Lake Tuggeranong research trial

Mesocosms Lake Tuggeranong

The Institute for Applied Ecology at the University of Canberra is conducting research in Lake Tuggeranong to identify in-lake treatments that could improve water quality and reduce the number of closures caused by blue-green algae.

Tuggeranong’s urban water filter

Lake Tuggeranong was created in 1987 as an attractive focal point for the new satellite town, a place for recreation, play, exercise and conservation. But it also has another critical role—to filter and clean the water flowing from Tuggeranong’s suburbs into the Murrumbidgee River.

Urban run-off is generally high in litter, organic matter, harmful nutrients and other pollutants and, after 30 years, the lake is literally overloaded. As a result, it is frequently closed to recreational use because of toxic blue-green algae outbreaks.

It is an ongoing challenge to manage Lake Tuggeranong so it can fulfil the recreational needs of the community while also protecting water quality downstream in the Murrumbidgee River. At the moment, the lake doesn’t appear to be performing either role effectively. Something has to be done.

What we know (and what we don’t)

Lake Tuggeranong is often closed for primary contact, generally because of algal blooms. In the 10 years to 2017, the lake was closed for an average of 93 days each year.

There are some gaps in our understanding of how and why blue-green algae forms in lakes. Key factors that can influence an outbreak include high nutrient levels (both phosphorus and nitrogen, but phosphorus is likely to be the most important of these), longer days and warm temperatures (which explains why lake closures generally occur in summer months), and stratification (warmer water on the top, colder water on the bottom).

Lake Tuggeranong appears to provide ideal conditions for algal blooms. Of all the factors listed above, nutrient load offers the best opportunity to change those conditions. Understanding where the nutrients come from and how they behave in the lake is the first step in determining what interventions will work best to improve water quality and reduce the frequency and severity of algal blooms.

The research project

The research is divided into two phases.

Phase 1 – Complete

Automated equipment was installed in the lake and in the streams that flow into the lake. This equipment collected water samples to measure pollutant loads, oxygen and water temperature. Researchers also collected weekly manual samples of the water in the lake.

All these samples were analysed at the University of Canberra by the research team at the Institute for Applied Ecology.

The Lake Tuggeranong research revealed that the lake suffers from both:

  • high loading of pollutants from the catchment during both base flow and stormflow conditions; and
  • high ‘internal loading’ (nutrient cycling between lake sediments and the water column).

Lake waters are saturated with phosphorus and conditions conducive to algal blooms are present all year round. When the lake is stratified during warm, still conditions, release of nutrients from the sediment layer in the bottom of the lake are exacerbating the problem, elevating nutrient levels in the lake and making nutrients released from the sediments available to blue green algae.

These results informed the management options trialled in Phase 2.

Phase 2 – In progress

Phase 2 has involved a suite of in-lake trials of three algal management options, and further research on light as a factor in algal growth. These were conducted as set of experimental ‘bag in lake’ mesocosms. The mesocosm trials consisted of:

  • a chemical treatment (Phoslock) that was hypothesized to bind phosphorous from the water column and limits the internal release of nutrients;
  • an addition of micro-nutrients (Diatomix) hypothesized to shift the phytoplankton community composition from cyanobacteria to diatom dominated;
  • an addition of hydrogen peroxide (H2O2) that was hypothesised to kill blue green algae; and
  • experimental shading to tease apart whether light or temperature are limiting growth of algal blooms in Lake Tuggeranong.

Research is ongoing and will continue into 2019/20.

Mesocosms

What’s square and orange and floating in Lake Tuggeranong? Nothing to worry about, and perhaps part of the blue-green algae solution.

The twelve ‘mesocosms’ visible from the Athllon Drive bridge are like giant bottomless plastic bags that isolate the water column from the lake surface to the lake bottom. Scientists use mesocosms to test how different treatment options affect water quality.

These orange mesocosms are part of a water quality research trial being conducted by the University of Canberra’s Institute for Applied Ecology. This is the second phase of the research, funded by ACT Healthy Waterways, to identify in-lake treatments that could improve water quality and reduce the number of closures caused by blue-green algae.

Associate Professor Fiona Dyer from the Institute for Applied Ecology at the University of Canberra said they would remain in place until the completion of the research project in June 2019.

“The mesocosms are like giant on-site test tubes in which we’re testing different products—including Phoslock, Diatomix and hydrogen peroxide—to see which is most effective at improving water quality.

“These treatments have had mixed success in other locations and we need to understand if they are effective treatments for Lake Tuggeranong specifically.

“Blue-green algae is susceptible to certain chemicals, including hydrogen peroxide, so we’re testing with small doses. The challenge is to not only determine which treatments will work but how much is required. What is the critical amount that will have an impact on the algae but not on plants and animals?” said Associate Professor Dyer.

Healthy Waterways manager, Ralph Ogden, said this was the beginning of a process that could take at least five years before results may be seen.

“There is no quick fix to the blue-green problem. Our research needs to be thorough. We need to be sure it will work—with no unintended consequences for other plants and animals.

“The mesocosms will help do this,” Dr Ogden said.

ACT Healthy Waterways is a joint initiative of the ACT and Australian governments to improve the quality of water entering our lakes and waterways and flowing downstream into the Murrumbidgee River system.

Apart from research trials, the project includes the construction of infrastructure—wetlands, ponds and rain gardens to filter urban stormwater flowing into our lakes and waterways—a community education campaign, and improvements to water monitoring practices. Together, these activities are improving water quality in the region’s waterways and providing a strong foundation for future efforts to do this.