Episodic flooding causes sudden deoxygenation shocks in human-dominated rivers

Area of Science:

• Environmental Science
• Ecology
• Hydrology

Background:

• Dissolved oxygen (DO) is crucial for riverine ecosystems.
• The impact of extreme river discharge (Q) on DO levels is not fully understood.
• High river discharge can increase aeration but also introduce oxygen-consuming pollutants.

Purpose of the Study:

• To investigate the effects of hydrological extremes, specifically floods, on dissolved oxygen (DO) and DO percent saturation (DO%sat) in Chinese rivers.
• To identify factors contributing to DO declines during floods.
• To assess the implications for aquatic ecosystems under changing climate conditions.

Main Methods:

• Analysis of daily DO, DO%sat, and Q data from 1156 Chinese rivers over three years.
• Statistical analysis to determine the relationship between Q and DO/DO%sat.
• Examination of flood events (Q > 95th percentile) and their impact on DO levels.

Main Results:

• DO and DO%sat decreased with rising Q in a majority of rivers (69.1% and 55.7%, respectively).
• Floods caused significant abrupt declines in DO (19.7%) and DO%sat (16.2%) in over 70% of rivers.
• Sharpest DO declines occurred in agricultural and urban areas, linked to increased ammonium and land-use intensity, leading to more frequent hypoxia.

Conclusions:

• Contrary to expectations, floods frequently reduce riverine oxygen levels, causing deoxygenation events.
• Urbanized regions showed faster DO recovery post-flood.
• Intensifying floods due to climate change pose a significant threat to aquatic ecosystems, especially in human-altered landscapes.