Measuring unseen contaminants: Dragonfly larvae as mercury biosentinels on U.S. National Wildlife Refuges

Background/Question/Methods

The United States National Wildlife Refuge System is the largest network of lands and waters conserved specifically for fish, wildlife and their habitats. National Wildlife Refuges (refuges) can be found in every state and many territories, providing a continental-scale system of conservation and crucial habitat for fish and wildlife, including many threatened or endangered species. Although refuges are protected, species can still be negatively affected by airborne pollutants (e.g., mercury) deposited in remote areas distant from emission sources, and by legacy pollution. Once delivered to aquatic environments, mercury can be converted to methylmercury, which biomagnifies through food webs and is harmful to humans and wildlife. Monitoring environmental mercury is complicated by factors influencing mercury methylation, including water management. Given the disconnect between inorganic mercury inputs and ecological risk, biosentinel organisms often provide better estimates of risk than measurements of mercury deposition alone. Dragonfly larvae are an excellent biosentinel since they are abundant in aquatic environments, easy to capture and identify, and are an intermediary predator in aquatic food webs. An ongoing research collaboration (Dragonfly Mercury Project) initiated in similarly protected lands (US national parks) was extended to refuges in 2021, with dragonfly larvae collected from multiple refuges for mercury analysis.

Results/Conclusions

Data were analyzed from 11 refuges located primarily in the southwestern region of the U.S. Preliminary findings indicate that mercury concentrations varied considerably among refuges and among sites within the same refuge. Individual dragonfly mercury concentrations in these refuges spanned the entire range observed in over 100 US national parks (< 5 to >1200 nanograms/gram dry weight). The lowest and highest mercury concentrations were recorded at different wildlife refuges in the same state (New Mexico). Based upon an integrated impairment index of mercury risk developed to inform managers where mercury concentrations are at levels of potential concern for wildlife and humans, approximately half of the refuges had moderate to high risk of impairment while one refuge was classified as severely impaired (Valle de Oro). This newly created refuge was formerly the site of numerous industrial activities and is being restored as an urban oasis for wildlife and people. Our results identify waterbodies where mercury may pose risks to wildlife and provide baseline measurements to monitor change on refuges. Wildlife managers can utilize this information to examine the influence of management actions on mercury bioaccumulation, ultimately identifying strategies to mitigate mercury contamination on refuges.