COS 295-5 - Effects of Fire Retardant on California Vernal Pool Community Composition

Abstract: Wildfires continue to threaten California and have recently become more frequent and intense. As a response, federal agencies such as the U.S. Forest Service (USFS) treat the perimeter of wildfires with fire retardant in attempt to control the fire. Previous studies have focused on the effects of fire retardant on individual species, but few have examined the effects on plant and invertebrate communities within an aquatic ecosystem. California vernal pools are seasonal wetlands that are imperiled and support dozens of threatened and endangered species. We investigated the effects of increasing concentrations of fire retardant on invertebrate and plant abundance and biodiversity in California vernal pool mesocosms. Mesocosms were lined with vernal pool soil that contained common vernal pool plant seeds and invertebrate eggs. Fire retardant solution was applied to 20 mesocosms at the California State University, Sacramento Arboretum. Each mesocosm was treated with a different fire retardant dilution ranging from 0% to 86%. Mesocosms filled with natural precipitation during the winter 2022/2023 rainy season, and sampling of organisms and water quality occurred every two weeks. Water quality measurements included temperature, pH, turbidity, conductivity, chlorophyll-a, nitrates and phosphates.

Fire retardant was found to have effects on vernal pool water quality, biodiversity, and community composition. Chlorophyll-a, nitrates, and phosphates increased with fire retardant addition. Total species richness and abundance increased over time and decreased with increasing fire-retardant concentration. Water quality shifted from oligotrophic to eutrophic conditions with increasing fire-retardant application. If vernal pools are subject to fire retardant misapplications during emergency fire retardant application, long term monitoring efforts will be needed to ensure that wildfire management does not come at the risk of biodiversity loss in associated freshwater ecosystems.