Temporal and Spatial Comparison of Mercury Bioaccumulation in the Lower Trophic Levels of a Post-glacial Lake Food Web

Abstract

Total mercury (Hg) concentrations and carbon (δ¹³C) and nitrogen (δ¹⁵N) stable isotopes were quantified among aquatic invertebrate and sediment samples collected from Keuka Lake in New York’s Finger Lakes region to evaluate temporal and spatial variability in Hg bioaccumulation and trophic ecology among these lower trophic levels. Hg concentrations ranged from 6.3 to 158.8 ng/g (dry wt) across dreissenid mussel, zooplankton, and juvenile (< 10 mm) and adult (≥ 10 mm) mysid shrimp (Mysis diluviana) samples. Hg concentrations were higher in samples collected from the western basin in 2015 relative to those for samples collected from this basin in 2022 (p < 0.001). While no specific mechanisms could be identified to explain this difference, higher δ¹⁵N values for zooplankton collected in 2015 support conclusions regarding the role of zooplankton trophic status on Hg concentrations in these populations. Spatial patterns in Hg concentrations were of generally low variability among samples collected from the lake’s east, west and south basins in 2022. Trophic positions as inferred by δ¹⁵N were represented by adult mysids > juvenile mysids > large zooplankton (≥ 500 µm) > dreissenid mussels ≥ small zooplankton (64–500 µm). Differences were evident among the regression slopes describing the relationships between sample Hg concentrations and δ¹⁵N values across the lake’s three basins (p = 0.028). However, this was primarily attributed to high δ¹⁵N values measured in dreissenid mussels collected from the south basin in 2022. Biota sediment accumulation factors ranged from 0.2 to 2.3 and were highest for adult M. diluviana but mysid δ¹³C values generally supported a pelagic pathway of Hg exposure relative to benthic sediments. Overall, these results provide additional support regarding the contributions of lower trophic levels to Hg biomagnification in aquatic food-webs.