Abstract
Wildfire is a common natural disturbance that occurs in many ecosystems. In recent decades, wildfires have become larger and more severe. One of the most common wildfire response tools used to protect against loss of life and property is fire retardant. Fire retardant is a mixture of chemicals often including ammonium polyphosphate that reduces combustion. Avoidance buffers have been established to keep these chemicals out of aquatic habitat, but misapplications events are documented each year. Previous studies have documented negative effects of fire retardant on water quality, plants, and invertebrates but no studies have been conducted in California. This mesocosm study investigates the effects of fire retardant on water quality, and plant and invertebrate communities in California vernal pools.
Twenty mesocosms were lined with vernal pool soil and treated with one of 18 different fire retardant concentrations (0.6%-86%) using Phos-Chek Wildfire Home Defense Retardant. Invertebrate and water quality data was collected every two weeks from December 2022 to April 2023. Plant data was collected at the end of the experiment in May 2023. Water quality measurements included chlorophyll-a, conductivity, nitrates, pH, phosphates, temperature, and turbidity. Algae cover, plant species richness, invertebrate species richness, and invertebrate abundance were measured across the 18 fire retardant concentrations in order to determine how the community composition responded to fire retardant contamination.
This study found that fire retardant caused water quality conditions to shift from oligotrophic to eutrophic with strong correlations between fire retardant and phosphates, nitrates, pH, conductivity, and chlorophyll-a. Algal cover increased with added fire retardant concentration. Shifts in invertebrate and plant communities occurred even at relatively low fire retardant concentrations considered “safe” by the manufacturer (18% concentration). Invertebrates were categorized by dispersal depending on whether they are able to move from one habitat to the next on their own (active) or rely on other mechanisms such as wind or other animals for dispersal (passive). Abundance of passive dispersers peaked at medium concentrations of fire retardant but the relationship between abundance and fire retardant changed slightly over time. Active disperser abundance increased with increasing fire retardant concentration and invertebrate richness peaked at medium concentrations. This is consistent with previous studies investigating the effects of added nutrients (nitrogen and phosphorus) on aquatic ecosystems. Even when the product is applied during the dry phase, California vernal pool plant and invertebrate communities are likely to experience decreased species richness and shifts in community composition after a fire retardant misapplication event.