Fire and Flow: Assessing the Long-Term Effects of Wildfires and Impact of High Flow Events on Phosphorus Concentrations in Mountain Streams

Climate change has led to significant shifts in the Earth’s weather patterns, often leading to longer, more intense droughts, irregular but extreme storms, and more severe wildfires with longer burn durations. These weather pattern changes have frequently led to shifts in ecosystem dynamics, impacting aspects such as nutrient flux, species diversity, and overall habitat health. Regarding nutrient flux specifically, changes in phosphorus (P) concentrations can negatively impact stream systems as elevated levels can lead to toxic algal blooms, which can cause habitat degradation, loss of usable recreational areas, and large fish kills. A common trigger of these P spikes is the occurrence of wildfires. As fire burns plants and other organic matter, it can often release trapped P, making it available for uptake by flora and fauna. However, if it is not immediately taken up, it can be transported to streams via storm runoff. Furthermore, as droughts, another common trigger, continue to get more severe, the likelihood of P accumulation throughout undisturbed water pathways and riverbed sediments significantly increases. Then, once a storm finally occurs and the flow pathways are disturbed, the accumulated P is mobilized and transported to streams via runoff, further contributing to spikes in P levels. In this study, we explored the influence of riverbed disturbance on stream P concentrations as well as the potential existence of any long-term repercussions of wildfire on P mobility. For one spring snowmelt season and two summer monsoon seasons, we collected water samples during riverbed-disturbing high flow events and deployed P catching devices in streams located in a 9-year-old burn scar. We found that riverbed sediments disturbed by high intensity rainstorms can influence the overall source of P in water columns, which switched between proximal and distal sources. We also found that despite short-term spikes typically found after a wildfire, stream P levels can significantly decrease long-term. This knowledge is important because a better understanding of these trends could foster improvements of strategies for water quality monitoring, restoration efforts, and the protection of natural resources by managing agencies