When wildfire smoke drifts over the ocean, what happens beneath the waves? As wildfires change in nature and become more frequent, it’s increasingly important to understand how ash deposition affects the ocean’s smallest, yet most essential, inhabitants.
Figure 1. Conceptual illustration of coastal wildfires. Coarse-mode smoke including ash, rich in organic matter and low in minerals, is likely to settle near the fire source. Fine-mode smoke, with lower organic content and higher mineral composition, disperses farther. Wildfire smoke deposition can introduce both fertilizing nutrients, such as inorganic nitrogen and iron, and more toxic compounds, including dissolved organic matter (DOM) species like aromatic hydrocarbons, affecting marine trophic levels. Additionally, wildfire smoke on the ocean surface may alter sunlight penetration, impacting phytoplankton photosynthesis.
In a recent study, the authors investigated how wildfire ash leachate influences coastal microbial communities. Through field incubations along the California coast, we found that ash-derived dissolved organic matter (DOM) increased bacterioplankton specific growth rates and organic matter remineralization, while leaving bacterial growth efficiency unchanged. This suggests that the added DOM was primarily used to fuel basic cellular functions rather than biomass production. Meanwhile, microzooplankton grazing declined, even as phytoplankton division rates remained stable, hinting at a decoupling of predator-prey dynamics that could promote phytoplankton accumulation.
Pre-existing phytoplankton biomass had a greater influence on microbial responses than the chemical composition of the ash itself. In low-biomass waters, bacteria more readily consumed the ash-derived DOM. In contrast, in high-biomass waters, the leachate was less bioavailable, potentially allowing more refractory ash-derived carbon to accumulate. These baseline differences appeared to influence phytoplankton size structure: smaller cells increased in high-biomass settings, while larger cells became more prevalent in low-biomass waters. These shifts may have implications for nutrient cycling, food web structure, and carbon export pathways, depending on how microbial activity and community composition respond in situ.
Authors
Nicholas Baetge (Oregon State University)
Kimberly Halsey (Oregon State University)
Erin Hanan (University of Nevada, Reno)
Michael Behrenfeld (Oregon State University)
Allen Milligan (Oregon State University)
Jason Graff (Oregon State University)
Parker Hansen (Oregon State University)
Craig Carlson (University of California, Santa Barbara)
Rene Boiteau (University of Minnesota)
Eleanor Arrington (University of California, Santa Barbara)
Jacqueline Comstock (University of California, Santa Barbara)
Elisa Halewood (University of California, Santa Barbara)
Elizabeth Harvey (University of New Hampshire)
Norm Nelson (University of California, Santa Barbara)
Keri Opalk (University of California, Santa Barbara)
Brian Ver Wey (Oregon State University)
