Anthropogenically enhanced sediment oxygen demand creates mosaic of oxygen deficient zones in the Ganga River: Implications for river health

Dissolved oxygen (DO) plays a major role in sustaining aquatic communities; its concentration and regulatory determinants are considered a key node predicting eutrophy, ecosystem health, and biogeochemical feedbacks. Here we report the status of dissolved oxygen deficit (DOD; hypoxia), and its mechanistic links with sediment oxygen demand (SOD) in the Ganga River. We conducted two independent but interlinked studies during summer low flows of three consecutive years (2016-2018) considering: 1) a 518 km middle segment of the Ganga River between Kanpur upstream and Varanasi downstream; and 2) trajectory analyses downstream two point sources, one flushing industrial effluents (Wazidpur drain) and the other with urban sewage (Assi drain). The concentration of DO at sediment-water interface (DO_sw) did appear < 2.0 mg L^-1 (hypoxia) at Jjmu; and up to 600 m and 800 m downstream Assi and Wazidpur drain respectively. The DOD at sediment-water interface (DOD_sw) was highest at Jjmu and did not show a significant decrease up to 300 m downstream to point sources. The SOD which varied between 2.03 and 13.16 (main river stem); 4.39 and 16.81 (Wazidpur drain); and between 2.00 and 13.50 g O₂ m^-2 d^-1 (Assi drain), was found to be a major contributor of DOD. Principal component analysis (PCA) and non-metric multi-dimensional scaling (NMDS) separated DO and alkaline phosphatase (AP) opposite to oxygen-consuming processes and sediment-P release. Using a dynamic fit model, we tested the dependence of sediment-P release on DO_sw and DOD_sw. A large increase in the sediment-P release with increasing DOD_sw and decreasing DO_sw indicated that the system may compromise its resilience in long-term future in terms of self-fertilization and P-eutrophy if the similar magnitude of anthropogenic pressure is continued. The study advances our understanding towards DOD associated habitat fragmentation, ecosystem resilience and niche opportunities useful for recovery and management of the Ganga River.