West Coast
Plumes of ash from the 2017 Thomas Fire extended over the northeastern Pacific Ocean, depositing large amounts of nutrients and fueling the growth of phytoplankton in coastal ecosystems.LIVE NEWS PLANETPIX/ALAMY

CA - How a California wildfire created a buffet for marine microbes

Ash falls could boost nutrients in coastal ecosystems—for better or worse

On the evening of 4 December 2017, a small patch of brush caught fire near Santa Paula, California. The blaze soon grew into one of the largest wildfires the state had ever experienced. By the time firefighters contained what became known as the Thomas Fire, it had devoured more than 1000 square kilometers of coastal woodland, killed two people, and left hundreds of homes in ruins.

The catastrophe wreaked havoc on land, but it also might have enabled coastal ecosystems to flourish, suggests research published today in the Proceedings of the Royal Society B. As the ash plume created by the fire billowed across the Pacific Ocean, it deposited a variety of chemical compounds, creating a veritable buffet for marine microbes.

The findings “demonstrate the interconnectedness of ecosystems not just locally, but regionally,” says Camille Stevens-Rumann, a fire ecologist at Colorado State University who wasn’t involved with the study.

When the Thomas Fire began, marine scientist Tanika Ladd of the Shannon Point Marine Center was pursuing her Ph.D. at the University of California, Santa Barbara. As part of her studies into phytoplankton—tiny, sunlight-fueled organisms that form the base of many aquatic food webs—she was preparing for a research cruise aboard the R/V Sally Ride. But as ash rained into the sea near the Santa Barbara Channel, Ladd and her colleagues realized they had an unexpected opportunity to study the impact of wildfire on marine life.

Some evidence already showed that burning forests and volcanic eruptions, which produce large amounts of ash and other airborne particles, can alter the chemistry of nearby aquatic ecosystems. These effects are often harmful because ash can contain high concentrations of toxic heavy metals. But ash can also have a fertilizing effect, because it is often rich in iron. Severe wildfires in Australia from 2019 to 2020, for example, may have helped trigger phytoplankton blooms thousands of kilometers away in the Southern Ocean.

Before boarding the Sally Ride, Ladd’s colleagues gathered ash that had fallen on car windshields across Santa Barbara. Once at sea, the team collected seawater and mixed in the ash. Over 4 days, they monitored the kinds and numbers of microbes present in the water, as well as the concentrations of various chemical compounds.

They found that the ash introduced high concentrations of many nutrients that phytoplankton rely on to grow and reproduce, including large amounts of nitrogen. And they discovered that a diverse array of microbial species was taking advantage of the abundance. “Everything seemed to benefit,” Ladd recalls.

The results suggest wildfire ash could help boost marine nutrient levels, which can vary by season. Off the coast of Southern California, for example, nitrogen levels drop in late summer and fall—just when fire season picks up.

But the experiment “has raised as many questions as it has answered,” says Sasha Kramer, a marine scientist at the Monterey Bay Aquarium Research Institute. Although microbes of all kinds flourished in the bottles, in-water measurements taken around the same time showed that dinoflagellates—a type of plankton associated with harmful algal blooms—were benefiting disproportionately. “Clearly, much more research … is required” to sort out whether ash falls help or harm marine ecosystems, says Kramer, who participated in the research cruise but wasn’t involved in the study.

Read more.