Australian Wildfires Triggered Massive Algal Blooms in Southern Ocean – Raises New Questions About Oceanic Carbon Uptake

2019-20 Australian Wildfires From Space

A satellite image shows smoke from the 2019-20 Australian wildfires covering parts of the Southern Ocean. Credit: Japan’s National Institute of Information and Communication Technology. Credit: Japan’s National Institute of Information and Communication Technology

Boom in carbon-sequestering phytoplankton raises new questions about oceanic carbon uptake and productivity.

Clouds of smoke and ash from wildfires that ravaged Australia in 2019 and 2020 triggered widespread algal 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 peer-reviewed study, published on September 15, 2021, 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.

The discovery raises intriguing new questions about the role wildfires may play in spurring the growth of microscopic marine algae known as 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.

“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 Nicolas Cassar, professor of biogeochemistry at Duke’s Nicholas School of the Environment.

The algal 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 algal 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.

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 DOI: 10.1038/s41586-021-03805-8

Cassar and Richard Matear of Australia’s national science agency, CSIRO, were corresponding authors of the study, which was conducted by researchers from the University of Tasmania, Duke, the Barcelona Supercomputing Center, the CSIRO Oceans and Atmosphere program, and the Plymouth Marine Laboratory.

The scientists used satellite observations, robotic ocean floats, atmospheric transport modeling and measurements of atmospheric chemistry to track the spread of pyrogenic iron aerosols from the Australian wildfires and measure their impacts on marine productivity.

Funding came from France’s “Laboratoire d’Excellence” LabexMER and Investissements d’Avenir programs; a Harry H. Hess Postdoctoral Fellowship; the Australian Research Council; the CSIRO Decadal Climate Forecasting Project; the AXA Research Fund; and the European Union’s Horizon 2020 program.

In addition to his faculty post at Duke, Cassar holds a research appointment at the Laboratoire des Sciences de l’Environnement Marin of the Institut Universitaire Européen de la Mer in Brest, France

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