Water Quality Report Card

The Vasse–Wonnerup Wetlands are among the most nutrient enriched systems in Western Australia. During winter, rainfall delivers freshwater and nutrients from the surrounding catchment into the wetlands. These nutrients support aquatic plants and a productive ecosystem but can also drive the growth of macroalgae and phytoplankton during the warmer months. In summer, as these blooms break down, oxygen levels in the water can decline, leading to poor water quality and occasional fish kills.

The Water Quality Index tracks changes in nutrient concentrations and phytoplankton biomass within the wetlands. It 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. The index uses monthly water quality data to assess conditions across the wet (May–October) and dry (November–April) seasons. It draws on the concentrations of key indicators, dissolved inorganic nitrogen, phosphate, total nitrogen, total phosphorus, and chlorophyll a.

The index has been developed by the Department of Water and Environmental Regulation, with a detailed methodology report in development, providing a consistent and straightforward basis for tracking changes in water quality over time.