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Water Quality Report Card

Aquatic Plants

B

Aquatic Plants

Macroinvertebrates

B

Black Bream

A

Waterbirds

C

Fish Kills

A

Water Quality

A

The Vasse Wonnerup wetlands are of great ecological value with seagrass meadows, native fish populations and large numbers of waterbirds. Yet the wetlands also struggle with poor water quality. Excessive nutrients flowing into the wetlands from the catch-ment can fuel blooms of phytoplankton and macroalgae. As well as being a nuisance and discolouring the water these blooms can be toxic, and as they die and decompose oxygen can be stripped from the water, potentially leading to fish kills and creating areas that cannot support a healthy aquatic ecosystem.
The water quality index aims to track the changes in concentration of nutrients in the wetlands as well as the concentration of phyto-plankton growing in the water. The index has been developed specifically for the Vasse Wonnerup wetlands as it is a unique system with significant modifications including surge barriers on both estuaries and a seasonally opened sand bar at its mouth.
Routine monitoring data collected at least monthly from across the wetlands by the Department of Water and Environmental Regula-tion is used to calculate the index in each year for both the wet (May – October) and dry (November – December) seasons. The index was develop by DWER (with the methodology report in development) using the following parameters:

  • Dissolved inorganic nitrogen – the most bioavailable form of nitrogen that phytoplankton and macroalgae consume;
  • Phosphate – the most bioavailable form of phosphorus that phytoplankton consume;
  • Chlorophyll a – measures of the amount of phytoplankton in the water by measuring the amount of chlorophyll pigment.

2023 Water Quality Grades Across Seasons and Regions

Upper Vasse

Wet Season

B

Dry Season

B

Lower Vasse

Wet Season

D

Dry Season

B

Vasse Channel

Wet Season

D

Dry Season

A

Wonnerup Inlet

Wet Season

A

Dry Season

A

Lower Wonnerup

Wet Season

B

Dry Season

A

Upper Wonnerup

Wet Season

B

Dry Season

B

Key Findings

  • The overall water quality throughout the wetlands in both the wet and dry seasons of 2023 was ‘Very Good’.
  • The only regions with a ‘poor’ rating in 2023 were the Lower Vasse and Vasse Channel in the wet season due to high concentrations of dissolved inorganic nitrogen, in particular ammonium. The high concentrations of ammonium likely developed due to low water movement in early winter. It took until August for the water level to be high enough to open the sand bar so water could flow out of the wetlands.
  • The very low rainfall in spring and summer of 2023/24 led to low water levels in both estuaries in early summer. Because of this the gates in the Wonnerup surge barrier were opened earlier than normal and the earlier introduction of seawater led to an improvement in the water quality index for the Lower Wonnerup in the dry season.
  • Due to the low rainfall the maximum salinities in both estuaries reached exceedingly high values with a maximum average salinity of 106 ppt and 87 ppt in the Upper Wonnerup and Upper Vasse respectively. These values are significantly higher than 35 ppt, the salinity of seawater.

Water Quality Grades Over Time

Wet Season

SpringUpper VasseLower VasseVasse ChannelWonnerup InletLower WonnerupUpper WonnerupDownload Reports
2023BDDABBDownload Report
2022CDCABC
2021DDEABB
2020DDDBCD
2019CDEBDC
2018BDEDDD
2017BCCBAB

Dry Season

YearUpper VasseLower VasseVasse ChannelWonnerup InletLower WonnerupUpper WonnerupDownload Reports
2023BBAAABDownload Report
2022ECAABD
2021CCBABE
2020CBAACD
2019ABAACE
2018CCAACB
2017BBAAED

Grades

A

Excellent

B

Very Good

C

Fair

D

Poor

E

Very Poor

View other report cards

Aquatic Plants

B

Aquatic Plants

Macroinvertebrates

C

Black Bream

D

Waterbirds

D

Fish Kills

A