Future mercury levels in fish: model vs. observational predictions under different policy scenarios

Abstract. Mercury (Hg) poses a global threat due to its long-range transport and transformation into methylmercury (MeHg), a potent neurotoxin that bioaccumulates in aquatic food webs. While global and regional efforts to reduce anthropogenic Hg emissions are ongoing, the implications of these policies for future Hg deposition and consequent MeHg levels in fish remain uncertain. This study synthesizes published modeling studies to examine projected relationships among Hg emissions, atmospheric deposition, and lake fish MeHg concentrations in 2050 under various policy scenarios. While models reveal a strong linear relationship between emissions and deposition (R² = 0.79), and a moderate correlation between Hg deposition and fish MeHg (R² = 0.63), these trends contrast with observational data, which often show nonlinear or more complex responses. Modeled atmospheric deposition and lake area emerged as key predictors, with higher deposition and smaller lakes associated with higher modeled fish MeHg levels. Notably, despite wide variation in model structures, including differences in atmospheric chemistry, emission inventories, legacy emissions, meteorological drivers, methylation-demethylation kinetics, and food web dynamics, the linear trends persisted. This apparent linearity underscores robust large-scale cause–effect patterns but also calls for caution: do current models truly capture the complexity of real atmospheric and ecosystem processes, or might they oversimplify mercury’s nonlinear cycling and ecological responses? These findings highlight the need to remain open to processes and interactions not yet fully represented in models, ensuring that future mercury assessments and policy strategies reflect the true complexity of natural systems.