The path to preventing toxic Blue-Green Algal blooms

During our long hot summer days, one of our favourite things to do is enjoy the water. We can’t do this when our lakes are closed.

A few people walking on a paved path near some grass next to a large lake. A couple of them are walking their dogs.

Scientific research is underway to understand how toxic algal blooms in Canberra’s urban waterways such as Lake Tuggeranong can be prevented. For example, consider work led by University of Canberra’s Fiona Dyer and Rodney Urbrihien. They have been monitoring inflow points, monitoring lake water quality and experimenting to understand a range of possible paths that could be considered to prevent toxic algal blooms. You can read the details here.

The science community knows the conditions needed for toxic algae to bloom and grow. 

Toxic blue green algae grows when:

  1. Water temperatures go above 25° Celsius
  2. Supportive thermal stratification conditions form
  3. Total phosphorous (TP) levels exceed 0.03[1] mg L -1
  4. There is nitrogen. The problem here is that toxic blue green algae can draw nitrogen from the air if it isn’t present in the water. Fiona Dyer explains easily here.

Like any living thing understanding how toxic blue green algae (cyanobacteria) grows is the starting point to understand what we can do to stop it.

Lab technician analysing water sample
A large body of water next to a tree filled with green algae.

Lowering the temperature of a lake isn’t possible and mixing an entire lake to disturb the stratification is extremely difficult in a lake the size of Lake Tuggeranong. Therefore, the key lies in the other vital ingredients.

What can we do?

Given nitrogen can be accessed by toxic blue green algae through the air, then the focus has to turn to Phosphorus (P)[2]P is one of the First Four ingredients that we can take away!

Stage 2 research identified that surface P levels in Lake Tuggeranong were 3x the TP concentrations needed for algal blooms.

A photo of Lake Tuggeranong's water surface with trees in the distance

Clearly, we have a big problem!

University of Canberra work continued identifying that the external nutrient load entering Lake Tuggeranong is high. Over the duration of the three-stage study, they calculated that 948 kg of phosphorus entered Lake Tuggeranong through three major inflows. Most of the phosphorus entered the lake in high flows during rainfall events.

Dead leaves clogging a street drain
Water flowing into a street drain

Phosphorous entering the lake occurs in two forms:

  1. Almost half is bioavailable. This form is called dissolved reactive phosphorus (DRP[3]), and when it enters Lake Tuggeranong, it can start feeding toxic Blue Green Algal blooms straight away when the other 3 of the First Four ingredients are present.
  2. The remaining phosphorus entering the lake comes in with organic matter that can be stored for later conversion to DRP.

Organic matter settles, creating a nutrient rich sediment layer. When conditions are ideal[4], including long dry periods[5] and during toxic algal blooms, phosphorous is released from the sediment layer delivering further fuel for the fire.

Work completed by the University of Canberra team identified that 111 and 113 kg of phosphorus was released from sediment in the lake in the 2017/18 and 2018/19 seasons, respectively.

Given that University of Canberra modelling suggests that approximately 80-100 kg of dissolved phosphorus is sufficient to maintain conditions conducive to algal outbreaks in Lake Tuggeranong, the next steps that people can take become quite clear. 

Actions any household can take are:

A person putting dead leaves into a green waste bin with autumn trees in the background.

Collect leaves. You can put leaves in your green bin, make a leaf tower, drop them off at designated drop off points, give them to community gardens or ask a service provider to remove them for you. Check out for options you can try. The added benefit of picking up leaves is that our gardens, paths, and streets look so much nicer when they are clear of leaves and removing leaves makes us bushfire ready.

A chicken next to a pile of dead leaves

Start composting. Grass clippings and leaves combine perfectly, breaking down to deliver nutrients your garden needs to survive and thrive. Take a look at how some Canberra gardeners compost here and here. Given over application of fertiliser is a key contributor of DRP, this also means you avoid spending money applying chemicals that can run off and do harm.

A landscape of grass with multiple trees in the distance

Source credit: ACT Waterways.

Create a rain garden. We can develop our gardens to stop excess stormwater runoff capturing organic matter in our garden beds.

A water tank sitting next to a house

Install a rainwater tank. External loads during large rain events contribute to the highest concentration of dissolved nutrients in the lake. By using rainwater, you can make big $$$ savings on your water account.

There is no doubt there is too much Phosphorous in Canberra waterways, such as Lake Tuggeranong. The sooner we all act to reduce P, the quicker we can get back to enjoying open lakes year-round. 

While science will continue to fit more missing pieces of the jigsaw puzzle together to further our understanding, we no longer need to sit back thinking someone else has got this. 

Urban landscapes are key contributor to P loads[6], and we can’t sit back hoping someone else will fix the problem. Each and every one of us needs to take action to lower P in Lake Tuggeranong.

Collective action will reduce the amount of P entering Canberra’s lakes.

[1] Some scientists set lower TP benchmarks suggesting levels need to be below 0.025 or 0.02 (see page 13, Ubrihien et al., 2019). [2] Phosphorous (P) is present in plant and animal cells and P is vital for all plants for harvesting the sun’s energy to grow and produce.  When plants die, P is returned to the soil, or water, where it can be used by soil microorganisms and other plants (including blue green algae). [3] DRP comes from organic matter such as algae, plant and animal tissue, waste solids, or other organic matter. Microbial decomposition of organic compounds converts organic particulate P to DRP.  [4] For those wanting the facts more scientifically, nutrients can be released from the sediment of the lake over summer when the lake is thermally stratified, and anoxic conditions occur in the bottom waters of the lake. [5] A period of time between rain events (November 2019 through to a large rainfall event on February 4th 2020 where over 50mm of rain fell) showed that DRP went up, indicating sediment in the Lake’s base released DRP (see page 34 Ubrihien et al. 2019). [6] Other key contributors are our much loved and used sporting grounds.

Written by Sharyn Rundle-Thiele

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