Low dissolved oxygen and oxygenation

Low dissolved oxygen

Oxygen is required by the majority of aquatic organisms for respiration and an oxygen deficiency in water poses a threat to our aquatic ecosystems. The effect of low (>2 -4 mg/L) to hypoxic (>0 to 2.0 mg/L) dissolved oxygen concentrations upon fish and other aquatic life is highly dependent on the duration and extent of hypoxia, the sensitivity of individual species, their life stage and mobility, but it can cause acute stress and death (fish kills) or may impact on the healthy functioning of the ecosystem through various sub lethal effects.

Oxygenation

Artificial oxygenation was extensively trialled and is now an effective management tool in the Swan and Canning Rivers. Artificial oxygenation provides a refuge and promotes the ecological health of the rivers by improving water quality in terms of increased availability of dissolved oxygen, which is essential to support fish and other aquatic life. The maintenance of an oxygenated water column promotes healthy ecological processes that break down organic matter and recycle nutrients, which in turn enhances the public amenity of the Swan and Canning rivers by reducing unpleasant odours and occurrence of nuisance algal blooms.

Oxygenation concept diagram

Five oxygenation plants are managed and operated by Department of Water in partnership with the Swan River Trust, two on the upper Swan at Guildford and Caversham, and three on the Canning River above the Kent St weir at Bacon St, Camsell Way and Nicholson Rd.These locations experience hypoxia for an extended period of the year owing to the oxygen demand of the ecosystem exceeding the natural rate of oxygen replenishment achieved through the processes of wind mixing, diffusion and photosynthesis. Low dissolved oxygen concentrations can arise through natural phenomena that include seasonality, changes in river flow, and both saline and thermal stratification of the water column. However, the main drivers of the elevated oxygen demand in the Swan and Canning River estuary are nutrient enrichment and organic loading that the rivers receive from the catchment.

Map of oxygenation plant locations

Oxygenating rivers and estuaries for environmental objectives is novel and highly specialised; there are no other known cases of estuarine oxygenation in Australia. The oxygenation plants utilise concepts and technology developed internationally, particularly for the treatment of stratified reservoirs. A major design challenge for the use of artificial oxygenation in the Swan and Canning river estuary was selecting an appropriate technology for application to a shallow, warm and in the case of the Swan River, a seasonally saline water body. Side stream super-saturation (SSS) was chosen because it is not dependent upon extended contact time in-situ between oxygen bubbles and the water column. The plants work by pumping water from the river, supersaturating it with oxygen within a land based plant, before returning the now oxygen rich water back to the river via distribution pipes. Guildford oxygenation plant dissolver

The Department of Water monitors the response of the river to the operation of the plants via real time monitoring buoy data and weekly physical profiling of the water column. Operational regimes and oxygen delivery rates of the plants are adjusted to ensure that dissolved oxygen concentrations within the area of influence are maintained at optimal levels to support fish and other aquatic life.

The Department of Water has assessed the effectiveness of the plants via experimental studies and modelling (see Water Science Note #001). For example, in 2012 water column profiling at Caversham undertaken prior, during and post plant operation demonstrated that dissolved oxygen concentrations increased incrementally each day until the plant was turned off, from which point a rapid decline in oxygen concentrations was observed. The impact of plant operations was larger at sites closer to the plant, for example at the Caversham (CAV) monitoring site 0.2 km upstream but sites further away, for example at Middle Swan Bridge (MSB) 3.2 km upstream, were also affected by the operation of the plant.

Effective operation of the Swan River and Canning River oxygenation plants can maintain adequate oxygen concentrations in the both the upper Swan River estuary and the Canning River upstream of the Kent St weir. Without operation of the oxygenation plants, hypoxic conditions would prevail for long periods.

Caversham oxygenation plant water column profiles before during and after operation.

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