Episodic flooding causes sudden deoxygenation shocks in human-dominated rivers

Dissolved oxygen (DO) sustains many biological and chemical processes in rivers, yet little is known about how changing hydroclimate alters lotic oxygen content. Elevated river discharge (Q) favors turbulent mixing of river ecosystems and influx of atmospheric oxygen; however, floods also increase influx of nutrients and oxygen-consuming materials from land, making it difficult to predict the net effects of hydrological extremes on lotic DO regimes over large geographic areas. Here, we analyzed daily records of DO, percent oxygen saturation (DO%sat), and Q over three years in 1156 Chinese rivers. We found that both DO concentrations and DO%sat declined with increasing Q in 69.1% and 55.7% of the rivers, respectively. Flood events (Q > 95th percentile) caused abrupt declines in both DO (19.7%) and DO%sat (16.2%) in 80.1% and 69.4% of the rivers, respectively. Land use practices had an important influence on these sudden deoxygenation events, with the magnitude of oxygen decline during floods correlated positively with coeval changes in lotic ammonium content (p < 0.001) and agricultural and urban land use (r2 = 0.39, p < 0.001). Further, the frequency of anoxic or hypoxic events was greatest in rivers within agricultural or urbanized regions, while their deoxygenation shocks were more intense. Despite the expectation that elevated Q would increase water aeration, we found that episodic floods caused rapid deoxygenation of rivers in agricultural and urbanized landscapes. Given that future atmospheric warming will increase the frequency and severity of flood events, sudden deoxygenation shocks are likely to reduce biodiversity and degrade the food web integrity of rivers within developed landscapes.