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Cyanobacteria blooms are a concern because they can produce toxins which can kills
animals in the lakes. Lake Monger (Galup) has cyanobacteria blooms most summers when
waters are warm. This results in little or no birdlife, something we should do to everything to
avoid in Perry Lakes.
Before the Herdsman Main was diverted to West Lake in May 2023, phosphorus inputs
came mainly from road runoff, from fertilisers used on the surrounding parkland and from
native and introduced animal faeces. Road runoff also contains fertilisers from household
gardens.
Various studies of stormwater quality in Perth have reported lower values than from runoff in
most cities. This could be because our urban areas are not as dense and our sandy soils
infiltrate rainfall rather than produce runoff and soil that can contain P. Regular street
sweeping has also been shown to reduce nutrients in stormwater.
Toxic cyanobacterium or blue-green algae (Anabaena) that is affecting all of Lake Monger (Galup).
It is important to understand the difference between P concentrations and contents. PerryLakes no longer overflows into the Subiaco Main Drain so even low concentrations of P entering the lakes can build up over time (i.e. increasing its content). For example, the average concentration of P in the Herdsman Main Drain is only 0.14 mg/L but if 2 million litres per day is diverted into West Lake, the amount of P that is added is about 100 kg per annum. The pumps can deliver about 3.5 million litres per day so this estimate may be conservative.
The lakes discharge water through their beds into the underlying Superficial Aquifer which will remove some P. Losses are most likely when the lake beds are dry, and organic P is oxidised to inorganic P which is more mobile.
Analyses of sediments in June 2019 showed that total P ranged between 557 and 1,110mg/kg in East Lake and 222 and 584 mg/kg in West Lake which was significantly more than concentrations in the surface water. The accumulation of nutrients in the sediments can serve as a source for algal growth if disturbed. Carp can remobilise P through their feeding habits.
A study has shown that the nitrogen in groundwater was greatest downstream of Camel Lake, then West Lake and finally East Lake reflecting their degrees of drying and release of nutrients from the lake beds. Phosphorus is largely removed when passing through the aquifer because of the presence of lime and in some cases iron oxyhydroxides in yellow sand.
The Town of Cambridge has been monitoring the levels of analytes in the Herdsman Main Drain and in each lake. The results are shown in Table 1. Total N and P are shown because organic forms can be mineralised into more plant-available inorganic forms if conditions are suitable as described above.
Toxic blue-green algae, Microcystis, forming its telltale red scum in Lake Jualbup, Shenton Park.
Table 1.
The table left indicates percentage of times each level exceeded the 95% trigger level for toxicants in freshwater (ANZG 2018) and range in concentrations between 6 June 2018 and 19 December 2023.
In essence, it shows that the phosphorus levels in the main drain are slightly less than in either lake, but levels can still exceed the trigger level for eutrophication.