Point Source-Driven Seasonal Hypoxia Signals Habitat Fragmentation and Ecosystem Change in River Ganga

The development of dissolved oxygen deficit (DOD; hypoxia) has been reported to expand over 2.45 × 10⁵ km² area of the ocean from over 400 different areas worldwide¹. Although cultural eutrophication has greatly accelerated DOD in estuaries and semi-enclosed seas^2,3, it is not a common phenomenon in large rivers⁴. Hydrological continuum reinforces oxygenation, and therefore, development of hypoxia (dissolved oxygen (DO) < 2.0 mg l^–1) is less critical in large rivers. River ecosystems usually respond to gradual changes in a smooth manner. However, smooth and continuous changes can be interrupted by sudden abrupt switches to a mosaic of alternative states leading to loss of resilience⁵. Such shifts are most often driven externally, for instance, point source flushing, but they can trigger internal feedbacks leading the system to behave chaostically even in the absence of external forcing⁶.