Wildfire ash is changing the ocean in ways scientists are only beginning to understand

As wildfires grow more extreme, scientists are exploring what their ash means for marine food webs, ocean productivity and the global carbon cycle. 

For Nicholas Baetge, the connection between wildfires and ocean science came into focus unexpectedly during a student-led research cruise in 2017. 

Baetge and his fellow graduate students had proposed the cruise in part to give researchers who did not normally go to sea the chance to experience oceanographic fieldwork. Their proposal earned them six days aboard R/V Sally Ride. Then, just days before the ship departed, the Thomas Fire erupted across Santa Barbara and Ventura counties, filling the air with smoke and ash.

“We started wondering what was happening in the ocean,” Baetge recalled. “We were seeing all this ash on the ground and realized that it must also be deposited on the ocean.”

That observation opened a new line of inquiry: What happens when wildfire ash reaches seawater? How do marine microbes respond? And what becomes of the carbon and nutrients the ash carries? 

Those questions have since become one strand of Baetge’s broader research into how microbes regulate the movement of carbon and nutrients through the ocean.

Looking beyond the satellite signal

Much of scientists’ early understanding of how wildfires affect the ocean came from satellite observations. From space, researchers can track smoke plumes and sometimes observe a subsequent rise in ocean chlorophyll, a pigment associated with phytoplankton. But a satellite can show only that a change occurred, not what caused it.

Ash is chemically complex. It can deliver nutrients such as nitrogen, phosphorus and trace metals that can stimulate some organisms. It can also carry organic compounds, metals, or other substances that inhibit them. Its composition depends on what burned, how intensely it burned and whether a fire moved beyond vegetation and soil into homes and infrastructure. 

The receiving ecosystem matters just as much. An ash input into nutrient-poor “blue water” may have a very different effect from the same input into a productive coastal community. Baetge and his collaborators brought those variables into controlled shipboard experiments. They exposed coastal plankton communities to leachates made from particles collected during the 2017 Thomas Fire and from vegetation burned under controlled conditions in Oregon. The Thomas Fire material was particularly enriched in dissolved organic carbon, nutrients, nickel and copper, while the experimental burns supplied relatively more iron and manganese.  

The experiments revealed a response more nuanced than simple fertilization. Ash-derived material increased bacterial growth and the breakdown of dissolved organic matter. But it did not consistently make phytoplankton divide faster. Instead, ash leachate reduced grazing by microzooplankton, the tiny consumers that normally keep phytoplankton populations in check. In low-biomass water, this release from grazing pressure allowed some phytoplankton to accumulate. 

In other words, the amount of phytoplankton present before the ash arrived helped determine what happened next. Pre-existing community conditions sometimes mattered more than differences among the ash sources themselves. 

“It isn’t simply a question of whether ash contains nutrients,” Baetge said. “We have to understand the community it enters and the balance among growth, consumption, and carbon recycling.”

That distinction matters. A rise in chlorophyll seen from space may look like faster phytoplankton growth, when it could instead reflect slower grazing, or a combination of processes. Experiments provide the measurements needed to interpret the satellite signal.

Following the carbon

Phytoplankton are a natural and essential foundation of marine food webs. Through photosynthesis, they take up carbon dioxide and convert it into organic matter. Some of that carbon moves through the food web, some is recycled by bacteria and some may be transported into the deep ocean. 

Wildfire inputs can alter each of those pathways. Bacteria may convert ash-derived organic matter back to carbon dioxide. Phytoplankton may incorporate wildfire-supplied nutrients into new biomass. Other compounds may persist in seawater or sink. A change in grazing can further influence whether carbon is recycled near the surface or passed to larger organisms and particles. 

These pathways make wildfire deposition relevant not only to marine ecology, but also to the global carbon cycle. Yet, scientists still do not know how much wildfire-derived carbon reaches the ocean, how long it remains there or how its fate varies among ecosystems. 

That question is becoming more urgent as extreme fires intensify. A 2024 study in Nature Ecology & Evolution found that the frequency of the most extreme wildfire events increased 2.2-fold between 2003 and 2023. Smoke and ash can also travel far beyond a burn scar: during major Australian wildfires, smoke plumes crossed the South Atlantic toward South America, carrying wildfire-derived material thousands of miles from its source.

A natural laboratory in Bermuda

Baetge recently joined ASU BIOS as a faculty member and assistant research scientist. Since arriving in Bermuda in May 2026, he has helped establish new capabilities, including flow cytometry for studying microbial communities and bio-optical instruments that can be deployed aboard R/V Atlantic Explorer and smaller vessels.

Together, these tools can connect fine-scale changes in seawater with patterns observed from space. Flow cytometry allows researchers to count and distinguish groups of microscopic cells, while bio-optical measurements reveal how particles and organisms interact with light, the same fundamental signal detected by ocean-color satellites. 

Bermuda offers a rare site in which to combine these approaches. Long-running ocean observations, regular access to the Atlantic and atmospheric observations at Tudor Hill create opportunities to follow material from the atmosphere into seawater and then into microbial communities and carbon-cycling processes.

Wildfire-ocean interactions are not the sole focus of Baetge’s research. They fit within a larger program aimed at understanding how microbial communities process organic matter and shape the ocean carbon cycle. That broader expertise and ASU BIOS’s combination of ships, time-series observations and laboratory infrastructure position the institute to help answer questions that span land, atmosphere, and sea. 

The ocean has always received occasional inputs from fire, and marine ecosystems have ways of responding. The emerging question is how those responses may change as extreme fires become more frequent, intense and geographically widespread.

“We don’t know what the response will be,” Baetge said. “That is why it is so important to understand the mechanisms.”

A fire may begin on land, but its effects do not stop at the shoreline. By following ash from the atmosphere into ocean food webs and carbon pathways, Baetge and his collaborators are helping reveal how a changing fire regime could reverberate across the sea.