Wildfire smoke link to Arctic algal bloom raises fresh questions

A wildfire in Siberia likely contributed to an unusually large and long-lasting algal bloom in the Arctic Ocean during the summer of 2014, according to a new study.

The work highlights an understudied impact of smoke, which may become more important as climate change causes wildfires to become more frequent and severe.

Burning forests and peatlands are known to release a lot of carbon, but smoke plumes also carry a variety of aerosols, including nutrients such as phosphorus, nitrogen and iron.

Scientists have theorized that these aerosols could reach nutrient-limited parts of the ocean, where they might act like fertilizers and promote the growth of phytoplankton – microscopic plant-like organisms that form the base of the ocean’s food web.

Until recently, researchers had only predicted this might happen in computer models.

“It’s one thing to do it in the models,” said Douglas Hamilton, assistant professor of marine, earth and atmospheric sciences at North Carolina State University in the United States. Hamilton co-led the study as a postdoctoral researcher at Cornell University.

“It’s quite another thing to actually observe it in the real world,” they said.

Last year, a different team of scientists linked a massive algal bloom off the coast of Antarctica to iron deposited by smoke from severe Australian wildfires in 2019-2020, Forbes reported.

The new study, published in the journal Communications Earth & Environment in September, is the first to document the phenomenon in the Arctic Ocean, the researchers say.

“We think this is going on in several places in the world, but it’s hard to detect,” said Joan Llort, a biogeochemical oceanographer at the Barcelona Supercomputing Center in Spain, who co-authored the study about the algal bloom in the southern hemisphere but was not involved in the recent work.

Although the new findings are not necessarily surprising, Llort said, the fact that researchers observed the phenomenon in the Arctic for the first time is relevant. It’s nice to see researchers put the pieces together, he said.

Process of elimination

Unlike other parts of the ocean, the Arctic is limited in nitrogen rather than iron. So when Hamilton and their colleagues noticed an unexpectedly large algal bloom about 850 km south of the North Pole in 2014, they looked for potential explanations.

It was a bit like a Sherlock Holmes-style analysis, Hamilton said. The team first ruled out oceanic mechanisms that are known to bring in nitrogen, such as inputs from rivers, ocean upwelling and storms. Next, they looked at possible sources of nitrogen from the atmosphere.

Downwind of the algal bloom, a wildfire had burned roughly 1.5 million hectares in Siberia, an area about half the size of Vancouver Island. (This was also the same summer as the NWT’s “megafire” season of 2014.) The Siberian fire’s timing aligned with the bloom, and satellite data revealed that the smoke had been transported to the bloom area.

The researchers ran computer simulations to estimate the amount of nitrogen being deposited at the site of the bloom. These simulations are known to underestimate emissions from boreal fires, however, and they don’t include emissions from peat, which is nitrogen-rich, Hamilton said. Adjusting their estimates to take these factors into account, the researchers found that the Siberian wildfire contributed between 12 and 100 percent of the nitrogen needed to support the bloom.

Although the work doesn’t prove that the bloom was entirely caused by the wildfire, it does suggest the fire played a significant role, the researchers reported.

As permafrost thaws and more peatlands burn in northern regions – including in Canada – a more sustained supply of nitrogen may reach Arctic waters and increase the production of phytoplankton, according to Hamilton.

“But what it actually means for the delicate balances of the Arctic ecosystem, we don’t know yet,” they said.

Like plants, phytoplankton produce oxygen and take up carbon. That may have climate implications, said Sasha Kramer, a postdoctoral researcher at Monterey Bay Aquarium Research Institute, who was not involved in the research but has examined wildfire’s effects on phytoplankton in California.

A change in productivity might affect food webs and ecosystems, too. But what blooms matters, Kramer said. “It’s really hard to theorize about what might happen with phytoplankton bloom, in part because there are thousands of species of phytoplankton.”

According to Llort, it’s also difficult to know what’s in the ash that emanates from wildfires. As a result, in the new work, the researchers had to rely on remote sensing and modelling, which means they had to make some assumptions. “It’s not ideal,” he said, but it’s currently the best they can do.

Many questions remain. For instance, it’s still unclear if the 2014 bloom in the Arctic was a rare occurrence or a sign of how the ecosystem may shift in a warming world, Hamilton said.

Researchers will have to study more of these events, they said, to draw firm conclusions about the link between algal blooms and wildfires, and begin answering questions about their impacts.

This article is produced under a Creative Commons CC BY-ND 4.0 licence through the Wilfrid Laurier University Climate Change Journalism Fellowship.