Stable isotope variation in loggerhead turtles reveals Pacific–Atlantic oceanographic differences

Denitrification and nitrogen-fixation processes in the marine environment have been intensively studied, particularly how these processes affect the nitrogen stable-isotope signature (δ15N) of inorganic nutrients and organisms at the base of the food web. However, the assumption that these δ15N differences at the base of food webs are reflected in higher trophic-level organisms has not been widely investigated. In the present study, we evaluated whether an ocean-basin δ15N variation was evident in oceanic juvenile loggerhead turtles Caretta caretta by analyzing their stable-isotope signatures in the Pacific and Atlantic oceans. Skin samples from oceanic juvenile loggerheads were collected from Peruvian waters in the southeast Pacific and from waters around the Azores Archipelago in the northeast Atlantic and analyzed for δ15N and carbon stable-isotope signature (δ13C). Our results showed that turtles in the 2 ocean regions have mean δ13C signatures of –16.3 and –16.7‰, which reflects the oceanic feeding behavior of these loggerhead populations. However, the δ15N signatures in Pacific loggerheads are significantly higher (mean ± SD = 17.1 ± 0.9‰) than those of Atlantic loggerheads (7.6 ± 0.5‰). This inter-ocean difference in δ15N values was also observed in organisms at the base of the food web in the 2 study areas. The δ15N at the base of the food web, which is determined by the predominant process of the nitrogen cycle in each ocean region, is subsequently transferred to higher trophic levels. Stable isotope signatures in high trophic-level organisms, such as oceanic-stage sea turtles, can reveal differences in oceanographic processes.