Duckweed in McDougall lake
Journalist’s background: I had a tip off today from a member of the public, concerned about the lake at Neil McDougall Park. It appears to have quite a bit of algae on the surface, I'm unsure if this is usual for the lake. Anyway, I've prepared some questions - could I get a response back by noon on Thursday.
1. Is it algae on the surface of the lake, if not what is it?
2. Is it normal for so much algae to be on the surface?
3. Is there more algae at the moment compared to this time of year in previous years?
4. Can this much algae have any impact on the wildlife, such as ducks?
5. Will the lake improve as winter comes?
6. What is the City of South Perth doing about the algae?
7. What has caused the algae?
8. Has the issue been going on for long (months, years)?
Please attribute the following quotes to City of South Perth Mayor Sue Doherty.
Is it algae on the surface of the lake, if not what is it?
No, it is not algae. It is an aquatic plant species called Lemna minor commonly known as duckweed which is not harmful to wildlife or people.
Is it normal for so much algae to be on the surface?
The growth of duckweed is 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. These nutrients are then deposited into the lake. The duckweed feeds off this nutrient and coupled with warm weather conditions it flourishes. It is not a mat forming plant, it moves around the lake with wind and animal movement.
Is there more algae at the moment compared to this time of year in previous years?
As a result of the rains in February this year and the continued warm weather further nutrient loading of the lake has occurred which has resulted in the duckweed growing for a longer period than previous years. As the weather cools the duckweed will reduce in density.
Can this much algae have any impact on the wildlife, such as ducks?
The duckweed provides habitat and food for wildlife including birds and turtles.
Will the lake improve as winter comes?
The duckweed prefers warmer weather and will continue to reduce in density as the weather cools over autumn and winter.
What is the City of South Perth doing about the algae?
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.
What has caused the algae?
High nutrient levels and warm weather are the cause of the flourishing duckweed in the lake.
Has the issue been going on for long (months, years)?
Duckweed occurs naturally, generally through the summer months of each year.
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