MERCURY BIOMAGNIFICATION IN AQUATIC FOOD WEBS OF GREAT SMOKY MOUNTAINS NATIONAL PARK

Mercury is a widespread pollutant threatening human, fish, and ecosystem health on a global scale. Biomagnification concentrates mercury in upper trophic level organisms including predatory fishes, a primary route of dietary mercury exposure for humans. However, mercury biomagnification is not well understood in stream ecosystems, especially in places with no known point sources of contamination. A 2016 study revealed that Smallmouth Bass Micropterus dolomeiu mercury concentrations varied between three streams in Great Smoky Mountains National Park (GSMNP), Tennessee USA. However, the reason for this spatial variation in mercury concentrations is not understood. Our objectives were to (1) measure environmental and organismal concentrations of mercury, (2) describe and compare trophic pathways, (3) and evaluate evidence for mechanisms leading to differential mercury contamination, in three streams of GSMNP; and (4) to compare Smallmouth Bass mercury concentrations in 2016 and 2022. We analyzed stable isotope ratios and mercury concentrations of eight food web components of GSMNP streams: leaf detritus, periphyton, and filamentous algae to represent basal resources; mayfly nymphs (Heptageniidae), Central Stoneroller Campostoma anomalum, dragonfly nymphs (Odonata), and crayfish (Cambaridae) to represent intermediate consumers; and Smallmouth Bass, an apex predator. Stable isotope analyses revealed similar food web structure across all three streams. Total mercury (THg) concentrations were lowest in basal resources and highest in Smallmouth Bass and crayfish. Compared to a previous study in 2016, mean Smallmouth Bass THg concentrations were lower in 2022 in all three streams, however, many individuals from Abrams Creek still exceeded the EPA threshold for safe consumption of 0.3 mg/kg, ww. In both years, Smallmouth Bass mercury concentrations were highest in Abrams Creek and lower in Little River and Little Pigeon. Mean THg concentrations of basal resources did not differ between streams, but the spatial pattern of concentrations of intermediate consumers mirrored Smallmouth Bass. The slope of the relationship between log-transformed contaminant concentrations and trophic level was positive for all three streams, indicating biomagnification is occurring in GSMNP. Our findings indicate that differences in mercury contamination of top predators in these systems are driven by differences in food web dynamics rather than differences in basal resource THg concentrations.