The Thomas Fire was the biggest wildfire California had ever experienced—at the time. It burned over 280,000 acres and destroyed more than a thousand structures in the final month of 2017. It painted the sky orange and brown, streaking across NASA’s satellite images of the state’s central and southern coast.

“Everyone was wearing masks because all this ash was falling.” Tanika Ladd, a graduate student at the University of California, Santa Barbara, was on campus as the fires raged. “We were walking around town, and everyone was wearing masks because all this ash was falling,” she said.

Ladd wondered how the ash might mingle with marine life offshore. So after a colleague collected fallen ash from the fire off car windows, she took the samples to the lab to find out. The tests suggest that nutrients leeched from the ash could spur phytoplankton growth, particularly during times of the year when the ocean is short on nutrients. The preliminary research is another step in uncovering wildfire’s evolving fingerprint on Earth’s landscape.

An Inhospitable Ocean

Volcanic eruptions pump iron-rich ash into the atmosphere, and it feeds phytoplankton downwind. Despite how “Planet Earth,” “Blue Planet,” and other documentaries depict the ocean, most of its surface is a barren, nutrient-poor wasteland.

Tiny floating organisms, called phytoplankton, rarely have the nutrients they need to grow in much of the ocean, and they take nutrients from wherever they can find them, even from atmospheric sources. Past studies on volcanoes have revealed how eruptions pumping iron-rich ash into the atmosphere could feed phytoplankton downwind, and dust drifting off the Sahara has long been recognized as a “sandy fertilizer” for ocean plants.

Much less attention has been paid to the impact of wildfire ash. In the case of Australia’s recent bushfires, which burned an area roughly the size of the state of South Carolina and killed at least 34 people, experts didn’t know how the ash accumulating along beaches affected marine life. And as wildfires in some places accelerate from drought, climate change, and forest management practices, this question may become more pressing.

Charred Fertilizer

Ladd and her colleagues concocted an experiment to test how plankton communities bobbing in the channel’s coastal waters would respond to an influx of ash-leeched chemicals. They mixed the ash with seawater, collected offshore in the Santa Barbara Channel (where ash clouds blew during the Thomas Fire), to create a yellowish mixture in the lab. After straining out the floating bits, researchers enriched tanks full of naturally occurring marine phytoplankton communities and let them grow outside in natural light conditions. At four different times over a week, they measured biomass and nutrients in the water. They repeated the experiment during each season.

In the experiments, the phytoplankton greedily sucked up the available organic and inorganic nitrogen coming off the ash in the form of nitrite, nitrate, and ammonium. Nitrogen is a major component needed for cells, but as Ladd explained, fire season, at least in the Santa Barbara Channel where she did the study, is a time when there are generally fewer nutrients in the system.

The additional nitrogen helped phytoplankton communities grow more than the controls during summer, fall, and winter, a trend Ladd could see by measuring the total biomass in the samples over time. During summer, fall, and winter, the ash-fueled phytoplankton had more than doubled the biomass than the controls. Plankton in the spring, on the other hand, showed less of an effect. The ocean has a huge influx of nutrients in the spring from ocean upwelling, so any seeding from the ash didn’t have as great an impact.

The ash didn’t leech phosphorus, which the ocean is often depleted of, but Ladd said the exact chemicals leeched from the ash will change by location. “My findings might be slightly different than [those of] someone else who does something with the Australian wildfires,” she said.

Questions Adrift

Ladd noted that the study is one of the first to link wildfire ash and marine systems, but many unknowns remain, such as the amount of ash deposited and what happens to it when it settles on the ocean. “If this is happening, then that atmospheric component of ash is likely a more important nutrient source for coastal systems,” she said.

Ash deposition “could have a global impact if it’s changing productivity or shifting communities.” Sasha Wagner, an assistant professor at Rensselaer Polytechnic Institute in Troy, N.Y., who did not contribute to the work, said ash deposition is an important source of nutrients of surface water for freshwater streams and lakes after a fire. “The fact that they were able to capture these samples and start asking these questions, I think, is really important to kind of push this kind of research forward.”

Nick Ward, a research scientist at Pacific Northwest National Laboratory in Sequim, Wash., who was not involved with the research, said he’s curious to know how wildfires might contribute to excess nutrients in the marine environment. With large fires in places like the Amazon, ash deposition “could have a global impact if it’s changing productivity or shifting communities,” he said.

Ladd plans to analyze the plankton’s DNA from the experiment to see whether the ash gave certain species an advantage over others. In a preliminary analysis using microscopes, Ladd found that the ash did not seem to change the abundance of one particular type of phytoplankton, but further analysis is needed. Ladd presented the work this month at Ocean Sciences Meeting 2020 in San Diego, Calif.

—Jenessa Duncombe (@jrdscience), News Writing and Production Fellow