Two years ago, in the southern Pacific Ocean, an explosion of algae grew to more than 2,000 miles wide — about the width of Australia.
Giant algal blooms are often tied to land pollution such as runoff from farmland, which is full of nutrients like nitrogen that these plant-like organisms need to thrive. But there were no nearby farms or factories here in the middle of the ocean.
The sprawling bloom was fueled instead by something faraway and unexpected: wildfires thousands of miles to the west.
In a landmark new study published in the journal Nature, researchers conclude that smoke rising from Australia’s historic 2019 wildfires drifted out to sea and fertilized vast communities of algae. The smoke, which contained the nutrient iron, gave rise to algal blooms that were together larger than Australia, the authors write.
“We know that these fires have catastrophic impacts on local ecosystems,” said Nicolas Cassar, a study co-author and professor of biogeochemistry at Duke University. But by spreading nutrients, he said, “they also impact ecosystems that are thousands of kilometers away.”
As regions from the American West to the Mediterranean grapple with another devastating wildfire season, the study adds a new dimension to our understanding of how climate-fueled disasters are transforming the planet. We’re learning that the impacts of fires go well beyond the human death toll, the leveled homes, and the charred forests. Extreme wildfires are also altering underwater ecosystems far from the flames.
How smoke from wildfires can fertilize the ocean
As anyone living in the US West knows, wildfire smoke is full of all kinds of stuff that’s toxic to inhale, including carbon monoxide and nitrogen oxides. Dirty air is one of the most serious public health threats and is known to shave years off of life expectancy.
But while harmful to humans, wildfire smoke — which typically contains nutrients like iron and phosphorus — can help algae and plants grow, just as fertilizer might help rescue a wilted house plant. Some of those nutrients come directly from burning forests or grasslands, while others are in dust that fires suck up and loft into the air, said Douglas Hamilton, a researcher at Cornell University who led a recent review on how aerosols affect ocean biogeochemistry.
In some cases, smoke settles over the land — as is common in the US, where the wind tends to blow smoke rising from western states toward the East Coast. That’s why the sky above New York was hazy this summer while wildfires were raging in California and Oregon.
In other regions, however, air currents send it out to sea. That’s what happened during Australia’s devastating wildfire season that began in 2019, according to the study. Smoke emanating from the fires — which burned 21 percent of the country’s temperate and broadleaf forests — traveled west into the southern Pacific Ocean, where it settled in waters with relatively low iron levels. There, any large source of iron is thought to be “an essential driver of oceanic primary production,” the authors wrote.
As a result, the researchers argue, the smoke spurred enormous algal blooms — concentrated in patches south of Australia and far off the western coast of South America. The blooms, shown in red in the below map, peaked in January 2020 and lasted for about four months, the authors said.
The results are especially surprising considering that in a typical year, the abundance of algae in these regions is actually lower during Australia’s summer (North America’s winter), according to the study.
Scientists have known for years that dust carried by the wind is a source of nutrients for algae, Hamilton said, but they had only an “inkling” that fire played a major role, too. This study is the first to connect large fires to large blooms, they said, which is what makes the work “groundbreaking.”
Are fires in California messing with the Pacific Ocean?
Massive wildfires have already burned hundreds of thousands of acres in California, the Brazilian Amazon, and Siberia this year. They’ve destroyed homes, wiped out wildlife, and flooded the air with greenhouse gases. Will they also change the biology of nearby oceans? That depends on several factors, experts say, such as the size of the blaze and the direction of the wind.
On occasion, for example, smoke from western wildfires will travel out over the Pacific Ocean. That’s what happened during the 2017 Thomas Fire in Southern California, then the largest in the state’s history. Smoke wafted over the Santa Barbara Channel, and research suggests it may have altered the marine ecosystem there. When the authors tested the water during the fire, they found an abnormally high number of certain marine algae called dinoflagellates, said Sasha Kramer, the study’s lead author and a doctoral student at the University of California Santa Barbara. It’s not clear what, exactly, that means for the health of the ecosystem, but different kinds of algae are known to be better or worse at sequestering the carbon they absorb, she said. (The authors didn’t find more algae overall.)
Elsewhere in the world, research has found a stronger connection between fires and phytoplankton. In 2003, a particularly notable paper linked a 1997 red tide — which choked a coral reef in Sumatra of oxygen, killing fish and the reef itself — to wildfires in Indonesia. “Iron fertilization by the 1997 Indonesian wildfires was sufficient to produce the extraordinary red tide,” the authors wrote, “leading to reef death by asphyxiation.”
Wildfire smoke can also cause algae to balloon in freshwater rivers and lakes. “It usually causes a really big bloom response because [the algae] is just sitting there getting fertilized in the sun,” Kramer said. In these cases, however, it’s typically phosphorus rather than iron that the algae need to bloom, Hamilton said.
Smoke from wildfires has even been shown to fertilize plants on land. One remarkable study from 2019 found that phosphorus from fires burning in southern Africa traveled all the way to the Amazon Basin, where it fertilized the rainforest.
How an increase in severe fires will shape the future of our oceans
While wildfires are becoming more frequent and severe due to climate change, scientists still aren’t sure what that means for the future of our oceans.
As the Nature study shows, fires can spur large algal blooms, but a lot depends on the local environment. If the ecosystem already has plenty of nutrients, for example, an influx of iron or phosphorus might not produce a bloom.
Even if there is a bloom, it’s not always clear if it’s good or bad. Phytoplankton, like plants, are photosynthetic, absorbing carbon dioxide (CO2) as they grow. In a basic sense, more algae mean fewer carbon emissions. That’s why some people have proposed fertilizing the ocean with iron as a way to combat climate change. But even big algal blooms may not absorb enough carbon to offset the CO2 stemming from the wildfires that fuel them. Some kinds of algae are also likely to release the carbon they store back into the atmosphere when they decay, whereas others, such as diatoms, are more likely to lock it up permanently, Kramer said.
“The best thing is to not rely on this carbon drawdown from phytoplankton, but to stop emitting CO2 in the first place,” Hamilton said.
Algal blooms can also wreak havoc on wildlife and throw ecosystems off balance. Blooms in Florida, for example, have killed thousands of fish, and even manatees. They have also caused “dead zones” around the world, including in parts of the Gulf of Mexico and the Chesapeake Bay, where there’s not enough oxygen for animals to survive. If nothing else, they’re likely altering the food chain, Hamilton said.
This research raises a lot of new questions, such as how fire-fueled phytoplankton affect ocean biology and carbon sinks, and stands to further complicate future climate models. It also highlights an uneasy juxtaposition: We can be both amazed by the science of catastrophe and gravely worried about what it tells us. It’s mind-boggling that fires in Australia are fertilizing oceans off the coast of South America — and troubling to think of what it might mean for the marine animals that live there.