Mercury biotransport via migratory birds and food web biomagnification influenced by geospatial patterns
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Intrinsic physico-chemical characteristics, trophic transfer and preexisting mercury (Hg) in aquatic ecosystems are ultimate drivers influencing accumulation and mercury concentrations ([Hg]) in organisms in a food web. Moreover, migration between aquatic systems is believed to greatly influence intraspecific [Hg] in fish-eating birds due to differential Hg accumulation during their annual migration cycle. Birds can therefore biologically transport Hg across long distances. Here, I investigated the geospatial variation of Hg pertaining to (1) Hg biomagnification in aquatic food webs, and (2) biologically-mediated transport of Hg via fish-eating birds through migration. First, I examined total Hg (THg) and methyl Hg (MeHg) biomagnification rates in aquatic food webs by compiling data from 205 sites worldwide. There was an effect of latitude on THg and MeHg food web biomagnification rates. Biomagnification rates in freshwater sites increased with dissolved organic carbon and decreased with total phosphorus and atmospheric Hg deposition. These results suggest that Hg biomagnification rates are highest in cold and low-productivity systems. Second, I examined the importance of migration on [Hg] in two migratory fish-eating bird species, the Double-crested Cormorant (Phalacrocorax auritus) and the Caspian Tern (Hydroprogne caspia) breeding in Lake Ontario. Temporal trends of [Hg] and stable isotopes of carbon (δ13C) and nitrogen (δ15N) during birds’ annual migration cycle was determined by sampling breast feathers and blood in recaptured individuals. I found Hg carryover in bird tissues from winter to summer, which is indicative of biologically-mediated transport of Hg. Slow changes in [Hg] over time were found for individuals with high winter exposure, suggesting a slow depuration rate and a fast uptake rate. I characterized (1) overwintering habitats by combining stable isotopes of sulfur (δ34S), δ13C, and δ15N, and (2) overwintering areas using stable isotopes of hydrogen (δ2H) in winter feathers. High [Hg] were found in birds overwintering in habitats characterized by food webs supported by benthic and C4 primary production. Higher [Hg] were found in birds overwintering in southernmost locations. Winter feather [Hg] were related to [Hg] in fish based on bird spatial assignments. Overall, these findings show that geospatial patterns and physico-chemistry are critically important to explain Hg biomagnification in aquatic food webs, and bioaccumulation and biotransport in fish-eating birds.