The City of South Perth said the green matter was not algae but duckweed, which was not harmful to humans or wildlife.
Gerry Coleman, who works in the area as a gardener, contacted the Southern Gazette with concerns about the condition of the lake.
“In a way it looks beautiful but when you consider that birds and wildlife and swimming in it, it doesn’t look very healthy,” he said.
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READ NOW“I think the council should be held accountable to fix it or even bring in the experts.”
Mayor Sue Doherty said the growth of duckweed was related to high nutrient levels in the lake.
“McDougall Lake is a receiving water body for stormwater in the area which picks up nutrients from the surrounding catchment area,” she said.
“These nutrients are then deposited into the lake. The duckweed feeds off this nutrient and coupled with warm weather conditions it flourishes.”
Ms Doherty said the result of rain in February and the continued warm weather further nutrient-loaded the lake, which enabled the duckweed to grow for a longer period.
“As the weather cools the duckweed will reduce in density,” she said.
“While duckweed is not harmful to wildlife or people, the City’s environment team monitor the occurrence and density of duckweed in McDougall Lake on a regular basis.
“In the 2017-2018 financial year a water sensitive urban design upgrade is planned for one of the inlets to help to reduce nutrients before they enter the lake.”
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