Lake Tuggeranong records lowest Waterwatch report score to date

High levels of pollutants from surrounding suburbs saw reduced biodiversity in the lake, with higher inflows from increased rainfall the biggest contributor.

Very high concentrations of nitrates were measured flowing from stormwater systems at the northern end of the lake, with large blue-green algae blooms around the town centre from summer until April.

Of 96 sections of the upper Murrumbidgee Catchment measured, Lake Tuggeranong was one of 18 reaches with declined health in 2021, as was Lake Ginninderra.

Ginninderra Creek, the most urbanised catchment, has been significantly impacted by the growth of Canberra's north, according to Waterwatch's annual report.

Gungahlin Pond and Lake Ginninderra both experienced dissolved oxygen levels and algal growth in 2021, the Catchment Health Indicator Program Report found.

Southern ACT catchment group coordinator Martin Lind said when high rainfall hits the suburban areas it tends to wash whatever has been lying around for 12 months into urban lakes.

Mr Lind said bacterial activity in the pollution traps around Lake Tuggeranong led to nitrates being quite high.

"So you've got a high load of pollutants coming in from the surrounding catchment, it's ending up in Lake Tuggeranong and it's sitting there for a while like a fish tank," he said.

"If it's high enough it'll push through over the weir and end up in the Murrumbidgee which is a big, big fear."

The report relied on data from 2179 surveys collected by more than 200 citizen scientists at catchments across Cooma, Ginninderra, Molonglo, Southern ACT and Yass.

The report found phosphorus concentrations had declined since the bushfires, with rivers becoming clearer, and catchments like Hospital and Bogong creeks in Namadgi National Park improving markedly.

Water, Energy and Emissions Reduction Minister Shane Rattenbury said the report highlighted the importance of action to improve stormwater. This included cleaning leaves from gutters and minimising fertiliser use.

"It also shows the value of having more naturalised urban landscapes to treat stormwater before it enters rivers and streams, a function many of our wetlands deliver," Mr Rattenbury said.

Environment, Planning and Sustainable Development Directorate Executive Group Manager Ian Walker said the ACT has the capacity to develop more wetlands, as development continued.

Mr Walker said the eight-year-old program had required a $93 million investment to build infrastructure at 20 sites across the territory.

Infrastructure varied from above-ground creation of wetlands like the site at Holder to large below-ground installations, known as gross pollutant tracks, which capture large loads of rubbish, Mr Walker said.

"The opportunity to deliver more wetlands depends on community interest and for the ACT government and the Commonwealth government to make those investment decisions," he said.

"We've been really fortunate to have lots of rain post the bushfire and while that's created some issues around erosion and flooding of particular built assets, it's actually really helped the environment to recover and you can see across the territory now, vegetation growth is really good."

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While 2021 saw consistently better flows in the Cooma region, the upper reaches of the Murrumbidgee River declined in condition slightly with the report indicating degradation could be linked to feral horses.

In recent years, Waterwatch volunteers have observed increasingly pugged banks - destabilised by hard hooves - with riversides heavily grazed and algal growth and sedimentation increasing.

While overall water quality scored excellent for reaches in the region in 2021, some measurements began to show a decline, such as increased nutrient readings.

Waterbug surveys detected a reduction in some pollution-sensitive species, with some highly-sensitive bugs missing altogether.

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