Black Summer bushfires triggered Southern Ocean algal blooms bigger than Australia

By science reporter Belinda Smith
When ash and smoke from the Black Summer fires dropped into the Southern Ocean, it fuelled an enormous algal bloom. (Supplied: Japanese Meteorological Agency)

As Black Summer bushfires devastated huge swathes of south-east Australia, they were seeding new life in the ocean, hundreds of kilometres away.

Iron-rich ash and smoke dropped from the atmosphere and into the Southern Ocean, fuelling enormous blooms of algae between New Zealand and South America.

At their biggest, the blooms covered an area larger than Australia, according to a study published today in Nature.

Richard Matear, an oceans and climate scientist with the CSIRO and study co-author, says the work shows how events like bushfires can have profound effects on ecosystems much further afield.

"This is kind of a nice example of how the terrestrial biosphere connects to the ocean in an interesting way," Dr Matear said.

'We can't necessarily always treat these things as separate."

How algal blooms bloom

Algal blooms materialise when single-celled organisms — algae, also called phytoplankton — rapidly multiply to form massive congregations.

There are thousands of species of marine phytoplankton. (Supplied: Penny Ajani)

Algae may be microscopic, but their blooms can be seen from space.

And blooms can be seasonal, such as the "spring bloom" that's currently kicking off along the coast of New South Wales, said Penny Ajani, a marine biologist who studies phytoplankton at University of Technology Sydney, and who was not involved with the study.

With more daylight, warmer temperatures and an influx of cold, nutrient-rich water, the bloom is visible from the coast.

"When I walk along the beach, the ocean's looking a particularly green colour at the moment, and you can see a little green line along the shoreline," Dr Ajani said.

The NSW spring bloom is driven by the East Australian Current and dissipates after a couple of weeks, but algal blooms can also be triggered during their off-season by adding trace elements such as iron to seawater.

Iron is essential for photosynthetic organisms like algae to live and reproduce. It can be found in pollutants as well as natural sources such as volcanic ash, which can spawn huge algal blooms in the North Pacific, for instance.

In the 2010s, Dr Matear studied how a different phenomenon — dust whipped up during the Millennium Drought and blown offshore — caused algal blooms in the Tasman Sea, which separates Australia from New Zealand.

Australia is no stranger to thick dust storms, especially during times of drought. (ABC: Ryan Lahiff)

"But then we had the big bushfires in 2019-20," he said. 

"We went 'oh, my God, forget about the Tasman Sea. It's the whole of the southern sector of the Pacific Ocean that is going off'."

Fire's effects hundreds of kilometres offshore

Algal blooms started appearing in the usually desolate Southern Ocean in October 2019, ramping up to enormous proportions between December and the following April.

These mega blooms were seen by satellites, which measured the amount of green pigment in algae — called chlorophyll — in the water.

Underwater devices that regularly bob to the surface to beam information about seawater composition to satellites confirmed the detections.

The blooms were downwind from the Black Summer bushfires, which propelled tiny particles of smoke and ash kilometres into the atmosphere, where they were blown to the east.

Vast tan-coloured plumes of bushfire smoke drift off Australia's south-east coast in January 2020. (Supplied: NASA/Joshua Stevens, EOSDIS/LANCE and GIBS/Worldview)

Satellites tracked the plumes' movement across the planet.

Some of the smaller smoke particles circled the Earth, while larger fragments of ash were dragged down by gravity and dropped into the ocean.

Yet more fell in rain.

While the ash and smoke that billowed from the bushfires wasn't particularly high in iron compared to, say, iron ore dust, it contained enough to get algae in the Southern Ocean multiplying like mad.

Suddenly, blooms spread across the ocean.

But by March 2020, almost all fires in NSW and Victoria were extinguished or contained, and as the smoke subsided, so too did the algal blooms.

Fire-generated algal blooms aren't necessarily limited to the Southern Ocean either, Dr Matear said.

"We're hearing lots about bushfires in western North America. The same kind of argument could happen in the North Pacific as well."

Could algae be a carbon offset?

Being photosynthetic organisms, algae make food by sucking the greenhouse gas carbon dioxide out of the atmosphere — which is also emitted by fires.

Another paper also published in Nature today by Dutch researchers estimated about 715 million tonnes of carbon dioxide was released into the atmosphere by the Black Summer fires, between November 2019 and January 2020.

That's almost 200 million tonnes more than the amount of carbon dioxide produced by human activities in Australia in 2018.

Dr Matear and his colleagues calculated that the ensuing algal bloom potentially used the equivalent of almost all of the carbon dioxide emissions emitted by the fires between November and January.

"You don't have to add much iron to have a massive carbon uptake response," Dr Matear said.

Only a fraction of that carbon would likely be sequestered long-term, though.

Algae's ability to slurp up atmospheric carbon dioxide is what some climate scientists think helped drive the Last Glacial Maximum, when vast ice sheets covered parts of Asia, North America and Europe.

Back then, 25,000 years ago, carbon dioxide levels in the atmosphere were around 100 parts per million lower than in pre-industrial times, Dr Matear said.

It's no surprise, then, that artificially seeding algal blooms, simply by adding iron to seawater, has been floated as a way to fight climate change.

Large algal blooms, such as this one which formed off New Zealand's South Island in November 2017, can be spotted by satellites. (Supplied: NASA/Jeff Schmaltz, LANCE/EOSDIS Rapid Response)

And because algae are at the bottom of the food chain, blooms could be beneficial for fisheries — or so the thinking goes.

The problem is you don't know what species of algae will appear, Dr Ajani said.

"Out of the hundreds of thousands of phytoplankton species that exist, there's about 100 species that are toxic.

"We don't really know why they produce toxins, but they can affect things like oysters along our coastline, and we monitor for those toxic species all the time.

"Iron fertilisation" experiments were conducted from the 1990s, where people dumped iron into the Southern Ocean to see what would happen.

Sure enough, algae bloomed, but not much data was collected on, for instance, what species thrived, how much carbon dioxide they consumed or how long that carbon was sequestered.

Iron fertilisation's very limited ability to sequester carbon long-term and other unknowns means the scientific community has agreed on a moratorium on that process, Dr Matear said.

"The last iron fertilisation experiment that happened [a decade ago] was almost cancelled because of concern that by doing this experiment, trying to get some fundamental understanding, that it was being construed as possibly promoting the idea."

But as the world warms, destructive bushfires will likely happen more often, so it's certainly worth investigating links between land and sea, Dr Ajani said.

"With more extreme weather events, these are the kinds of studies that we need to show what these larger-scale impacts are going to be."


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