Anthropogenic impacts on estuarine oxygen dynamics: A model based evaluation

Abstract

Direct human interference has been shaping todays estuaries for centuries, and depletion of dissolved oxygen (DO) frequently occurs in such anthropogenically impacted estuaries. Whereas the role of nutrient input as a major human impact driving DO depletion is clear, the effect of bathymetric modifications as another human impact is less well-known. Here, we aim at a better understanding of how DO dynamics are influenced by bathymetric modifications and changed nutrient input. Therefore, we introduce a coupled hydrodynamic-biogeochemical model (Untrim-Delwaq) and develop an idealized one-dimensional set-up of a vertically well-mixed, funnel-shaped estuary with strong human impact. The set-up dimensions are inspired by the Elbe Estuary (Germany) and model results show good agreement in comparison to observational hydrodynamic and water quality data. In particular, the model reproduces the dynamics of the summer oxygen minimum which regularly develops in the estuarine freshwater part. Analysis of our model runs shows that the estuarine biogeochemistry is dominated by heterotrophic degradation processes rather than primary production because of severe light limitation. A scenario analysis indicates that a reduction in input load scales down all biogeochemical processes. In contrast, a bathymetric change affects the estuarine system and its DO in a more complex way. In particular, the interplay between surface-to-volume ratio and the degradability state of the organic material is the most important factor which determines the capacity to recover high DO mineralization losses by atmospheric input. Thus, our study demonstrates the relevance of bathymetric factors during the assessment of human interference on DO dynamics and biogeochemical processes in estuaries.