Wildfire Influences on Stream Network Hydro-Biogeochemistry Are Related to Watershed Properties, Burn Severity, and Organic Matter Chemistry
Authors
Allison Myers-Pigg1* (allison.myers-pigg@pnnl.gov), Alan Roebuck Jr.1, Vanessa Garayburu-Caruso2, Katie Wampler3, Jake Cavaiani1, Emily Graham4, Kevin Bladon3, Tim Scheibe2, RC-SFA Team
Institutions
1Marine and Coastal Research Laboratory, Pacific Northwest National Laboratory, Sequim, WA; 2Pacific Northwest National Laboratory, Richland, WA; 3Oregon State University, Corvallis, OR; 4Pacific Northwest National Laboratory, Seattle, WA
URLs
Abstract
Wildfires impact river corridor hydrobiogeochemical function by altering the availability of organic matter (OM) and nutrients within the landscape and the hydrological mechanisms responsible for their delivery to aquatic systems. A shift in post-fire input to aquatic systems can have cascading impacts on carbon and nitrogen stoichiometry that may impact stream metabolism. Developing a predictive understanding of wildfire impacts remains a challenge, due to the complex array of spatial and temporal drivers that influence stream biogeochemistry post-fire. To better understand localized drivers of dissolved organic carbon (DOC) concentrations across a stream network, researchers used a Spatial Stream Network (SSN) model to study a stream network impacted by fire with ~100 sites above, within, and below the burn perimeter across seasons. The SSN model indicated that wildfire impacts were masked by the variability of site-level landscape characteristics across the watershed. During dry periods burn severity was not a major influencing factor, but as the basin seasonally rewet, burn severity did influence observed DOC concentrations, showing a decrease in DOC with increasing burn severity. In streams with drainage areas entirely within the burn perimeter, the spatial drivers of OM chemistries (e.g., burn severity) were partially modulated by localized hydrological processes. Researchers have also found that thermodynamically predicted bioavailability of fire-altered OM may be higher than previously assumed. Together, these results suggest that spatiotemporal controls on the transport of fire-altered OM to the stream network influence in-stream biogeochemical processes, highlighting complexity in upscaling localized watershed scale processes across systems and scales. Confirming this complexity, researchers found that relationships between DOC, climate, and percentage of watershed burn area in fire impacted systems exhibited high spatial variability across the continental U.S. Collectively, this work has advanced the understanding of wildfire impacts on DOC quantity and OM quality post-fire, with implications for aquatic ecosystem function.