Australian wildfires caused unprecedented ocean algae blooms

Intensified by prolonged drought and high temperatures, the wildfires that blazed through Australia’s bush in 2019-20 were severe, torching around 74,000 km² of eucalyptus forest. These fires spewed more than twice as much carbon dioxide as previously estimated, increasing the country’s anthropogenic carbon emissions by more than a third over the year before, according to a new analysis (Nature 2021, DOI: 10.1038/s41586-021-03712-y).

The fires also released huge amounts of nutrient-containing aerosols into the atmosphere. Researchers have now linked these emissions to an extraordinary algal bloom bigger than the Australian continent that formed several thousand kilometers away in the Southern Ocean (Nature 2021. DOI: 10.1038/s41586-021-03805-8).

With new high-resolution satellite measurements, a team led by meteorologist Ivar Van der Velde of SRON Netherlands Institute for Space Research analyzed atmospheric carbon monoxide concentrations during the fires, which lasted from Nov. 2019 toJan. 2020. Based on the known ratio of CO to CO₂ emitted in wildfire smoke, the researchers estimate that the fires vented 715 million tons of CO₂ into the atmosphere. Five different fire inventories had previously estimated less than half this amount. Making accurate estimates of wildfire-caused CO₂ emissions is crucial to modelling future climate scenarios, the study authors note.

In a separate study, Weiyi Tang of Princeton University and biogeochemist Nicolas Cassar of Duke University focused on iron carried by fire-made aerosols. The Southern Ocean ecosystem is known to be iron limited, so the team wanted to see if they could link atmospheric iron concentrations from the wildfires to the massive algae bloom in the Southern Ocean that erupted shortly after the fires. The team studied satellite data to track the aerosols as they blew across the sea and to measure the ocean’s chlorophyll a content based on reflectance. High levels of chlorophyll a in the ocean indicate an algae bloom. A global network of robotic scientific instruments that drift in the ocean collected direct chlorophyll a measurements and other information that the researchers used to confirm the presence of algae and corroborate the extent of the algae bloom they had seen via satellite images. A monitoring station directly downwind of the fires provided measurements of aerosol iron levels. Taking all this information together, the researchers concluded that vaporized iron from the wildfires traveled through the atmosphere and deposited in the Southern Ocean, where it drove the massive algae bloom.

Cliffton Buck, a chemical oceanographer at University of Georgia’s Skidaway Institute of Oceanography who was not involved with the study, was shocked at the size of the bloom, which he says was “far beyond anything and of such a greater magnitude than I would have anticipated.” Algae absorb CO₂ from the atmosphere, but it is unclear where that captured carbon will go after the phytoplankton die. Buck is interested to see what happens as the bloom dissipates and how that will impact the carbon cycle and marine food webs.

“It’s unprecedented,” says Brian LaPointe, research professor at Florida Atlantic University’s Harbor Branch Oceanographic Institute. “It’s just incredible that the magnitude of these fires now is so big that they’re affecting open water ecosystems large distances away from the source of the fires.” LaPointe, who studies sargassum blooms in the Atlantic Ocean, adds that as climate change brings more and bigger fires, “the atmospheric pathway is becoming a much more important pathway for the transport of these key limiting nutrients.”

This is “another example of climate change and fires affecting things that have never been affected like that before,” LaPointe says, and it will take more research to understand the global ecosystem impacts.