High Hg biomagnification in North Atlantic coast ecosystems and limits to the use of δ15N to estimate trophic magnification factors

•Hg declined significantly in the North Atlantic coast of spain between 1990 and 2021.•Despite its overall low concentrations, hg biomagnified in the studied food chain.•Hg pollution can be hazardous even when found at low levels in lower trophic levels.•Nitrogen pollution differentially affected the δ15N signatures of the species studied.•The use of δ15N in biomagnification studies may be limited by nitrogen pollution.