Threats
Eutrophication
The risk of Perry Lakes experiencing severe algal blooms.
Dr Don McFarlane · Science Lead, FoPL
Photo: Ian Stewart
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The science
What is eutrophication?
Phosphorus is the main driver of eutrophication at Perry Lakes. When levels rise too high, conditions become favourable for toxic cyanobacteria — the same blue-green algal blooms that have made Lake Monger (Galup) largely uninhabitable for birdlife in summer. Once a lake experiences a bloom, cyanobacteria leave behind cysts that make future blooms more likely. Prevention is critical.
Perry Lakes no longer overflows into the Subiaco Main Drain, which means even low concentrations of phosphorus entering the lakes can accumulate over time. Since 2023, when the Herdsman Main Drain was diverted into West Lake, phosphorus inputs have increased significantly — the drain delivers an estimated 60–100 kg of phosphorus per year.
Perry Lakes no longer overflows into the Subiaco Main Drain, which means even low concentrations of phosphorus entering the lakes can accumulate over time. Since 2023, when the Herdsman Main Drain was diverted into West Lake, phosphorus inputs have increased significantly — the drain delivers an estimated 60–100 kg of phosphorus per year.
“Eutrophication is an excess of nutrients which causes a dense growth of plant life.”
Toxic cyanobacteria (Anabaena) at Lake Galup — a situation we are working to prevent at Perry Lakes. Photo: FoPL
The data
Current phosphorus status
How often key water-quality measures exceed safe trigger levels across both lakes and the inflowing drain. Higher percentages mean more frequent exceedance.
| Analyte | West Lake | East Lake | Herdsman Main Drain |
|---|---|---|---|
| Total P — % of time exceededTrigger 0.06 mg/L · range 0.04–43 mg/L | 67% | 50% | 44% |
| Total N — % of time exceededTrigger 1.5 mg/L · range 0.4–3.4 mg/L | 20% | 30% | 56% |
| Dissolved oxygen (saturation) — % below trigger | 64% | 50% | 71% |
| Total dissolved solids (salinity)mg/L | 638 | 673 | 469 |
| pH | 7.24 | 7.75 | 7.27 |
Percentage of times each analyte exceeded the 95% trigger level for freshwater toxicants (ANZG 2018). Data: 6 June 2018 – 19 December 2023. Source: Town of Cambridge. exceeds trigger within range
The phosphorus balance
To keep phosphorus levels stable, FoPL needs to remove as much phosphorus as enters the lake each year — approximately 100 kg. The most practical way to do this is by harvesting aquatic plants like bulrush (Typha) and water fern (Azolla), which naturally absorb phosphorus as they grow.
Compounding pressures
Other contributing factors
Carp disturbance
Bottom-feeding carp stir up lake sediments and remobilise stored phosphorus back into the water column, increasing bloom risk. Electrofishing management is required to keep numbers down.
Microcystis forms its telltale red scum at Lake Jualbup, Shenton Park. Photo: FoPL
Reed encroachment
Bulrush and water-fern growth, fuelled by excess nutrients, reduces the open water that waterbirds depend on. Harvesting this plant material also removes phosphorus from the system.
Bulrush (Typha) encroaching on open water at West Lake. Photo: FoPL
What can be done
Possible solutions
A combination of management actions, applied together, can hold phosphorus in balance and reduce the risk of a toxic bloom.
- Harvest and remove plant material — bulrush (Typha) is the most suitable. At least 100 kg of phosphorus must be removed annually to maintain the status quo.
- Adjust pumping rates so part of each lake dries each autumn, allowing oxidisation and phosphorus loss through the aquifer.
- Plant vegetation that shades the lake — toxic algae grows faster in warm, sunlit water.
- Reduce phosphorus additions to turf around the lakes, including Alderbury Reserve.
- Add crushed limestone and/or ironstone gravel to inlet areas of West and East Lake to remove phosphorus on entry.
- Ensure streets in the Subiaco Main Drain catchment are swept several times each year.
- Remove carp through electrofishing to reduce sediment disturbance and phosphorus remobilisation.
- Consider additions of PhosLock™ when phosphorus levels become excessive or when a bloom occurs.
- Investigate pumping nutrient-rich water from East Lake into the Subiaco Main Drain inlet.
References
Further reading
A combination of management actions, applied together, can hold phosphorus in balance and reduce the risk of a toxic bloom.
- Australian and New Zealand Government (2018). Guidelines for fresh and marine water quality. waterquality.gov.au
- GHD (2019). Perry Lakes Environmental Baseline Assessments: Baseline Surface Water and Sediment Quality. Report to the Town of Cambridge.
- McFarlane, D., Bourke, S. and de Castro Tayer, T. (2019). Hydrochemical implications of wetland drying and augmentation options at Perry Lakes. Report to the Town of Cambridge.