Did the Thomas Fire Fuel a Phytoplankton Community Shift in the Santa Barbara Channel?

The Thomas Fire ignited on 5 December 2017 and burned nearly 300,000 acres of land in Ventura and Santa Barbara counties before being contained on 12 January 2018, making it the largest wildfire in California history. During the fire, a persistent plume of ash, smoke, and soot extended up to 1,000 km offshore, depositing ash over the Santa Barbara Channel (SBC). The effect of this ash influx on the SBC surface ocean phytoplankton community was investigated with an Imaging FlowCytobot (IFCB) onboard R/V Sally Ride during a cruise from 15 to 22 December 2017. Both scattering and fluorescence triggers on the IFCB were used to image phytoplankton and particles, concluding ash and detritus. Over 100,000 images were collected and each image was manually classified, resulting in time series of phytoplankton cell counts and biovolume, as well as ash and detrital particle counts. The application of the IFCB to quantify ash particles, in addition to phytoplankton and detritus, represented a unique use of this instrument in a coastal environment during a period of significant environmental stress. One year later, when no fires were present, the SBC phytoplankton community and detrital particle counts were sampled again with the IFCB. Comparison with the Thomas Fire samples suggests notable shifts in the phytoplankton community during the fire. During the Thomas Fire, dinoflagellates dominated the phytoplankton community (comprising up to 70% of the total phytoplankton cell biovolume per sample), whereas one year after the fire, diatoms dominated the community. Ash and detrital particle counts were elevated in the surface ocean throughout the cruise, and the proportion of ash and detritus to all particles was highest nearshore. Wildfire severity and frequency are expected to increase in California in the future—indeed, the Mendocino Complex Fire surpassed the Thomas Fire to become the largest fire in California history in August 2018. Thus, this study provides an important baseline for the impacts of wildfire ash on phytoplankton community composition in the SBC and beyond, as well as a model for using the IFCB in novel ways to assess the contributions of wildfire ash to surface ocean particles.