Dangers Downstream

Controlled burns are critical for healthy forests as they eliminate leaf litter that can contribute to large wildfires, reduce the abundance of invasive species and rejuvenate the forest ecosystem. However, controlled burns, like all forest fires, may also have unanticipated consequences. One such consequence is the release of mercury into the environment. Mercury is a highly toxic heavy metal that is released into the atmosphere largely through the burning of fossil fuels. Once released, gaseous mercury is absorbed by plants, meaning forests store mercury that can be released into the surrounding soil, air and water during a forest fire. However, the amount and ultimate fate of mercury released during controlled burns is still largely unknown, including the degree to which released mercury may bioaccumulate in fish and aquatic insects living in streams that drain burned areas.

Students in the Stream Fish Mercury Dynamics in Managed Forests Creative Inquiry project, led by Dr. Troy Farmer and working closely with Dr. Alex Chow, both in the Department of Forestry and Environmental Conservation, are working at the U.S. Forest Service Santee Experimental Forest north of Charleston, SC to study mercury in fish and aquatic insects following controlled burns. As students trudged through small streams and swamps to collect fish and aquatic insects for the mercury study, they discovered another threat to the fish—a deadly pathogen moving from fish to fish like wildfire.

In the fall of 2021, the team collected fish and aquatic insects to sample for mercury contamination in both control watersheds and those treated with controlled burns. Students collected fish by backpack electrofishing, a process that stuns fish by sending small pulses of electricity through the water. The samples were sent to the University of North Carolina at Greensboro to be analyzed for mercury. While collecting fish, the team noticed sick and dying fish in the treatment watershed that had recently experienced a controlled burn and timber harvesting. Students preserved diseased fish and shipped them to Auburn University where fish pathologist Dr. Tim Bruce confirmed the pathogen was Aphanomyces invadans, which threatens fish health in freshwaters along the eastern coast of the United States. This discovery caused the team to pivot their focus to address the presence of this newfound pathogen. Students collected water samples for nutrient analysis and investigated long-term changes in water quality to try and understand how forest management practices may have affected water quality and, subsequently, fish health. Before the next sampling session during spring 2022 the team learned everything they could about A. invadans.

During the spring 2022 sampling event, the team returned to sample both treatment and control watersheds. They found no evidence of A. invadans during their sampling, but will continue monitoring the sites as they collect samples for the mercury study during 2022 and 2023. Even in a year’s time the students have gained a much deeper understanding of how quickly aquatic systems can be affected by disturbances. “A really cool part of this [Creative Inquiry project] is seeing how interconnected everything is and communicating to people who are not in our major how this can affect everyone’s lives,” Emily Davidson, a junior biological sciences major, said.

While diseased fish in a low country watershed 242 miles away from Clemson may seem insignificant, the events that occurred in the Santee Experimental Forest provide a chance to better understand linkages between forest management, water quality and fish health. The Creative Inquiry students’ prompt attention to this unexpected development allowed for valuable data to be collected that will advance this goal. Ultimately, knowledge gained from this study may improve the health of fish in watersheds managed for forest products and lead to better quality of water in all of South Carolina, keeping both people and fish healthy and worry-free.