Date of Award

Winter 12-18-2019

Level of Access

Open-Access Thesis

Degree Name

Master of Science (MS)


Ecology and Environmental Sciences


Sarah Nelson

Second Committee Member

Hamish Greig

Third Committee Member

Collin A. Eagles-Smith

Additional Committee Members

Amanda Klemmer


Mercury (Hg) is a widespread water quality concern because it is a potent neurotoxin that biomagnifies through food webs, posing risk to biota higher in the food chain. Aquatic insects are effective for characterizing relative Hg risk in freshwater food webs because they live and forage in aquatic habitats where Hg is present and converted to its more toxic bioavailable form, methylmercury (MeHg). Specifically, dragonfly nymphs (Odonata) are increasingly being used as a Hg bioindicator. Therefore, I (Chapter 1) conducted a systematic review of the literature to evaluate the utility of dragonflies as biosentinels for Hg. Dragonfly nymphs have been used throughout the world to provide baseline Hg concentrations within waterbodies, analyze factors affecting bioaccumulation in invertebrates, and determine Hg concentrations within food webs. Dragonfly adults have been used for monitoring Hg concentrations as vectors into and among terrestrial habitats and food webs. There has been an increase in the use of dragonflies for Hg monitoring in the last 10 years, with more than half of the studies in the review conducted after 2009. However, research evaluating temporal Hg variability in dragonfly nymphs is limited, which can have important implications for monitoring programs and risk assessment because risk to wildlife may be under- or overestimated due to sample timing asynchrony with representative life-history events or body burden variation. Thus, I (Chapter 2) assessed the temporal variation in total mercury (THg) concentrations in similar aged cordulegastrid dragonfly nymphs from two coastal Maine streams over the course of one year. When we accounted for stream, there was no overall temporal pattern in dragonfly THg concentrations throughout the year. We found that the two streams had similar annual mean THg concentrations; however, within stream variation of THg among months was high and differed among months. Therefore in temperate streams, multiple collections, ideally in different seasons, could be warranted to obtain a robust estimate of Hg concentrations within a lotic ecosystem. Besides dragonfly nymphs, other aquatic insects also serve as an important link in the MeHg pathway in that they transfer MeHg from single-celled organisms to larger predators in the aquatic environment. Additionally, emergent aquatic insects make up a substantial proportion of the diet of many riparian terrestrial consumers and are a primary mechanism by which MeHg is incorporated into terrestrial systems. Therefore, I (Chapter 3) aimed to determine taxonomic composition, biomass, temporal trends, MeHg concentrations, and MeHg flux of emerging aquatic insects in two coastal streams in Maine using weekly emergence trap collections. Different stream reaches harbored different aquatic insect communities and thus, yielded differences in cross-ecosystem subsidies biomass and temporal flux with respect to aquatic insect emergence. These differences shed light on which organisms were vectors for MeHg and the timing of MeHg pulses to the terrestrial environment in natural systems. My research indicate that remote protected natural streams, such as those in Acadia National Park, can yield high MeHg concentrations in biota and thus, large amounts of MeHg flux to the terrestrial environment and predators.