Hydrodynamic controls on oxygen dynamics in a riverine salt wedge estuary, the Yarra River estuary, Australia

摘要

Oxygen depletion in coastal and estuarine waters has been increasing rapidlyaround the globe over the past several decades, leading to decline in waterquality and ecological health. In this study we apply a numerical model tounderstand how salt wedge dynamics, changes in river flow and temperaturetogether control oxygen depletion in a micro-tidal riverine estuary, theYarra River estuary, Australia. Coupled physical–biogeochemical models havebeen previously applied to study how hydrodynamics impact upon seasonalhypoxia; however, their application to relatively shallow, narrow riverineestuaries with highly transient patterns of river inputs and sporadicperiods of oxygen depletion has remained challenging, largely due todifficulty in accurately simulating salt wedge dynamics in morphologicallycomplex areas. In this study we overcome this issue through application of aflexible mesh 3-D hydrodynamic–biogeochemical model in order to predict theextent of salt wedge intrusion and consequent patterns of oxygen depletion.The extent of the salt wedge responded quickly to the sporadic riverineflows, with the strength of stratification and vertical density gradientsheavily influenced by morphological features corresponding to shallow pointsin regions of tight curvature ("horseshoe" bends). The spatiotemporalpatterns of stratification led to the emergence of two "hot spots" ofanoxia, the first downstream of a shallow region of tight curvature and thesecond downstream of a sill. Whilst these areas corresponded to regions ofintense stratification, it was found that antecedent conditions related tothe placement of the salt wedge played a major role in the recovery ofanoxic regions following episodic high flow events. Furthermore, whilst athreshold salt wedge intrusion was a requirement for oxygen depletion,analysis of the results allowed us to quantify the effect of temperature indetermining the overall severity and extent of hypoxia and anoxia. Climatewarming scenarios highlighted that oxygen depletion is likely to beexacerbated through changes in flow regimes and warming temperatures;however, the increasing risk of hypoxia and anoxia can be mitigated throughmanagement of minimum flow allocations and targeted reductions in organicmatter loading. A simple statistical model (R2 > 0.65) issuggested to relate riverine flow and temperature to the extent ofestuary-wide anoxia.