Nitrogen-limitation Overrides Impacts of Wildfire Ash on Primary Production in Naturally Eutrophic, Grassland Lakes
摘要
Wildfires can produce large amounts of phosphorus-rich ash, providing a potentially limiting nutrient to lake ecosystems. Although fire is ubiquitous in grasslands, little is known about the bioavailability of nutrients in ash, nor its impacts on lakes in grasslands. We present data on the chemical composition of grassland ash and determine the short-term effects of ash on algal growth and nutrient limitation in productive, grassland lakes of the Nebraska Sandhills. We added grassland-derived ash (0 – 1,000 mg L−1) with and without additional nutrients (phosphorus (P), nitrogen (N)) to multiple lakes using bottle bioassays and determined changes in algal biomass, phytoplankton metabolism and nutrient availability. We then leveraged remotely sensed lake color data to contrast the effects of wildfire on water quality in grassland and forest lakes. Although we observed limited changes in algal biomass or metabolism in response to ash, we found that grassland ash was rich in P. In one lake, ash combined with the N treatment increased algal biomass. We observed no changes in lake color following wildfires and observed that grassland lakes were more productive than forest lakes. Our results indicate that wildfires can transport P to grassland lakes, but the impact on algal growth is mediated by lake trophic state and degree of nutrient limitation. Our research highlights important distinctions between lakes in forests and grasslands, and future assessments of wildfire impacts on aquatic ecosystems should determine whether differences in fire regimes, catchment characteristics and limnological properties shape lake ecosystem response to wildfires on multiple timescales.