Observations of Phytoplankton Community Composition in the Santa Barbara Channel During the Thomas Fire

AbstractThe Thomas Fire ignited on December 5, 2017 and burned nearly 300,000 acres of land in Ventura and Santa Barbara counties until January 12, 2018, making it the largest wildfire in California history at the time. During the fire, a persistent plume of ash, smoke, and soot extended up to 1,000 km offshore over the Santa Barbara Channel (SBC). The effect of this ash influx on the SBC phytoplankton community was investigated with an Imaging FlowCytobot (IFCB) onboard R/V Sally Ride during a research cruise in mid‐December 2017. Over 100,000 images of phytoplankton and nonliving particles were collected and each image was manually classified, resulting in time series of phytoplankton cells and detrital particles, including ash. Comparing the Thomas Fire conditions to historical monthly sampling in the SBC in December reveals significant differences in sea surface temperature, nutrient concentration, and phytoplankton community composition (PCC) in December 2017. During the Thomas Fire, dinoflagellates dominated the phytoplankton community (comprising up to 90% of the total phytoplankton cell biovolume per sample), while diatoms and picophytoplankton typically dominate surface ocean PCC in the SBC in December. While this study was not able to demonstrate a correlational relationship between PCC and wildfire ash concentration, the significant differences in surface ocean biogeochemistry in December 2017 compared to past winters is notable. Wildfire severity and frequency are annually increasing in California and globally with unknown impacts on the marine ecosystem; thus, this study provides an important baseline assessment for the SBC.