Environmental and food web determinants of Lake Trout mercury concentrations in Ontario lakes

Prey composition and availability is considered a primary predictor of lake trout ( Salvelinus namaycush) mercury (Hg) concentrations. Evidence from other freshwater fishes suggests that environmental and landscape factors likely also contribute to fish Hg dynamics, yet comprehensive, contemporary assessments for lake trout from boreal and north-temperate lakes are lacking. Here, we reassess the importance of prey characteristics using both previously published and contemporary data, incorporating additional variables and model complexity to better understand factors influencing Hg dynamics of Ontario lake trout. Our analyses indicate that (1) lake trout Hg concentrations are primarily associated with individual body size, (2) high dissolved organic carbon (DOC) concentrations elevate Hg for fish of a given size, and (3) a coarse categorization of food chain length, specifically the presence of Mysis diluviana, informs Hg biomagnification slopes. The inclusion of DOC was vital for assessing human consumption risk, as lake trout in high DOC lakes were more likely to exceed Hg guidelines at sizes often harvested by anglers. Drivers of lake trout Hg levels in boreal and north-temperate lakes closely match those reported to affect other fishes in the region, regardless of feeding, thermal, and habitat strategies.