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Low oxygen conditions

Adequate dissolved oxygen in the water is crucial for aquatic life. When levels of oxygen in the water become less than ideal, many organisms can become stressed and in extreme cases, where oxygen levels drop to or near zero, this can result in fish deaths. Very low levels of oxygen in the rivers over prolonged periods can also cause unpleasant odours and result in excessive nutrient release from sediments, which encourages algal blooms.

Dissolved oxygen levels are categorised as:

  • Well oxygenated (> 6 mg/L)
  • Oxygenated (> 4 and ≤ 6 mg/L)
  • Low oxygen (> 2 and ≤ 4 mg/L)
  • Hypoxic (> 0 and ≤ 2 mg/L)
  • Anoxic (0 mg/L)

Low oxygen conditions in the rivers are exacerbated by the decomposition of excess organic matter (e.g. algae blooms, leaves, debris, pet/livestock waste, wastewater overflows), through microbial action and low water movement.

When organic material enters the river system, naturally occurring microbes break it down. These microbes need oxygen to metabolise, so the more active they are, the more oxygen they remove from the water. Historical river and catchment management practices, combined with a harsh climate of droughts and flooding rains, result in large amounts of organic matter being periodically deposited into the river. When this happens, microbial activity can increase rapidly to break down the new  load of organic matter and oxygen levels in the water often drop quickly as a result.

Stratification occurs when water at the surface of a lake or river does not mix with the bottom waters. This also affects oxygen levels in the Swan Canning river system. A salt wedge gradually moves upstream from the estuary mouth in Fremantle during summer when rains have ceased and freshwater flows diminish. Because salt water is heavier than fresh water, it sinks to the bottom of the estuary and does not mix with the fresher surface waters, which are oxygenated through diffusion at the air-water interface. As a result, this salty bottom water can become hypoxic or anoxic.

Tackling low oxygen

Parks and Wildlife combines medium and short-term approaches to improving low oxygen levels.

Medium-term strategies to reduce the nutrient and organic load entering the rivers include the Drainage and Nutrient Intervention Program, Swan Canning Water Quality Improvement Plan and local water quality improvement plans.

In the shorter-term, Parks and Wildlife works in partnership with the Department of Water to deliver oxygen relief to targeted areas of the Swan Canning Riverpark. The oxygenation program began in 1998 in response to low oxygen conditions and severe algae blooms in the Canning River, upstream of the Kent Street Weir. There are now three plants oxygenating a 5km stretch of the Canning River above the weir via a piped distribution network. There are also two oxygenation plants located at Caversham and Guildford in the Swan River that can provide oxygen to a 10km zone, depending on tidal movement.

The oxygenation program aims to maintain a concentration of 4mg/L of oxygen or higher in the water column within each of the oxygenation zones. This threshold concentration is set at a level that supports a wide range of aquatic life. Many organisms can survive in or will move away from environments where oxygen drops below 4 mg/L, but can become stressed if low oxygen conditions continue for prolonged periods.

Oxygenation works by supersaturating river water with oxygen. Water is taken from the oxygen-depleted bottom waters of the river and mixed under pressure with oxygen gas in the plants. The highly oxygenated water is then piped back into the bottom waters of the river where it mixes, distributing dissolved oxygen throughout the water column. The effectiveness of the oxygenation program is monitored every week, to help inform the operational management of the plants. The most recent oxygen levels in the Swan and Canning rivers are available in weekly water quality reports.