Study Finds Hidden Effects Of Wildfires On Water Systems

Highlights:

• Wildfires disrupt microbial communities on land and can also alter aquatic systems when dissolved organic matter on burnt lands is carried into rivers, lakes or drinking water supplies.
• In a new study, researchers found that the impact of wildfires on water systems could lead to nutrient imbalances, depleting oxygen levels or harmful algal blooms.
• More complex treatment methods may be needed to ensure impacted water is safe for consumption or recreation.

Wildfires profoundly influence the unseen microbial world within our waters, directly impacting water quality and ecosystem health, according to a new study presented at ASM Microbe 2025, the annual meeting of the American Society for Microbiology.

“Understanding these critical, subtle changes is key to developing effective strategies for protecting our water resources in an era of increasing wildfire activity,” said presenting study author Courtney Gardner, Ph.D., assistant professor in the Maseeh Department of Civil, Architectural and Environmental Engineering at The University of Texas at Austin.

The researchers conducted the new study because the increasing frequency and intensity of wildfires pose a growing threat, not only to land, but also to vital water resources. “When rain falls on burnt landscapes, it carries dissolved organic matter, often referred to as ‘pyrogenic organic matter,’ into rivers, lakes and even drinking water supplies,” Gardner said. “A critical gap in our understanding was how this fire-derived material specifically impacts the aquatic microbial communities that are essential for maintaining water quality and cycling nutrients like nitrogen. Unbalanced microbial ecosystems can lead to compromised water health, affecting everything from aquatic life to the safety and treatability of our drinking water. Our goal was to uncover these subtle, yet significant, consequences.”

The new study involved setting up controlled experiments in the laboratory that mimicked what happens in natural water bodies. The scientists prepared water samples and then introduced different forms of dissolved organic matter: some from unburnt plant material, and some from plant material that had been combusted at low (250°C) or moderate (450°C) temperatures, reflecting different wildfire conditions. For 42 days, the researchers meticulously observed how the existing microbial communities in these water samples responded. This involved analyzing changes in their composition—which types of microbes were present and in what numbers—and examining the abundance of specific genes involved in the nitrogen cycle. By comparing these responses across the different water treatments, they could discern the direct effects of wildfire-derived organic matter.

They found the extent of burning dramatically influences the impact on aquatic microbes. Specifically, dissolved organic matter originating from fires at around 250°C led to a decreased diversity of microbial life and fostered less growth of microbial communities compared to organic matter from hotter burns. This suggests that the moderate heat processing creates more bioavailable forms of carbon that these microbes can readily utilize. Furthermore, the scientists observed significant decreases in the capacity for microbial nitrification, a crucial part of the nitrogen cycle that converts ammonia (a form of nitrogen that can be harmful in high concentrations) into nitrate (a form of nitrogen that plants can use).

“These findings carry important implications for how we manage water resources in wildfire-prone regions,” Gardner said. “The observed alterations in microbial communities and nitrogen-cycling processes can disrupt the natural balance of aquatic ecosystems. Such disruptions might lead to imbalances in nutrient availability, potentially fueling harmful algal blooms, depleting oxygen levels essential for aquatic life or increasing the presence of undesirable compounds.” For communities, this means that post-wildfire water bodies may require more complex and potentially more expensive treatment methods to ensure water remains safe for consumption and recreational use. “Our research provides foundational knowledge to anticipate and address these challenges.”

The researchers said it's crucial to underscore that the new research highlights the interconnectedness of terrestrial and aquatic ecosystems. Wildfires have far-reaching effects on water that are often underestimated. “This study provides scientific evidence that reinforces the need for integrated land and water management strategies in regions vulnerable to wildfires,” Gardner said. “By focusing on fundamental scientific understanding, we can inform the development of more resilient communities and healthier environments in the face of escalating climate challenges.”

About The American Society for Microbiology
The American Society for Microbiology is one of the largest professional societies dedicated to the life sciences and is composed of over 37,000 scientists and health practitioners. ASM's mission is to promote and advance the microbial sciences. 

ASM advances the microbial sciences through conferences, publications, certifications, educational opportunities and advocacy efforts. It enhances laboratory capacity around the globe through training and resources. It provides a network for scientists in academia, industry and clinical settings. Additionally, ASM promotes a deeper understanding of the microbial sciences to all audiences.