Effects of heatwave events on dissolved oxygen in the Elbe Estuary

Extreme heatwaves significantly disrupt estuarine hydrodynamics and biogeochemical processes, reducing dissolved oxygen (DO) levels and posing a serious threat to water quality and ecosystem health. The Elbe Estuary is an example of the impact of high nutrient loads from the river basin on a land-sea continuum heavily used by humans. In the Elbe Estuary, especially downstream of Hamburg, low DO occurs frequently in summer, ever more often undercutting the normative minimum concentration of 7 mg/L (219 mmol/m³) for transitional waters according to national legislation. However, the mechanism responsible for DO changes in bottom waters under the influence of heatwaves is not clear. In this study, we simulate the spatial distribution characteristics of DO in the Elbe Estuary during the extreme heatwave in 2018 using the SCHISM-ECOSMO 3D coupled hydrodynamic-biogeochemical model and explore the predicted response to extremely high temperatures. The results show four heatwave events occurred in 2018 based on observational data, with the third event simulated to cause the most severe hypoxia in the Port of Hamburg. Heatwaves intensify stratification, suppress vertical mixing, and facilitate the lateral transport of low-oxygen water from the port basin to the main channel, leading to lower DO levels in the bathymetric jump region than in subsequent, even warmer, non-heatwave periods. The lowest DO location in the main channel shifts upstream due to reduced river discharge and the migration of the tidal convergence zone. The study reveals the effects of heatwave events on the distribution of DO in the estuary, providing an important scientific basis for estuarine environmental protection and water quality management.