Fish eye lens chronologies reveal reservoir-driven changes in carbon biogeochemistry and mercury bioaccumulation

Reservoir impoundments alter river carbon (C) biogeochemistry as submerged terrestrial material is decomposed, which enhances mercury (Hg) methylation. Methylmercury (MeHg) is a neurotoxin that bioaccumulates in aquatic organisms and biomagnifies along food webs, exposing wildlife and human consumers. Reservoir characteristics (ie., water residence time and land flooded area) regulate C and Hg dynamics, yet their temporal dynamics remain poorly resolved. Fish eye lenses grow incrementally and preserve fine-scale temporal records of Hg bioaccumulation and C biogeochemistry. Lenses from the Romaine hydroelectric reservoirs (Québec, Canada) and an upstream control fluvial lake were analyzed to reconstruct high resolution histories of Hg bioaccumulation, C biogeochemistry (carbon isotopic signature; δ¹³C), and trophic position (nitrogen isotopic signature; δ¹⁵N) in Northern Pike (Esox lucius), Lake Whitefish (Coregonus clupeaformis), and White Suckers (Catostomus commersonii). δ¹³C remained stable over time in the control lake but was lower and temporally variable in reservoirs, indicating that C biogeochemical change was associated with impoundment and altered by characteristics (land flooded area and water residence time). Mercury was negatively associated with δ¹³C, suggesting increased C processing drives MeHg production. Consistent with this, higher Hg bioaccumulation rates (change in log₁₀Hg over time) in Lake Whitefish occurred in reservoirs with larger land flooded areas and longer water residence times. Species-specific δ¹³C patterns demonstrated that C biogeochemical change differed among environmental compartments (littoral and limnetic) and trophic timescales. Hg bioaccumulation rates were highest in Northern Pike, consistent with their high trophic position and the positive association between Hg and δ¹⁵N.