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Lower Vasse River and Toby Inlet Science

The science of the Lower Vasse River and Toby Inlet focused on trials to improve water quality and visual amenity over summer months and developed models to increase flows and/or flushing and improve water quality.  The Reconnecting Rivers model was developed to investigate options for increasing flows into the Lower Vasse River and the Reconnecting Toby Inlet model was developed to investigate options for increasing tidal flushing. 

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Water treatment trials

An innovative water treatment trial was carried out in the Lower Vasse River over the 2016/17 summer to assess the effectiveness of a newly developed phosphorous-binding hydrotalcite-clay (HT-clay) to reduce algal blooms and improve water quality.

The HT-clay works by removing phosphorus from the water column while preventing phosphorus release from the sediments. To test the effectiveness of the clay, different doses were applied to fifteen mesocosm tanks that were embedded in the sediment of the river upstream of the Causeway Road Bridge. 

The Department of Water and Environmental Regulation coordinated the trial and carried out weekly monitoring measuring water quality parameters, nutrients and phytoplankton (algae). 

Key findings of the 2016/17 HT-clay trial include:

  • The clay treatment reduced algae growth and improved water quality. This was evident in both, visual assessment and evaluation of algal growth indicators such as chlorophyll concentration and turbidity.
  • FRP and total phosphorus (TP) concentrations remained below the recommended water-management guideline thresholds over the entire 5 month trial-period in all mesocosms treated with appropriate HT-clay doses.
  • The clay treatment reduced the soluble phosphorus fraction (measured as filterable reactive phosphorus, FRP) by up to 95% with the highest tested clay dose.
  • Application of the clay resulted in a noticeably improved visual appearance.

After the successful mesocosm trial, the trial was up-scaled in summer 2017-18 to a
larger treatment area in the river with treatments separated by PVC curtains. This trial also showed that HT-clay was effective at reducing algal blooms within hours of application, even after the bloom was established. More information can be found in the project factsheet.

Water lily investigation

The Water lily investigation was carried out over the 2017 summer with the aim to better understand the impacts of the introduced Mexican waterlily (Nymphaea mexicana) on water quality and ecology of the Lower Vasse River.

The Mexican water lily is a plant native to the United States and Mexico and an introduced species in the Lower Vasse River.  Anecdotal evidence suggests that the lily has been present in the Lower Vasse River and the New River Wetlands for nearly 20 years occurring as small isolated infestations. Over recent years the spread of the water lily has increased rapidly throughout the river raising concern in the local community and with local authorities. 

In response to this concern, the Department of Water and Environmental Regulation commissioned Ottelia Ecology to undertake this study. 

The study took place in the Lower Vasse River downstream of the Busselton Bypass through to the lower section of the river adjacent to the Busselton museum. View Map.  Twenty seven sites were established along the river in areas upstream, downstream and within water lily stands. Water quality, sediment nutrients, aquatic vertebrate and water lily biomass and nutrient content were measured at representative sites. 

Some of the key findings of this study to date include:

  • Dissolved oxygen levels were extremely low below water lily stands, with measurements consistently below critical levels for aquatic fauna (2.0 mg/l)
  • Despite lower oxygen levels, water lily sites supported higher abundance and diversity of invertebrates than other areas in the river
  • Total phosphorous concentrations were very high at all sites, but were significantly lower downstream of water lily stands
  • Algal blooms were considerably higher downstream of water lily tands, evidenced by very high chlorophyll a values and high turbidly
  • Some water quality parameters (algae levels, water clarity and visual amenity) were considerably better within and upstream of water lily stands, suggesting that the lilies did improve water quality, although the mechanisms for how they did was not determined in this study

A report and factsheet are available on the study. Findings from this study will be used to develop future management strategies of the Mexican water lily in the Lower Vasse River.