A spatiotemporal framework for assessing hypoxic risk to riverine aquatic life: from summary data to actionable...

Riverine hypoxia, oxygen depletion in flowing waters, poses ecological risk, yet its spatial and temporal dynamics remain poorly characterized. This study presents a novel risk-based spatiotemporal framework for assessing riverine hypoxia across Indian river systems, utilizing 18,976 site-year summary data points (annual minimum, mean, maximum) across 87 sub-basins over 17 years. We conceptualized 'contextual hypoxic risk' as the probability of summary statistics falling below a critical threshold (4.0 mg L⁻¹), distinguishing emerging, intermittent, and perennial hypoxic conditions. Hypoxic risks were estimated under an empirical Bayes (Poisson-Gamma hierarchical framework) to address overdispersion and stabilize standard error. Sub-basin-level aggregation mitigated site selection bias, yielding hydrologically meaningful and analytical units that facilitates policy decision. Of observations, 47% (8914) were emerging hypoxic events, 7% (1328) intermittent, and 1% (190) perennial. At least one severe hypoxic event (DO < 2.0 mg L⁻¹) occurred at approximately 25% of sites-significantly exceeding the global average (12.6%). Spatial analysis revealed pronounced heterogeneity, with hypoxic risk ranging from 0 to 50%, and northwestern and Gangetic sub-basins exhibiting high hypoxic risks and faster intensification. While hypoxic occurrences revealed upward national trend, hypoxic risk showed transient patterns. Space-time interaction analysis indicated statistically significant accelerating risk trajectories-particularly for perennial and intermittent types-across six key sub-basins. By coupling probabilistic risk modelling with spatial smoothing and trajectory steepness diagnostics, this framework quantifies both spatial variability and temporal intensification of riverine hypoxia. The scalable, reproducible framework is applicable to other developing regions. Findings underscore urgent need for adaptive, sub-basin-scale management to prevent progressive establishment of chronic hypoxic regimes in India's river systems.