Temporal and spatial scales of oxygen depletion in a shallow tributary estuary in response to altered nutrient loading and elevated temperature

Numerical modeling tools in estuarine and coastal environments have become increasingly sophisticated and have readily reproduced seasonal and inter-annual dynamics of dissolved oxygen in large ecosystems. Less numerous are high resolution models that simulate carbon, oxygen, and nutrient variability in shallow (<5 m) environments, including the hourly-scale processes (e.g., diel cycling hypoxia) and high degree of pelagic-benthic coupling that characterize these systems. In this study, we implemented a coupled hydrodynamic-biogeochemical model (ROMS-RCA) in the Chester River estuary, a small, shallow, and turbid tributary within the Chesapeake Bay estuarine complex. We used the model to quantify oxygen dynamics in both vertically-stratified and mixed locations, and to determine the magnitude of oxygen depletion in each environment to altered external forcing. To address how warming temperatures will counterbalance proposed nutrient reduction strategies, we simulated a range of potential future conditions associated with elevated warming, elevated and reduced nutrient inputs, and altered seasonal variations in nutrient loads. The results indicate that oxygen concentrations respond strongly to external physical forcing in both environments, but that the reproduction of diel cycling hypoxia does not readily emerge from existing model formulations for plankton and sediment biogeochemical dynamics We address this latter challenge with a quantification of the relative contributions of water-column and sediment oxygen consumption rates to oxygen depletion across depth gradients in the estuary.