Bay Nature magazineWinter 2021


How Do Wildfires Affect Ocean Ecosystems?

As wildfire seasons expand, scientists in Santa Barbara and beyond investigate how falling ash impacts the plankton in our waterways.

January 3, 2021

One December day in 2017, I lined up with friends at a Santa Barbara theater under a “Holiday Concert” marquee in a wintry scene replete with whirling white flakes. But there would be no music, only coughing and distant sirens. It was wildfire ash falling onto the holly wreaths that day, and we were in line for concert cancellation refunds. The ash was from the Thomas Fire, which would go on to claim almost 282,000 acres and more than a thousand structures, growing into what was then the largest wildfire in California history.

Elsewhere in town as the ash fell, marine biology graduate student Tanika Ladd saw an opportunity to explore a mystery that has proven tough to resolve: How does wildfire ash affect the organisms in our waterways? Since plankton form the foundation of our coastal ecosystems, Ladd wondered, what would happen if their equilibrium was disrupted? 

Ladd’s research, completed during and after the Thomas Fire and presented at the American Geophysical Union’s Ocean Sciences Meeting in February 2020, adds depth to our understanding of how lengthening fire seasons affect plankton off California’s coast.

ocean beach orange sky
Ocean Beach at noon on September 9, 2020. (Photo by Eric Simons)

What we already know is that, “Ash can contain minerals that act as fertilizer (e.g. nitrogen and phosphorus) causing algal blooms,” San Francisco State University biologist Jonathan Stillman wrote. “Which has runon effects, like harmful algae or deoxygenation (when algae die). Ash can also cause smothering when a thick layer settles on sedentary animals living in low-flow environments.” 

In Santa Barbara, Ladd’s fellow graduate student swept ash piling up on car windshields into jars, and then Ladd added ocean water from the Santa Barbara Channel, and waited. The result: the phytoplankton community more than doubled its biomass, feeding on the nutrients in the ash. This dynamic, she noticed as they repeated the experiment, was significantly seasonal. The ocean upwelling that occurs in spring in the Santa Barbara Channel provides necessary nutrients for phytoplankton, but during other seasons the channel is nutrient-depleted. It was in these summer, fall, and winter water samples that the most dramatic plankton growth occurred.

Closer to the Bay, Melissa Foley of the San Francisco Estuary Institute’s Bay Regional Monitoring Program studied the effects of ash from the 2008 Basin Fire on ocean waters near Big Sur, finding that a phytoplankton bloom followed the blaze. Foley has a theory about what caused this type of bloom, which can lead to large populations of harmful algae or even depleted oxygen in the water. Big Sur seawater contains the nitrogen needed for cell growth but not much iron. “It’s possible that the iron in the ash from that high-intensity fire contributed,” she says. 

Since no two ashfalls are the same—nor any two aquatic ecosystems—Santa Barbara’s circumstances differ from those we see farther north. Different stretches of coastline have their own topography, weather, and upwelling intensity. Foley and Ladd were asking the same question in different circumstances, but their answers were markedly similar.

As for the Bay, it is nutrient-rich all year, not just in spring. It also hosts much smaller and less diverse populations of plankton than do our coastal waters. After this fall’s apocalyptic orange skies, “there were no documented fish die-offs, or anything dramatic like that,” Foley says. She did notice some effects through measurements of both chlorophyll and dissolved oxygen in the water. Her data suggest that the dark skies may have disturbed phytoplankton growth and therefore oxygen levels. Periods of low oxygen are common in the lower South Bay, she says, “but it is likely that the September event caused those episodes to be prolonged.” She notes that the phenomenon was caused by an inversion of smoke and fog that blocked sunlight, meaning the ash only indirectly affected the plankton, by disrupting their photosynthesis. Ladd says inspiration for her research came from a group of fellow graduate students, who were embarking on a research cruise in the Santa Barbara Channel when the Thomas Fire broke out. Instead of sailing, they faced evacuation orders and air quality warnings. “We replanned the entire cruise to try to answer this question,” she says.

Sadly, it didn’t take long for that fire to lose its “largest in history” title. In just three years, it’s dropped to seventh place. Meanwhile, clues to this mystery continue to fall from the sky.

About the Author

Kathy Jean Schultz is a freelance medical writer who covers new experimental research.