Hypoxia and associated feedbacks at sediment-water interface as an early warning signal of resilience shift in an anthropogenically impacted river.

Multiple human perturbations in the large rivers often cause habitat fragmentation creating patches of unpredictable structural and functional attributes. The resilience has been largely neglected in riverine studies, despite its pivotal importance in ecosystem recovery. We expect that a shift in sub-habitat conditions along a river transect subjected to frequent oxygen fluctuation and release of carbon, nutrients and other substances generate feedbacks to overstep the resilience and constrain ecosystem recovery. Because dissolved oxygen (DO) plays a regulatory role in ecosystem structure and functioning and feedbacks the denitrification and sediment-P release, we consider the mechanistic links among DOsw, denitrification and sediment-P release to identify resilience level and to construct a dynamic fit model to uncover the level of resilience and critical transitions in the river. We investigated 180 sites downstream two point sources and two tributaries, each with a 1.4 km river segment, covering 630 km length of the Ganga River. The dynamic fit model intersecting the DOsw at <1.5 mg L-1, sediment-P release >7.03 mg m-2 d-1 and denitrification rate >1.0 mg N m-2 hr-1 at 25 m reach downstream point sources indicated a threat to natural/self-recovery of the Ganga River. The non-metric multidimensional scaling (NMDS) and neighbor-joining analysis indicated that locations up to 700 m downstream Wazidpur drain have overstepped the ecosystem resilience. We found almost similar results downstream Assi drain and study confluences. Our explicit incorporation of DOsw, sediment-P release, and denitrification in an organized framework provides key insights to detect resilience and critical transitions in an anthropogenically impacted river ecosystem. Given the importance of the Ganga River for national water security and supply across several major states in India, research on the factors and status of resilience underpinning its recovery should be high on our national agenda.