Sediment-water oxygen and nutrient fluxes in a hypoxic estuary

Abstract

Examining sediment-water fluxes can be essential for understanding aquatic chemistry and ecosystems, but the numerous coupled processes near the sediment-water interface make this a difficult system to analyse. This is especially pertinent in periodically hypoxic, nutrient-rich waterways, where the sediment oxygen demand and rates of sediment nutrient flux interact with the water column chemistry and ecology. In this paper, a region of the seasonally-hypoxic Swan River estuary in Perth, Western Australia, is examined to determine nutrient fluxes and the effect of oscillating bottom water oxygen conditions. The oxygen fluctuations investigated are designed to capture variability on seasonal scales as well as with high frequency (daily) changes in water column oxygen concentrations due to pumping by an oxygenation plant. A numerical sediment diagenesis model is used to examine the system under natural and pumping conditions. The model is validated against available datasets of porewater depth profiles and fluxes of NH4 + , NO3 - and dissolved inorganic carbon. The simulated flux of NH4 + is found to be 16.3 mmol N d -1 . The peak flux of PO4 3- corresponds to the period of anoxia. The depth of oxygen penetration into the sediment is between 1 and 1.5 mm, which shows little variation between oxic and anoxic bottom water conditions. It was found that there was limited time lag between the bottom water oxygen concentration and the sediment porewater concentration and therefore the sediment oxygen demand was largely insensitive to antecedent conditions. The model was used to develop a simple oxygen flux relationship for use in water quality models of the area. Areas for improvement of the model include the need for refining the timescale of oxygen fluctuation to capture the effects of changes shorter than one day; specifying separate organic matter rate constants for different reaction pathways; changing the effects of bioturbation and bioirrigation to reflect hypoxic environments; and including the effects of feedbacks into the water column, which can next be achieved by coupling this sediment model with a pelagic hydrodynamic and ecosystem model.