Typhoon-induced depth-dependent nitrogen release from reservoir sediments: Insights from high-resolution ROV monitoring

Typhoon-induced extreme rainfall events have become more frequent recently due to climate change. However, their specific effects on endogenous nitrogen release at the sediment–water interface, particularly in deep-water environments, remain poorly understood. Using a custom remotely operated vehicle (ROV), we conducted high-frequency underwater sampling and real-time monitoring of depth-dependent sediment nitrogen (TN, NH₄-N, NO₃-N) release from July to October 2023 in Gankeng Reservoir (Shenzhen, China), spanning a once-in-a-century typhoon rainstorm. In deep sediments (7–9 m), TN and NH₄-N concentrations in pore water increased sharply, 2.2 to 4.8 times above pre-typhoon levels, while NO₃-N in shallow sediments (3–5 m) decreased significantly. Although hydrochemical parameters returned to pre-storm levels within 43 days, TN and NH₄-N in deep pore water remained ∼20 % higher than before the event. Analyses of nitrogen speciation, enzyme activity, and correlations indicate that enhanced NH₄-N release was driven by typhoon-induced shifts in hydrochemistry and microbial activity. PCA and hierarchical clustering further highlight distinct nitrogen release patterns between shallow and deep sediments. These findings demonstrate that typhoon rainstorms can substantially accelerate TN and NH₄-N emissions from sediments, offering critical insights for proactive water quality management to mitigate endogenous eutrophication risks in reservoirs under a changing climate.