Origin of Sulfur in Diet Drives Spatial and Temporal Mercury Trends in Seabird Eggs From Pacific Canada 1968-2015.

Mercury (Hg) is a neurotoxin that can be particularly harmful to top predators because it biomagnifies through the food web. Due to variation in the food web structure, variation in Hg exposure in predators may represent variation in diet rather than Hg availability. We measured Hg in eggs from six seabird species (N = 537) over 47 years. In contrast to expectation, storm-petrels feeding partially on invertebrates had the highest Hg burden while herons feeding on large fish had the lowest Hg burden. A multiple regression showed that Hg correlated with δ34S (R2 = 0.86) rather than trophic level (δ15N of "trophic" amino acids). Sulfate-rich environments (high δ34S) have sulfate-reducing bacteria that produce methylmercury. Variation in Hg within and among seabirds near the top of the food web was associated with variation in δ34S at the base of the food web more so than trophic position within the food web. Hg levels in seabirds only changed over time for those species where δ34S also varied in tandem; after accounting for diet (δ34S), there was no variation in Hg levels. Variation in Hg in seabirds across space and time was associated with the origin of sulfur in the diet.