What data are needed to detect wildfire effects on coastal ecosystems? A case study during the Thomas Fire

Abstract

Wildfires are growing in frequency and severity worldwide, with anthropogenic climate change predicted to worsen the effects of wildfires in the future. While most wildfire impacts occur on land, coastal fires can also affect the ocean via smoke production and ash deposition. The impacts of wildfires on marine ecology and biogeochemistry have been studied infrequently. Increasingly, remote sensing measurements are used to study the impacts of wildfires on marine ecosystems through optical proxies, such as chlorophyll-a from remote sensing reflectance (Rrs).Here, we evaluate the potential and limitations of remote sensing measurements to describe the effects of wildfires on optics and phytoplankton observations. Using samples collected in the Santa Barbara Channel (California, USA) during the Thomas Fire in December 2017, we consider the impact of wildfire smoke and ash particles on chlorophyll-a retrievals from in situ Rrs vs. from satellites. Our results identify a persistent overestimation of chlorophyll-a concentrations from MODIS-Aqua compared to chlorophyll-a concentrations from in situ Rrs and direct measurements. We compare the spectral shapes and magnitudes of inherent optical properties (IOPs) derived from inverted Rrs vs. the IOPs measured in situ, finding an underestimate of colored dissolved organic matter (CDOM) absorption from optical modeling. Satellites will remain an important tool to measure the impacts of wildfires on marine ecosystems. This analysis helps to quantify the impacts of smoke and ash on remote sensing observations for studying changes in ocean ecology and biogeochemistry from wildfires and provides suggestions for improved interpretation of remote sensing results during wildfire events.