Artificially oxygenating the Swan River estuary increases dissolved oxygen concentrations in the water and at the sediment interface

Abstract

The upper reaches of the Swan River estuary (Perth, Australia) has a history of eutrophication-related oxygen depletion, which has contributed to poor water quality and fish deaths. To alleviate hypoxic conditions, a trial side-stream supersaturation (SSS) oxygenation plant was established at Guildford (39 km upstream of the estuary mouth) in 2009. After notable success, a second plant was constructed at Caversham (44.2 km upstream of the estuary mouth) in 2011. Oxygenation plants have more commonly been used to treat deep, freshwater lakes and reservoirs and this is a pioneer application to a shallow estuary. We report on the effect of the Caversham plant on water and sediment condition over a 12-day experiment: before, during and post-plant operation. We monitored several physical and chemical parameters collected from daily longitudinal transects, moored continuous loggers, an acoustic Doppler current profiler and an in-situ sediment microprofiler. Oxygenation immediately improved dissolved oxygen concentrations in the water column and the distance over which the effect was observed was strongly influenced by the hydrodynamics of the estuary. After five days of oxygenation, water column dissolved oxygen had increased over a distance in excess of 11.5 km. In addition, oxygenation improved dissolved oxygen concentrations at the sediment-water interface, thereby increasing oxygen fluxes into the sediment. Ultimately, artificially oxygenating the Swan River estuary provides a refuge for fauna while facilitating aerobic decomposition of organic matter and enhancing nutrient cycling at the sediment-water interface. In light of the increasingly critical state of urbanised estuaries world-wide, results from this study highlights 1) the effectiveness of oxygenation in improving water quality and its potential for facilitating ecosystem restoration, and 2) the diversity of environments in which artificial oxygenation can be applied.