Island pruning

In 2024, the City engaged a specialist consultant to prepare a Maylands Lakes Environmental Restoration Masterplan that will approach the water quality issues in a holistic manner.

As part of developing the Masterplan, the consultant Alluvium has been provided with all previous research and data from the City and undertaken additional analysis to better understand the water quality challenges.

This has included reviewing lake water and groundwater assessments undertaken by GHD, and others associated with the Maylands Lakes project. GHD had previously developed estimates of water balance that Alluvium has further refined by considering additional catchment runoff and nutrient concentrations.

Alluvium has also considered international literature around the rates of contributions from birds (Hahn et al 2007) and estimates of cormorant numbers from observations and those reported by community members. The Hahn paper highlighted a per capita loading rate of non-breeders of between 4.5–11.5g Nitrogen per day and 2.1–3.2g Phosphorus per day in great cormorants. Using groundwater contributions of 0.23mg/L and surface runoff contributions of 0.1 mg/L for total phosphorus, Alluvium used the water balance estimations to calculate the loads from the various water sources (groundwater, surface water and lake water), and then estimates of the bird numbers, which ranged from 30 to 300 birds, conservatively estimating 100 birds present for nine months of the year. Alluvium also investigated similar estimates for total nitrogen.

This research has shown the bird contribution is significant (up to a third), and if bird numbers were much lower, the contribution would still be a factor in the overall function of the lakes. Advice received from ecologists, water quality scientists and hydrogeologists indicate algal blooms in Perth occur whenever the water conditions are favourable and there is sufficient phosphorus to ‘feed’ the blooms. Any source of phosphorus, whether it be from groundwater, surface water, sediments or birds, is going to provide the necessary ‘food’ for algal blooms to occur.

Reducing these inputs is a primary focus of the City’s actions, therefore focusing on reducing nutrient inputs, including those from birds, will all help make conditions less favourable for future algal blooms. The City will be planting vegetation on the islands across our winter planting seasons. These works will include establishing appropriate endemic species on the islands to support a more diverse habitat, including sedges which will also act as a natural filter to remove nutrients.