Monitoring aquatic organisms is critical to inform management of mercury contamination within food webs and across landscapes. Predatory invertebrates are increasingly being used to gauge biotic mercury exposure, given their import as both prey and predator, as well as their site fidelity, widespread distribution, and biphasic linkage of aquatic and terrestrial habitats. Quantitative understanding of mercury bioaccumulation, biomagnification, and toxicity to predatory invertebrates remains limited, requiring further research in order to estimate population and community level impacts. To quantify dietary bioaccumulation over time, we collected dragonfly larvae (Libellula pulchella, L. forensis, Plathemis lydia, and Pachydiplax longipennis) from a single pond and reared them in the laboratory on a clean diet to foster depuration of background mercury for seven weeks. After this period, larvae were fed California blackworms (Lumbriculus variegatus) dosed at four ecologically relevant concentrations of methylmercury. Dosing continued for eight weeks with regular larval subsampling to quantify total- and methyl-mercury accumulation over time, with assays performed to quantify feeding rate, predator avoidance behavior, and immune response.
Results/Conclusions
Preliminary data suggest that an 8-week diet of 3.4±0.8, 16.3±1.4, 80.3±4.9, and 390.4±17.8 ppb methylmercury led to dragonfly total mercury tissue concentrations of 31.0±5.7, 42.2±4.3, 120.1±4.4, and 507.6±37.4 ppb respectively. Low, medium, and high treatment tissue concentrations were estimated to be 1.5, 4.6, and 18.6 times higher respectively than control levels (p<0.001, Dunnett’s test). Trophic transfer factors decreased with increasing mercury dose, with values of 9.1, 2.6, 1.5, and 1.3 for control, low, medium, and high, indicating higher bioaccumulation efficiency at lower doses. Bioaccumulation rate gradually decreased over time, appearing to reach steady state by week 6. The trophic transfer rates quantified here inform how different environmentally relevant levels of mercury contamination influence assimilation to secondary consumers, and how this accumulation changes over time. Additionally, these findings augment the utility of the US National Park Service’s comprehensive Dragonfly Mercury Project dataset by empowering inference of contamination at lower trophic levels from dragonfly tissue.