Continuous in situ monitoring provides new evidence of storm‐induced riverine hypoxia following climatic extremes

Abstract June 2023, the warmest June on record at that point in the United Kingdom, was marked by an exceptional warm‐dry period followed by convective storms that triggered pronounced dissolved oxygen (DO) depletion across four river sites spanning a 25–5000 km 2 basin gradient. Our continuous in situ monitoring revealed spatial variability in the onset, duration, and magnitude of hypoxia, with rapid DO crashes in urban‐influenced catchments and delayed responses in more permeable agricultural systems. For the sites with supporting fluorescence measurements, increases in dissolved organic carbon (DOC) loading indicated substantial rises in overall DOM (dissolved organic matter) loading during stormflow, and these periods of elevated DOM load coincided with DO depletion. Laboratory PARAFAC analysis and in situ fluorescence measurements further showed that this DOM pulse was dominated by protein‐like, labile dissolved organic matter (LDOM), whereas humic‐like fractions contributed less to the overall DOM pool during stormflow. Together, our observations provide new empirical high‐frequency in situ evidence linking storm‐mobilized LDOM to riverine hypoxia under extreme hydroclimatic conditions. While observations presented here were made possible by a fortuitous opportunity, it is important to acknowledge such occurrences can be crucial for advancing scientific discoveries. Our findings therefore underscore the value of continuous high‐frequency in situ monitoring, as it enables the detection of transient disturbances that can produce outsized impacts on aquatic ecosystem properties.