Identification of sources and bioaccumulation pathways of MeHg in subantarctic penguins: a stable isotopic investigation

Abstract

Seabirds are widely used as bioindicators of mercury (Hg) contamination in marine ecosystems and the investigation of their foraging strategies is of key importance to better understand methylmercury (MeHg) exposure pathways and environmental sources within the different ecosystems. Here we report stable isotopic composition for both Hg mass-dependent (e.g. δ202Hg) and mass-independent (e.g. Δ199Hg) fractionation (proxies of Hg sources and transformations), carbon (δ13C, proxy of foraging habitat) and nitrogen (δ15N, proxy of trophic position) in blood of four species of sympatric penguins breeding at the subantarctic Crozet Islands (Southern Indian Ocean). Penguins have species-specific foraging strategies, from coastal to oceanic waters and from benthic to pelagic dives, and feed on different prey. A progressive increase to heavier Hg isotopic composition (δ202Hg and Δ199Hg, respectively) was observed from benthic (1.45 ± 0.12 and 1.41 ± 0.06‰) to epipelagic (1.93 ± 0.18 and 1.77 ± 0.13‰) penguins, indicating a benthic-pelagic gradient of MeHg sources close to Crozet Islands. The relative variations of MeHg concentration, δ202Hg and Δ199Hg with pelagic penguins feeding in Polar Front circumpolar waters (1.66 ± 0.11 and 1.54 ± 0.06‰) support that different MeHg sources occur at large scales in Southern Ocean deep waters.

Mercury isotopes of key tissues document mercury metabolic processes in seabirds

“…Δ 199 Hg/Δ 201 Hg slopes have been experimentally estimated during MeHg photodemethylation (1.36 ± 0.02) and Hg(II) photoreduction (1.00 ± 0.02) in aquatic compartments (Bergquist and Blum, 2007). The Δ 199 Hg/Δ 201 Hg ratios calculated for the tissues of the three seabird species ( Fig S2) are in line with slopes previously reported in marine organisms (Day et al, 2012;Masbou et al, 2015Perrot et al, 2016;Renedo et al, 2018aRenedo et al, , 2018b. Since Hg MIF is thought to be unaltered by Hg metabolic processes, no changes in Δ 199 Hg values were expected among seabird tissues.…”

“…However, this seabird is known to migrate to warmer subtropical waters during interbreeding periods (Cherel et al, 2016) and they assimilate MeHg from different marine pools both at the breeding site (subantarctic and Antarctic during their chick-rearing period) and during their winter migration far from their colonies (subtropical). A recent study on resident seabirds showed higher (~ 0.6 ‰) blood Δ 199 Hg values in subtropical populations relative to Antarctic populations as a result of higher extent of MeHg photodemethylation at subtropical latitudes (Renedo et al, 2018b). This observation supports that the higher Δ 199 Hg values in feathers could be associated to the assimilation of more photochemically impacted MeHg from subtropical habitats during the inter-breeding period.…”

“…Ocean (Blum et al, 2013), the dilution of surface MeHg with non-photochemically impacted MeHg produced at depth leads to a consistent decrease of Hg isotopes (both MDF and MIF) in fish according to foraging depths. Therefore, changes of feeding behaviour or distinct foraging zones of seabirds among their annual cycle could induce inter-tissue Δ 199 Hg variability, as previously observed in blood and feathers of adult penguins (Renedo et al, 2018b). Since Antarctic prions have a constant pelagic crustacean-based diet over the year (Cherel et al, 2016), the higher Δ 199 Hg values in their feathers do not seem to be related to changing foraging depths during their annual cycle.…”

Using blood and feathers to investigate large-scale Hg contamination in Arctic seabirds: A review

“…Furthermore, Hg isotopes have been widely used in the last decades as a new and powerful perspective to explore the cycle of this element (Blum et al, 2014) and could thus be used to track the trophic and spatial origins of Hg in polar seabirds (Renedo et al, 2018). Hg has seven naturally occurring stable isotopes that undergo both mass-dependent fractionation (MDF) and mass-independent fractionation (MIF), both related to distinct Hg isotopic ratios.…”

Influence of sexual dimorphism on stable isotopes and trace element concentrations in the greater hooked squid Moroteuthopsis ingens from New Zealand waters

“…Furthermore, deep-sea organisms have been reported to contain higher Hg concentrations relative to epipelagic animals (Chouvelon et al, 2012;Choy et al, 2017;Monteiro et al, 1996), which also contributes to an increased exposure of Hg for squids migrating towards deeper waters. Further investigations are needed to validate this hypothesis, including the use of stable isotopes of Hg (e.g., Blum et al, 2013;Renedo et al, 2018).…”