A 3D hydrodynamic-biogeochemical model for assessing artificial oxygenation in a riverine salt-wedge estuary

Abstract

The presence of anoxia and hypoxia in bottom waters is regularly observed in salt wedge riverine estuaries. The extent of oxygen depletion depends both on physical circulation patterns and biogeochemical processes such as organic matter mineralization, photosynthesis and sediment oxygen demand. Over the past decade, reduced flows in the Swan-Avon River catchments have led to reduced flushing of nutrients and organic matter in the Swan-Canning estuarine system in Western Australia (WA), and more intense stratification that has been able to penetrate further upstream. As a result, hypoxia and anoxia in the upper reaches is a now a persistent management challenge due to detrimental effects on estuarine biodiversity and overall amenity and health of the river. As part of a multi-pronged strategy to tackle the problem of eutrophication in the estuary, several oxygenation plants have been installed in the upper reaches of the river that use micro-diffusers to re-oxygenate anoxic and hypoxic waters. In this study we have aimed to simulate oxygen dynamics and assess the overall efficiency of the oxygenation plants through development of a finite volume coupled hydrodynamic - biogeochemical model of the Upper Swan estuary (Figure 1). The model accurately captured the salt-wedge dynamics and extent and severity of hypoxia and anoxia. The model has been used to explore budgets of oxygen and nutrients for different plant operational regimes, and has allowed us to define the spatiotemporal benefit of these scenarios. The results are used to inform the most cost effective way to achieve the desired improvements in river habitat.