Implications of direct anthropogenic pressures on dissolved oxygen dynamics in a well-mixed estuary
This thesis shows that the impact of man-made bathymetric modification on DO deficits in well-mixed estuaries is smaller than the eutrophication effect due to anthropogenic nutrient enrich-ment. The application of a tailored hydrodynamic and biogeochemical model configuration on an ideal-ized estuarine model domain demonstrates that DO dynamics are dominated by oxygen-consuming processes. A scenario analysis indicates that (i) a reduction in riverine nutrient and organic matter load scales down all biogeochemical processes, and (ii) the interplay between water surface-to-volume ratio and the degradability state of the organic material is the most important factor to determine the severity of the DO minimum. The latter emphasizes the rele-vance of bathymetric factors during the assessment of human interference on DO dynamics and biogeochemical processes in estuaries. The realistic effect of a modification which reflects 40 years of human activities is analyzed in a detailed 3D model domain of the Elbe Estuary. The impact of the anthropogenically higher im-pacted bathymetry exhibits an about 7% shorter water age, a 10 – 15 % lower ratio of water surface to underlying water volume and an about 5% lower oxygen saturation level in the estua-rine oxygen minimum zone. In comparison, a variation in nutrient and organic matter load of 50% at the river boundary has a much larger impact on the estuarine DO deficit. Nevertheless, bathymetric modifications pose an additional stressor to the oxygen system and the cumulative effect of many bathymetric modifications over centuries may be particularly relevant, especially in combination with the human activities related to the maintenance of the artificially deeper bathymetry.