Spotting algal blooms from space
In the early months of 2016, a 1650 kilometre stretch of the Murray River turned murky green.
The culprit was a massive outbreak of blue-green algae that lingered from February through to June that year. It had a major impact on water usage from the river.
Blue-green algae are a growing environmental issue in Australia, as warmer temperatures, abundant nutrients and reduced rainfall encourage blooms of a primitive form of phytoplankton known as cyanobacteria, many of which produce toxins harmful to humans and other animals including livestock, fish and shellfish.
Cyanobacteria are naturally occurring in most aquatic systems, but a range of factors such as higher temperatures, nutrients and stable conditions can trigger a surge in their numbers, leading to a bloom that can have devastating consequences for other organisms in these ecosystems.
One of the big challenges is keeping track of these outbreaks so authorities can take steps to limit the health risks whilst at the same time minimising the economic damage to the regions affected. Until now, the only way water authorities could do this was by regular physical sampling and laboratory analysis of the thousands of lakes, rivers and dams around the country.
In a world-first, CSIRO scientists – in partnership with the NSW Department of Primary Industries – Water – have now developed a satellite-based blue-green algal alert system.
Satellite monitoring of water bodies is nothing new; it’s already used to monitor the health of the Great Barrier Reef. But this is the first time an eye-in-the-sky has been turned toward inland water bodies. These pose a particular challenge for satellites because many water bodies are relatively small compared to something the size of the Reef.
But with advances in satellite-based remote sensing, we now have the technology to enable monitoring down to a much smaller scale.
The United States Geological Survey launched the Earth-observing Landsat satellite program in more than four decades ago. The most recent Landsat satellite carries equipment capable of measuring the levels of turbidity in water. Turbidity doesn’t necessarily equate to blue-green algae blooms, but in the absence of other possible causes such as sediment stirred by recent rainfall, it’s a reasonably accurate proxy.
But this is only one piece of the puzzle. The bigger challenge was working out how to make this data available – and accessible – to the institutions that have need of it. Satellite data sets are enormous, and often require specialist knowledge and significant computing resources to process.
The Australian Geosciences Data Cube is an innovative solution to that problem. A joint project between Geoscience Australia, CSIRO and the National Computing Infrastructure, the data cube does the processing hard work and produces seamless, non-overlapping coverage of surface reflectance data for the entire Australian continent. It is set up in such a way that, for even a single pixel – which corresponds to an area approximately 25 x 25 metres in size – researchers can drill down through 30 years of Landsat data for that area.
Furthermore, the Data Cube comes with an interface that permits users’ own algorithmic programs to directly interact with the data. Using this, we have been able to slice out all the lakes, rivers and dams in New South Wales and categorise in them the likelihood of the presence of algal blooms as green, amber or red alerts and show the spatial distribution of the threat.
The data isn’t quite real-time – Landsat passes overhead every sixteen days, and the satellite data will take around five days to be uploaded – but it’s a quantum leap advance compared to manually testing every single water body in the state on a weekly basis.
It means that water authorities can spot potential algal blooms much earlier and implement responses such as warning signage and notifications to the community.
And this is just the start. In 2015, the European Space Agency launched the first of two new Earth-observing satellites – Sentinel-2A and Sentinel-2B. These are equipped with imaging technology that will allow us to detect chlorophyll, which will give us far greater accuracy in distinguishing between the turbidity caused by sediment and the turbidity caused by algal blooms. There are also plans to expand alert system to the rest of Australia.
Another significant advance would be if we were able to predict in advance when an algal bloom might be likely. This would require incorporating meteorological data with an understanding of the hydrodynamics of water bodies, and models of how algae might behave under various conditions. But this ability may be just over the horizon.
With a warmer, drier future likely for large parts of the Australian continent, these alert and early-warning systems will enable us to keep a closer eye on the health and safety of our vital water bodies.
Dr Tim Malthus is Research Group Leader of CSIRO’s Coastal Monitoring, Modelling and Informatics Group. He previously led the Environmental Earth Observation Program in the CSIRO.