Geographical variation in carbon stable isotope signatures of marine predators: a tool to investigate their foraging areas in the Southern Ocean

Stable carbon isotope signatures (delta C-13) are increasingly being used to determine the foraging habitats of consumers in the Southern Ocean. An underlying assumption is that a latitudinal gradient in VC values at the base of the food chain should also be reflected in organisms at higher trophic levels. Our main objective was to test that assumption by using penguin chicks (7 taxa) as predator models, because the feeding habits of provisioning adult penguins are well known during the chick-rearing period. As expected, a strong negative correlation was found between latitude and delta C-13 values of whole blood of penguin chicks. delta C-13 values ranged from -24.8 +/- 0.5%. in Adelie penguins living in Antarctica (67 degrees S), to -19.5 +/- 0.3 parts per thousand in northern rockhopper penguins living in the subtropics (38 S). Unlike VC values, stable nitrogen isotope signatures (delta N-15) were not related to latitude but instead were strongly affected by penguins' diet, with fish-eaters having higher delta N-15 values than crustacean eaters. 613C values also reflect more local spatial foraging segregation of penguins at a subantarctic archipelago (Kerguelen Islands), where they varied from -22.3 +/- 0.2%. for king penguins feeding in oceanic waters to -15.9 +/- 0.3%. for gentoo penguins foraging in an enclosed bay. Blood delta C-13 values moreover allowed differentiation of 2 sub-populations of gentoo penguins foraging in 2 different environments at Kerguelen. The method has 2 limitations related to the lack of longitudinal variations and to overlapping delta C-13 values due to different isotopic gradients (latitudinal and inshore/offshore) in the marine environment. The study nevertheless shows that the delta C-13 values of marine predators are efficient indicators of the foraging habitats at various spatial scales, both in terms of latitude in oceanic waters, and in terms of inshore/offshore gradients in waters surrounding subantarctic islands.