The U.S. Geological Survey is researching the effect of wildfires on the release of stored mercury from forest watersheds and subsequent transport, methylmercury formation, and bioaccumulation post-fire. Wildfires not only alter the re-release of mercury into the environment but also affect its reactivity and availability to the food web, complicating predictions of mercury exposure risks to wildlife and humans amid other co-occurring stressors.
Mercury is a toxic heavy metal that poses significant wildlife and human health risks. It is converted into methylmercury, a highly toxic organic form, by microorganisms in aquatic environments. This toxic mercury form accumulates in the tissues of fish and other animals, leading to higher concentrations at higher trophic levels (biomagnification). Although considerable research and monitoring have focused on sources, transformations, and bioaccumulation of mercury, the effect of additional stressors, such as wildfires, is not as well understood. Wildfires can potentially disturb soil and water systems, releasing previously stored mercury back into the environment, thereby complicating existing issues related to mercury contamination. Understanding these interactions is crucial for effective management and mitigation strategies.
The release, movement, and transformations of mercury owing to wildfires are complex and can significantly affect ecosystems and human health. Wildfires reduce the mercury content in the soils via evasion to the atmosphere, which might initially seem to decrease mercury transport and accumulation in affected ecosystems; however, the destruction of vegetation, along with changes to soil properties and hydrologic flow paths, can actually lead to increased soil erosion. This erosion facilitates the transport of mercury and other contaminants into water bodies, complicating the ecological consequences of wildfires.
Headwater streams, which are intricately connected to their surrounding watersheds, are particularly vulnerable to these changes. Wildfires can alter vegetation, light levels, water flows, and overall productivity, all of which influence the food webs within these streams. Although mercury is a well-researched contaminant, the effect of wildfires and changes in mercury transport make it difficult to predict changes in the food web mercury concentrations.
To better understand the effects of wildfires on mercury concentrations and bioaccumulation in the food web, researchers conducted a comprehensive study across 36 headwater-stream watersheds in Washington, Oregon, and Idaho. These watersheds were assessed 1 year post-wildfire and compared with samples from 21 similar unburned watersheds. Concentrations of total mercury and methylmercury in particles suspended in the water were 89 and 178 percent higher, respectively, in the burned watersheds. The concentrations increased with burn severity, which was associated with increased soil erosion. Although total mercury levels in filtered water samples remained relatively consistent between burned and unburned watersheds, methylmercury concentrations were notably 51 percent higher in the burned areas. Of particular concern were aquatic insect species within these streams, which exhibited methylmercury concentrations that were sometimes as much as 251 percent higher in burned watersheds compared to their unburned counterparts.
This study underscores the critical downstream effects that wildfires can have on mercury mobility, methylmercury formation, and subsequent bioaccumulation in aquatic food webs. If the frequency of wildfires continues to rise, it is likely that we will witness significant shifts in mercury cycling, which can affect current management and mitigation strategies locally and globally. Understanding these interactions is crucial for developing effective responses to the growing threat of wildfires and their implications for environmental health and public safety.
This study was supported by the U.S. Geological Survey Ecosystems Mission Area, through the Environmental Health Program (Contaminant Biology and Toxic Substances Hydrology) and the U.S. Geological Survey Northwest Pacific Islands Region.
Publication:
Wildfires influence mercury transport, methylation, and bioaccumulation in headwater streams of the Pacific Northwest
