under these impacted conditions (TP 0.02 – 0.09 mgl-1, TN 0.33 – 0.96 mgl-1, FNU 0.62 - 5.43) population densities and biomass of macroinvertebrates were high (5293 ± 409 individuals m-2 and 2631 ± 153 mg dry weight m-2) and stable. In contrast, macroinvertebrate community structure showed significant negative linear relationships (Pearson’s r) for taxa richness (r = -0.79), Shannon Index of Diversity (r = -0.85), Evenness (r = -0.81), relative abundance of EPT individuals (r = -0.88) and relative biomass of hard substrate colonisers. At the same time, relative biomass of Chironomidae and Oligochaeta (r = 0.76) was positively correlated to mean turbidity values. Our results indicate a key impact of suspended fine sediment loads on macroinvertebrate community structure and to a lesser extent on habitat complexity at low rates of hyporheic fine sediment intrusion (mean values 0.9 - 1.6 g DW l-1d-1) in the Kharaa River. Hence, the implementation of effective regional management strategies aiming at to reduce erosion processes to ecologically tolerable levels deserves high priority.
