Biosentinels have been used to indicate relative risk of mercury (Hg), a contaminant that has led to fish consumption advisories in all 50 states, tribes, and territories, to food webs across the US. Dragonfly larvae have been employed as biosentinels for Hg at the national scale, with good correspondence to fish, amphibian, and biogeochemical drivers. Although intra-annual variability in Hg concentrations in dragonflies is typically low relative to spatial variation, few studies have investigated the effects of extreme hydrologic events on Hg bioaccumulation. Regionally, the Northeastern US has experienced long-term atmospheric deposition of Hg from long-range transport, and has also seen the greatest increases in frequency of extreme precipitation events in the US. Such events can influence Hg via increases in dissolved organic carbon (DOC) mobilization, or potentially redistribution of legacy Hg from historic deposition or industrial pollution. The Merrimack River Watershed in New England was the earliest industrialized area in the US, with known in-watershed Hg releases including use of kyanized (Hg-treated) lumber in the 1800s to construct a canal and lock system, and the Nyanza Superfund site that used Hg in dye manufacturing through the 1970s. The Merrimack Watershed also includes remote, forested landscapes in its headwaters in the White Mountain National Forest (WMNF), NH. This project used dragonfly larvae, biosentinels for Hg in food webs, to investigate spatial patterns in Hg within the Merrimack River Watershed, with focus on both the industrialized areas within the watershed (Lowell and Lawrence, MA) and headwater areas in WMNF. Over five years of dragonfly collections, the watershed experienced extreme drought and wet years (e.g., July 2023 was the fourth wettest July since recordkeeping began for the Northeast; in 2025, New Hampshire had its highest coverage of extreme drought in the history of the U.S. Drought Monitor), providing hydrologic extremes that reveal how these climate-related events influence potential Hg changes in food webs. Although previous research has shown limited inter-annual variability in dragonfly Hg, such extreme hydrologic events did not occur in that study period. Thus, the current study offers the opportunity to refine our understanding of how these types of events could inform interpretation of long-term trends and underscores the need for long-term monitoring.