Historical landscape management, an increase in ignitions, and climate change have led to an increase in wildfires globally. Wildfires are expected to increase in size, frequency and severity as global temperature increases. Direct effects from wildfires can affect fundamental hydrologic and biogeochemical transformations from land to water (e.g., groundwater, lakes, streams) while smoke plume emissions from fires can alter the ecology of ecosystems far beyond burned watersheds. This session will focus on understanding wildfire impacts on aquatic ecosystems at different scales of time (seasonal to decadal) and space (single ecosystems to continental and global). We will also consider impacts across biological scales, from physiological impacts on organisms to biogeochemical cycling. We seek to unravel how wildfires affect fundamental functional processes within aquatic ecosystems and to understand the resilience of ecosystems to wildfire disturbances. Simply, in this session we will explore what happens when fire and water mix?
Lead Organizer: Sudeep Chandra, University of Nevada- Reno (sudeep@unr.edu)
Co-organizers:
Jessica Corman, Unviersity of Nebraska, Lincoln (jcorman3@unl.edu)
Adriane Smits, UC Davis (adriannesmits@gmail.com)
Janice Brahney Brahney, Utah State University (janice.brahney@usu.edu)
Steve Sadro, UC Davis (ssadro@ucdavis.edu)
Facundo Scordo, Instituto Argentino de Oceanografía, Universidad Nacional del Sur (UNS)-CONICET, Bahía Blanca, Buenos Aires, Argentina (scordo@agro.uba.ar)
Presentations
09:00 AM
Wildfires change dissolved organic carbon composition in boreal headwater streams (8212)
Primary Presenter: Erin Matula, Trent University (erinmatula@trentu.ca)
Boreal forests are one of the largest terrestrial carbon sinks which export large amounts of carbon annually into downstream waters as dissolved organic material (DOM). Once in water, DOM can be readily consumed and respired by microbes depending on its concentration and composition, thereby offsetting carbon sequestration on land. Increasing wildfire frequency associated with climate change has the potential to change the concentration and composition of DOM exported from land into receiving waters. We tested how wildfire changes the concentration and composition of DOM exported from boreal forests into boreal shield headwater streams in northwestern Ontario, Canada over a summer growing season. We compared the composition of DOM between catchments that were either burned in the last four years (n = 10) or had been undisturbed for at least 20 years (n = 10) using optical spectroscopy and ultra-high-resolution mass spectrometry. We found a 28% increase, on average, in dissolved organic carbon concentrations in burned streams by August. Burned streams had lower H:C and higher O:C, indicating less bioavailable DOM, likely because of black carbon from the burned catchments entering the streams. These results will inform forest management policy and improve carbon accounting in Canada to meet international obligations by providing novel results on carbon movement from terrestrial to aquatic systems in context of wildfires.
09:15 AM
Dissolved organic matter character across a wildfire affected stream network (7978)
Primary Presenter: Katie Wampler, Oregon State University (katie.wampler@oregonstate.edu)
There has been an increase in large, high severity wildfires in recent years, leading to potential landscape, hydrologic, and water quality impacts. In particular, there is increasing concern about the effects of wildfires on dissolved organic matter (DOM) due to its role in aquatic ecosystem health and drinking water treatment. While there’s evidence that wildfire can alter the chemical composition of DOM, there has been substantial variability in observations. We aim to increase our understanding of post-fire changes in DOM character by using excitation emission matrices (EEMs) data from high spatial resolution stream sampling. Following a large 2020 wildfire in Oregon, we sampled >120 stream sites across the fire affected stream network. Sampling was repeated across seasonal hydrologic conditions to capture variation in hydrologic pathways and organic matter sources. We will use spatial stream network models to explore seasonal and spatial patterns of DOM character across the basin and to understand factors contributing to variability in character metrics across the sub-basin. Our work will contribute to a mechanistic understanding of how wildfire impacts DOM sources and character, which is critical to predict wildfire effects on DOM and preserve aquatic ecosystem health and source water quality.
09:30 AM
Watershed Properties and Burn Severity Influence River Corridor Biogeochemistry After Wildfire (7998)
Primary Presenter: Allison Myers-Pigg, Pacific Northwest National Laboratory (allison.myers-pigg@pnnl.gov)
Wildfires impact river corridors by altering both the availability of organic matter (OM) and nutrients within the landscape and the hydrological mechanisms responsible for OM delivery to aquatic systems. A shift in post-fire inputs to aquatic systems can have cascading impacts on OM content and composition that may impact stream metabolism. In fire-impacted systems across North America, we found no significant differences in dissolved organic carbon (DOC) compared to non-fire impacted systems across climates and percent of watershed area burned. Rather, DOC shifts in burned catchments exhibited high spatial variability and were best explained by catchment characteristics. In companion research across a stream network in the Pacific Northwest (PNW), we found that wildfire impacts on DOC were masked by variability of site-level landscape characteristics. During dry periods, burn severity was not a major influencing factor. As the basin rewet seasonally, we observed an inverse relationship between catchment burn severity and DOC concentrations. In streams with drainage areas entirely within the burn perimeter, burn severity was a major spatial driver of OM chemistries during the first storm post-fire, but was partially modulated by localized hydrological processes. Together, our results suggest that spatiotemporal controls on the transport of fire-altered OM to the stream network influence in-stream OM quantity and composition, highlighting complexity in upscaling localized watershed scale processes across systems and scales.
