The Gulf Stream frontal system: A key oceanographic feature in the habitat selection of the leatherback turtle?
Résumé
Although some associations between the leatherback turtle Dermochelys coriacea and the Gulf Stream currenthave been previously suggested, no study has to date demonstrated strong affinities between leatherbackmovements and this particular frontal system using thorough oceanographic data in both the horizontal andvertical dimensions. The importance of the Gulf Stream frontal system in the selection of high residence time(HRT) areas by the North Atlantic leatherback turtle is assessed here for the first time using state-of-the-artocean reanalysis products. Ten adult females from the Eastern French Guianese rookery were satellite trackedduring post-nesting migration to relate (1) their horizontal movements to physical gradients (Sea SurfaceTemperature (SST), Sea Surface Height (SSH) and filaments) and biological variables (micronekton andchlorophyll a), and (2) their diving behaviour to vertical structures within the water column (mixed layer,thermocline, halocline and nutricline). All the turtles migrated northward towards the Gulf Stream north wall.Although their HRT areas were geographically remote (spread between 80–30 °W and 28–45 °N), all the turtlestargeted similar habitats in terms of physical structures, i.e. strong gradients of SST, SSH and a deep mixedlayer. This close association with the Gulf Stream frontal system highlights the first substantial synchronizationever observed in this species, as the HRTs were observed in close match with the autumn phytoplankton bloom.Turtles remained within the enriched mixed layer at depths of 38.5 ± 7.9 m when diving in HRT areas, likely tohave an easier access to their prey and maximize therefore the energy gain. These depths were shallow incomparison to those attained within the thermocline (82.4 ± 5.6 m) while crossing the nutrient-poor subtropicalgyre, probably to reach cooler temperatures and save energy during the transit. In a context of climate change,anticipating the evolution of such frontal structure under the influence of global warming is crucial to ensure theconservation of this vulnerable species.