Monitoring mercury across the National Wildlife Refuge System using a biosentinel approach

The National Wildlife Refuge System in the United States is the world's largest network of lands and waters conserved specifically for fish, wildlife, and their habitats. National Wildlife Refuges (hereafter, refuges) are distributed nationwide and although these lands are protected, species within them remain vulnerable to airborne pollutants like mercury that can be deposited in areas far away from emission sources. Mercury can be converted to methylmercury in aquatic environments, which biomagnifies through food webs and is harmful to fish, wildlife, and humans. Mercury assessment is complicated by factors influencing methylation, and biosentinel organisms such as dragonfly larvae often provide better estimates of risk than measurements of mercury deposition alone. An ongoing research collaboration (the Dragonfly Mercury Project) that utilizes dragonfly larvae to assess mercury concentration on protected lands was implemented on refuges beginning in 2021. We analyzed mercury concentration in 1,356 dragonfly larvae collected from 30 refuges located throughout the United States between 2021 and 2023. Individual dragonfly mercury concentrations varied considerably among refuges and spanned the entire range observed in other protected lands (<3 to >2,200 ng/g dry weight). Some of the lowest and highest mercury concentrations were recorded at different refuges in New Mexico. Based upon an integrated impairment index of mercury risk developed to inform managers where mercury concentrations are of potential concern, 80% of the refuges sampled had sites classified as moderate or higher risk. Our results identify waterbodies where mercury may pose higher risks to fish, wildlife, and humans and provide baseline measurements to monitor change. This information can be incorporated into future research designed to examine the influence of management actions on mercury bioaccumulation, ultimately identifying strategies to mitigate mercury contamination.