A $63,000 project to disrupt the build-up of toxic blue-green algae in Lake Burley Griffin had no measurable impact, the National Capital Authority has concluded.
For 18 months, the National Capital Authority has trialled the use of ultrasonic pontoons to control the growth of blue-green algae.
The pontoons were installed in Lotus Bay and West Basin in late 2022 in the hope that the devices could reduce the amount of harmful algae at popular swimming spots.
"This was an opportunity to test a new innovative way to reduce blue-green algae in Lake Burley Griffin," a spokesperson said.
The NCA continued collecting water samples during the recreation season from October to April around the lake and found that while levels of blue-green algae did reduce where the pontoon installed, there was a catch.
The levels went down across the lake, and the sites with the high-tech solution - which destroys the algae by causing disruptions in the water column - were no different to the trend across the lake.
"Overall, there was a decrease of around 40 per cent in the levels of BGA at both test sites compared with the previous recreation seasons," the agency's annual report states.
"However, all other recreation sites around the lake showed a similar reduction in BGA levels over the same period.
"These results suggest that reduction in total BGA is likely due to environmental factors, [rather] than directly attributable to the introduction of ultrasonic pontoons."
Closures of swimming spots due to high levels of algae occurred at the same rate whether or not a pontoon was present.
"Given the results, it is difficult to definitively state that introduction of the ultrasonic pontoons at the test sites had a positive impact on water quality at those locations," the NCA report states.
With the trial having concluded in June this year, the NCA says it will continue to monitor water quality, to see if water quality worsens at Lotus Bay and West Basin after the pontoons are removed.
The NCA will continue to look for other ways to limit blue-green algae, which occurs naturally in Canberra's freshwater lakes.
Previous trials of other solutions that targeted the algae were also unsuccessful, including a solar-powered circulator installed in 2011, with water experts noting there's no quick fix.
The ACT and federal governments have spent millions trying to stop blue-green algae on Canberra's waterways, however University of Canberra water expert Dr Rodney Ubrihien said solutions like the pontoons, or treating the water with copper or other chemicals, did not address the underlying problem.
Instead, a broader, catchment-based approach was needed.
"The long-term solution to blue-green algae is managing nutrients in the system," he said.
"This is something we can do [but] it's not easy."
Increasing development around the lake and along the streams that feed into the body of water increased the nutrients that caused algae to thrive.
Dr Ubrihien said to address the algae, the impact of these development decisions on water quality in the lake needed to be taken into account.
"The long-term solution is a hard one; a complex one," he said.
"Short-term solutions to manage blue-green algae in lakes, there's a lot of attempts that are generally not successful."