09:45 AM
WILDFIRE SMOKE AFFECTS LAKE AND POND THERMAL AND METABOLIC REGIMES: A WATERSHED-SCALE INVESTIGATION (8121)
Primary Presenter: Mary Farruggia, University of California Davis (maryjadefarruggia@gmail.com)
Climate change is increasing the likelihood of extreme fires globally. These wildfires produce large smoke plumes that can decrease solar radiation and deposit ash particles across ecosystems. Several key physical, chemical, and biological processes in aquatic ecosystems are controlled by factors affected by smoke. However, the effects of smoke on lake and pond ecosystems are still not well known. We studied effects of wildfire smoke on 2 lakes and 4 ponds in a representative high-elevation (2800-3230 m) watershed in the Sierra Nevada mountains of California. Smoke from major wildfires in 2020 and 2021 covered our study watershed for an average of 49 days between August and October. Using a network of high frequency measurements of water temperature and dissolved oxygen, we found that although lakes and ponds responded synchronously to smoke, the magnitude of effects varied. In general, thermal responses were clearest. Smoke reduced water temperature, with the magnitude of response controlled in part by waterbody size. Changes in rates of primary production associated with smoke were more varied, but also tended to scale with waterbody size. These results suggest that the impacts of smoke on small lakes and ponds are mediated by multiple factors, but that smaller waterbodies may be particularly vulnerable to the effects of smoke. Understanding the factors that mediate lake and pond sensitivity to smoke is a priority given current and projected wildfire regimes in a climate-modified world.
10:00 AM
NITROGEN-LIMITATION MODIFIES THE EFFECT OF WILDFIRE ASH ON PRIMARY PRODUCTION IN PRODUCTIVE GRASSLAND LAKES: INSIGHTS FROM BOTTLE BIOASSAYS AND A PROCESS-BASED MODEL (7811)
Primary Presenter: Daniel Gschwentner, University of Nebraska-Lincoln (dgschwentner2@huskers.unl.edu)
Wildfires produce phosphorus-rich ash which can alter light and nutrient availability in lakes, ultimately influencing ecosystem processes such as primary production or biogeochemical cycles. However, the immediate effects of wildfire-derived nutrients on lake ecosystems are unclear, particularly in grasslands which are understudied in the context of wildfire:lake interactions. In this study we used bottle bioassays and a process-based model to determine how ash concentrations of 0-100 ppm influenced primary production and nutrient limitation in productive grassland lakes in Western Nebraska. We hypothesized that ash-derived phosphorus would initially increase primary production but eventually shift lakes towards light or nitrogen limitation. Our approaches produced mixed results: although ash increased phosphorus availability in the bottle bioassays, pervasive nitrogen limitation constrained primary production. In contrast, ash inputs increased primary production and shifted lakes towards light- followed by nitrogen-limitation in the process-based model. For both the bioassays and process-based model, the effect of ash on lake on primary production and nutrient cycling was modified by the availability of nitrogen, indicating stoichiometric constraints on lake ecosystem processes. Our results suggest that the effects of wildfires on lake ecosystems are mediated by trophic state and nutrient stoichiometry.
10:15 AM
Wildfire smoke alters ecosystem metabolic rates in lakes (8047)
Primary Presenter: Adrianne Smits, University of California Davis (asmits@ucdavis.edu)
Increasingly severe wildfires release smoke plumes covering entire continents, depositing aerosols and reducing solar radiation fluxes to millions of lakes, yet little is known about impacts on inland waters. This large-scale study we 1) quantified annual and seasonal trends in the spatial extent of smoke cover in California, USA (2006 - 2022), and 2) assessed the impacts of smoke on gross primary production (GPP) and ecosystem respiration (R) in 10 lakes spanning a large gradient in trophic status, during the three smokiest years since 2006 (2018, 2020, 2021). The maximum spatial extent of medium or high-density smoke cover between June - October has increased to 70% of California’s area, with the greatest increases in August and September. In the three smokiest years, lakes experienced an average of 33 days of smoke between July and October, resulting in substantial reductions in shortwave radiation fluxes and 4 to 5-fold increases in atmospheric fine particulate matter concentrations (PM2.5). R decreased during smoke cover in oligotrophic lakes but not in eutrophic lakes, whereas responses of lake GPP were extremely variable. Effects of prolonged smoke exposure on inland waters will vary substantially due to mediating effects of lake attributes and seasonal timing of wildfires.
SS25 - Fire and Water: Towards an Understanding of Wildfire Impacts on Aquatic Ecosystems
Description
Time: 9:00 AM
Date: 4/6/2024
Room: Hall of Ideas G