More about algal blooms

Compiled from information provided by Dr Fiona Dyer and Clarissa Barbosa.

What is an algal bloom?

Algae is a broad term for a range of simple organisms that mostly obtain food via photosynthesis. When algae occur in large quantities it is known as an algal bloom.

Algae occur naturally in fresh and marine waters and algal blooms are often a normal part of the seasonal cycle. Algal blooms can change the smell, appearance or colour of water.

Why are algal blooms a concern?

For fish and other organisms to live or thrive, there must be oxygen in the water. Algal blooms can last days or even months, and some algae grow so fast that they use up all dissolved oxygen in the water. Even in the process of dying, algae can use oxygen.

Some algae are toxic or harmful in other ways. They can damage the tissues of fish or poison shellfish (and those who eat them). Harmful algae ingested by mammals (including humans) can lead to liver damage, liver disease, nerve damage and even death. Cyanobacteria (or ‘blue-green algae’) is among these toxic algae and is one of the most common causes of blooms in freshwater systems.

What causes algal blooms in our lakes?

In order to grow and ‘bloom’ algae needs the right combination of factors:

  • Water temperatures above 25 degrees which is why we see more lake closures in Canberra during the summer months, although blooms can still occur in cooler conditions.
  • Light. Blue-green algae are better adapted than other species to variable light conditions which means they can out-compete other species of algae in turbid (dirty) water.
  • Stable conditions. Still water that isn’t ‘mixed’ by winds, rain, regular or high flows provides ideal conditions for algae to grow. Thermal stratification (when the water in the lake forms layers) means the cooler bottom layer becomes anoxic (without oxygen), causing nutrients to be released from the sediment which feed the algae.

High levels of nitrogen and phosphorus seem to be a major contributing factor when it comes to algal blooms. While other algae get nitrogen and phosphorous from water, most blue-green algae can obtain nitrogen gas from the air - a clear advantage when competing for nutrients to grow.

Nutrients enter our waterways in sediment and eroded soil, in fertilisers and organic matter like leaves and grass clippings and even sewage effluent. Urban expansion and human activities have caused an increase in the concentrations of nutrients in many water bodies which leads to an increase in the frequency, intensity and duration of algal blooms.

Canberra’s lakes were all designed to receive stormwater from surrounding suburbs (the catchment), trap the nutrients and stop them flowing downstream into the Murrumbidgee River. As As a result, nutrient levels in the lakes have increased over the years and, when the conditions are right, they are released into the water and algae grows.

No one factor is responsible for triggering and maintaining algal blooms in Canberra’s lakes which makes it very difficult to reduce the frequency and severity of algal blooms.

What impact will climate change have?

Our climate is changing and in recent years the ACT has experienced some of the hottest temperatures on record over summer and autumn and historically high summer rainfall. The best climate predictions we have for the ACT are that we can expect to have longer, hotter summers and more frequent and severe storm events.

We know algal blooms occur when the temperature, nutrient and light conditions are right.  Longer, hotter summers mean more opportunities for algae to grow and bloom and longer, more frequent lake closures. We also know that more severe summer storms result in faster run-off which washes more nutrients and pollutants from the catchment into the stormwater system and on into the waterways and lakes.

So, in summary, climate change means more days at the right temperature and plenty of nutrients to help blue-green algae grow and bloom.

What can we do?

  • Reduce emissions so temperature increases are minimised.
  • Manage our land (particularly in urban areas) to ensure nutrients and pollutants (leaves, fertilisers, soil) don’t wash into our waterways and lakes.
  • Increase native vegetation along our lakes and waterways to keep the water cooler and provide a buffer zone that is better able to catch leaf litter and other pollutants.
  • Build water quality infrastructure (wetlands, ponds and rain gardens) to catch and treat stormwater before it flows into our lakes and waterways

Want to know more?

What can I do? Tips for clean stormwater H2OK Only rain down the stormwater drain

Are toxic blooms the new normal for Australia’s major rivers? The Conversation