Sensible sentinels: preliminary mercury data for dragonfly nymphs (Odonata: Anisoptera) across northern New England corroborate expected spatial pattern

Nelson, Sarah; Chen, Celia; Roebuck, Hannah; Zoellick, Bill

"Mercury (Hg) is a potent neurotoxin that is delivered to ecosystems via deposition from a global atmospheric pool, and ultimately bioaccumulates in aquatic and terrestrial foodwebs. Around the Gulf of Maine, research sites in ‘pristine’ areas have fish and other biota that exceed thresholds considered safe for human consumption or wildlife protection. All Maine, Vermont, and New Hampshire surface waters are under fish consumption advisory and are considered impaired with respect to Hg because of these patterns and the difficulty in predicting which systems are most affected. Together with a team of citizen scientists, we are evaluating the utility of dragonfly nymphs (Odonata: Anisoptera) as indicators of Hg status in the Gulf of Maine region. We propose that dragonfly nymphs will be good sentinels because they are: (1) widespread and found in most surface waters in the region, (2) long-lived in aquatic systems (1-5 yr as nymphs), (3) exhibit site philopatry, (4) important prey species for fish that are consumed by humans, and (5) simple to capture, process, and analyze at meaningful Hg concentrations. Specifically, because dragonfly nymphs are themselves predators, Hg concentrations are high enough for laboratory and statistical analyses to be meaningful. The average Hg concentration in dragonfly nymphs sampled across Maine was 0.097 ppm (wet weight basis), greater than the proposed wildlife safety criterion (0.077 ppm). At sites across the region (ME, NH, VT, and MA), we used our data to address hypotheses regarding whether Hg varied with body size or by family; these characteristics were less important than a field site’s landscape setting. Data from a survey of a variety of surface water sites in or near four National Park areas in ME (Acadia), MA (Boston Harbor Island and Saugus Ironworks), and VT (Marsh Billings Rockefeller) confirmed that Hg in dragonfly nymphs was more variable among sites than within a site, suggesting that they are useful indicators for Hg. Further, three years of research by citizen scientists has confirmed correlation between Hg in dragonfly nymphs and DOC in five streams within Sunkhaze National Wildlife Refuge. Hg in dragonfly nymphs were related to concentrations in other media at three long term monitoring sites within Acadia National Park where Hg in mature forests has been shown to be greater than early successional forests. More research on dragonfly larval life history will help to develop a mechanistic understanding of this spatial variability in Hg bioaccumulation." /STUDY LOCATIONS:/ /ACADIA NATIONAL PARK/