Extreme wildfire conditions shift coastal phytoplankton community structure in California

Abstract Extreme wildfires have increased in frequency and intensity globally, particularly in the Western United States. Here we examine how the 2020 Lightning Complex Fires in California influenced coastal phytoplankton communities using monitoring network data products in the Monterey Bay, the ocean region closest to this large fire. We observed no clear response in ocean chlorophyll a , often considered a proxy for total phytoplankton biomass, during and after the fires. However, using phytoplankton community composition observations we detected a shift in phytoplankton size and taxonomic structure that coincided with the timing of the fires. Small centric diatoms initially dominated, followed by a proliferation of chain‐forming diatoms, including Asterionellopsis , Skeletonema , Hemiaulus , Leptocylindrus , Thalassionema , and Thalassiosira . Cross‐correlation analysis and generalized additive models identified wildfire aerosols (PM2.5) as a significant predictor of these diatom blooms, though the precise mechanisms remain uncertain. We speculate that a combination of nutrient deposition, light limitation from smoke shading, interactions with oceanographic conditions, and differential mortality due to grazing or toxicity drove the observed phytoplankton shifts. This study provides rare observational evidence linking extreme wildfires to changes in coastal phytoplankton communities and underscores the need for sustained ocean monitoring, rapid‐response sampling, and mechanistic studies to unravel these complex wildfire–ocean interactions.