Seasonal mechanisms of low-oxygen events in the Elbe estuary

The Elbe estuary located in northern Germany plays a critical role in European maritime trade, primarily through the Port of Hamburg. In this area, however, low-oxygen events i.e., lower dissolved oxygen concentration than expected for an extended period of time, threaten the ecological state and the provision of ecosystem services, especially in summer. Here, we use long-term observations and numerical modelling to analyze the co-occurrence of marine heatwaves i.e., anomalously water warmer than expected for an extended period of time, and low-oxygen events in the tidal Elbe from 2017 to 2024. Statistical analyses reveal a great level of co-occurrence between heatwaves and low-oxygen events specially in spring (31%) and summer (42%).

The degree of co-occurrence varies spatially, with stations downstream of the Port of Hamburg being more susceptible to low-oxygen events during heatwaves. Our model analyses show that mineralization and nitrification increase during spring heatwaves, and primary production also decreases during summer heatwaves, suggesting that different mechanisms act in each season causing low-oxygen events due to heatwaves. Water temperature controls the main oxygen-consuming biogeochemical processes, which are accelerated with warm periods and lead to low oxygen conditions.