Australian wildfires triggered massive Phytoplankton blooms in the Southern Ocean

Clouds of smoke and ash from wildfires that ravaged Australia in 2019 and 2020 triggered widespread phytoplankton blooms in the Southern Ocean thousands of miles downwind to the east, a new Duke University-led study by an international team of scientists finds.

The study study, published in Nature, is the first to conclusively link a large-scale response in marine life to fertilization by pyrogenic—or fire-made—iron aerosols from a wildfire.

It shows that tiny aerosol particles of iron in the windborne smoke and ash fertilized the water as they fell into it, providing nutrients to fuel blooms at a scale unprecedented in that region.

Phytoplankton require iron for photosynthesis

The study’s co-author Prof Peter Strutton, of the University of Tasmania’s Institute for Marine and Antarctic Studies, likened the phytoplankton bloom to “the entire Sahara desert turning into a moderately productive grassland for a couple of months”. “The entire Southern Ocean is basically low in iron because it’s a long way from dust sources, so any small amount of iron that gets deposited there can cause a strong response,” Strutton said.

The discovery raises intriguing new questions about the role wildfires may play in spurring the growth of microscopic marine phytoplankton, which absorb large quantities of climate-warming carbon dioxide from Earth’s atmosphere through photosynthesis and are the foundation of the oceanic food web.

A satellite image shows smoke from the 2020-21 Australian wildfires covering parts of the Southern Ocean. Credit: Japan’s National Institute of Information and Communication Technology.

Our results provide strong evidence that pyrogenic iron from wildfires can fertilize the oceans, potentially leading to a significant increase in carbon uptake by phytoplankton,” said Professor Nicolas Cassar, of Duke’s Nicholas School of the Environment.

The phytoplankton blooms triggered by the Australian wildfires were so intense and extensive that the subsequent increase in photosynthesis may have temporarily offset a substantial fraction of the fires’ CO2 emissions, he said. But it’s still unclear how much of the carbon absorbed by that event, or by blooms triggered by other wildfires, remains safely stored away in the ocean and how much is released back into the atmosphere. Determining that is the next challenge, Cassar said.

Carbon uptake

The researchers estimate the amount of carbon taken up by phytoplankton cells as a result of the bloom was equivalent to around 95% of the emissions generated by the 2019-20 bushfires.

For that carbon to be permanently removed from the atmosphere, however, the phytoplankton cells would have to sink into the deep ocean and be stored there, Strutton said.

There’s a lot of recycling of energy and biomass that happens in the surface waters. It’s likely that a lot of that carbon that was initially taken up might have been re-released to the atmosphere when those phytoplankton cells started to break down or were eaten.

Large wildfires, like the record-breaking blazes that devastated parts of Australia between 2019 and 2020 and the fires now raging in the western U.S., Siberia, the Amazon, the Mediterranean and elsewhere, are projected to occur more and more frequently with climate change, noted Weiyi Tang, a postdoctoral fellow in geosciences at Princeton University, who co-led the study as a doctoral candidate in Cassar’s lab at Duke.

These fires represent an unexpected and previously under-documented impact of climate change on the marine environment, with potential feedbacks on our global climate.

Pyrogenic aerosols are produced when trees, brush and other forms of biomass are burned. Aerosol particles are light enough to be carried in a fire’s windborne smoke and ash for months, often over long distances.

More information: Widespread phytoplankton blooms triggered by 2019–2020 Australian wildfires, Nature (2021). DOI: 10.1038/s41586-021-03805-8 , www.nature.com/articles/s41586-021-03805-8

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