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 by 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. Our analysis suggests that these relationships are partly shaped by shared structural assumptions within current modeling frameworks, particularly the treatment of atmospheric deposition as the dominant driver of future fish MeHg change and the representation of relatively short response times between changing Hg inputs and fish exposure, with limited representation of the time-lagged buffering provided by legacy Hg stores. Within these constraints, 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, and food web dynamics, these linear trends persisted. By identifying these consistent patterns and the assumptions that support them, this study provides a benchmark for integrating currently underrepresented processes such as ecological complexity and climate-driven ecosystem feedbacks, into future assessments supporting the Minamata Convention.