Post-wildfire changes to water quality: a review with implications for harmful algal blooms and reservoir resiliency

Abstract Wildfires wreak havoc on both urban and natural ecosystems and are expected to worsen under future climate conditions. Wildfires have the potential for long-term impacts on the health of downstream reservoirs. This study seeks to compile current literature worldwide regarding impacts of wildfires on reservoir water quality, with a special focus on the implications for harmful algal blooms. The overarching purpose was to fully understand wildfire impact on aquatic health and identify key gaps in research that are needed for accurate management of reservoirs. Due to changes in watershed conditions post-wildfire (mainly temperature, light, and nutrient dynamics), burned waterbodies are more susceptible to blooms in the months to years following a wildfire, with forested areas having recorded algae in burned waterways as high as 5–71× than that of unburned and simulated forested reservoirs showing up to 3× the peak chlorophyll-a values as their simulated unburned counterparts. Changes in algae are mainly attributed to changes to soil properties, increased nutrient loads, and increased light availability and temperatures caused by burning of surrounding vegetation. Factors that influence algae can be short-term, such as light limitation from smoke and large influxes of nutrients and metals, or longer-term, such as changes to canopy cover, legacy nutrients in sediments, surrounding runoff, and sediment chemistry. These external changes can influence harmful algae blooms (HABs) through alteration of bloom timing, extent, and community composition. There is still a need for longer-term studies of nutrient and algal dynamics, as most studies are limited to monitoring algae immediately following a wildfire. It is essential that reservoir managers anticipate how aquatic ecosystems are likely to respond to higher frequency and severity of wildfires and how their management strategies will need to be adapted.