Mercury\nStable Isotopes Reveal the Vertical Distribution\nand Trophic Ecology of Deep-Pelagic Organisms over the North-East\nAtlantic Ocean Continental Slope

Deep-pelagic species are central\nto marine ecosystems and increasingly\nvulnerable to global change and human exploitation. To date, our understanding\nof these communities remains limited mainly due to the difficulty\nof observations, calling for complementary innovative tools to better\ncharacterize their ecology. We used mercury (Δ<sup>199</sup>Hg, δ<sup>202</sup>Hg, Δ<sup>201</sup>Hg, and Δ<sup>200</sup>Hg), carbon (δ<sup>13</sup>C), and nitrogen (δ<sup>15</sup>N) stable isotope compositions to segregate deep-pelagic\nspecies caught on the continental slope of the Bay of Biscay (NE Atlantic)\naccording to their foraging depth and trophic ecology. Decreasing\nfish Δ<sup>199</sup>Hg values with corresponding depth estimates\nfrom the surface to down to 1,800 m confirmed that mercury isotopes\nare able to segregate deep species over a large vertical gradient\naccording to their foraging depth. Results from isotopic compositions\nalso identified different mercury sources, likely reflecting different\ntrophic assemblages over the continental slope, in particular, the\ndemersal influence for some species, compared to purely oceanic species.\nOverall, our results demonstrate how mercury stable isotopes can inform\nthe vertical foraging habitat of little-known species and communities\nfeeding in the deep.