Algal Blooms in Lakes Increase After Wildfire Smoke Events in the Contiguous United States

Area burned by wildfire has increased in the contiguous United States and in many places globally in recent years, impacting communities and ecosystems nearby and even far downwind of fires. We examined the potential effects of smoke on surface chlorophyll-a concentrations in lakes, as algal blooms can adversely impact drinking water supplies and aquatic ecosystems. We linked remotely sensed surface chlorophyll-a concentrations from over 2000 lakes across the contiguous U.S. with airborne smoke plume locations and densities for 2018. Chlorophyll-a values for each lake were fitted to seasonal curves, and the residuals were tested between smoke versus no smoke periods. Changes in chlorophyll-a were also compared between the weeks before and after smoke events for each lake. Lastly, we categorized lakes based on elevation and trophic status to further discern fire effects on water quality. Mean residual surface chlorophyll-a concentrations during and within 2 weeks after smoke periods were significantly elevated (1.5 μg/L) compared to values from no smoke periods (-0.3 μg/L). Moreover, surface chlorophyll-a responses increased significantly with higher smoke density, and in hundreds of the lakes, increased by amounts exceeding global drinking water alerts for chlorophyll-a. We observed significant increases in responses for lakes at higher elevation (> 538 m) and for eutrophic lakes. We hypothesize that fire-driven smoke effects are contributing to the formation of algal blooms given the proper antecedent conditions, like elevated water temperatures. Algal blooms can occur after ephemeral change in lake trophic status and produce cyanotoxins and other adverse impacts. Because wildfire smoke can affect large regions, our findings have implications for drinking water and recreational lakes across the U.S. and globally, as well as for lake ecology, particularly in higher elevation systems with otherwise limited nutrient inputs.