Diet and foraging niche flexibility in green and hawksbill turtles

We used stable isotopes to investigate isotopic niche size, overlap, and diet composition in green (black and yellow morphotype Chelonia mydas ; 50.0 to 95.0 cm curved carapace length, CCL) and hawksbill turtles ( Eretmochelys imbricata ; 38.5 to 83.0 cm CCL) in a recently described foraging habitat in North Pacific Costa Rica. We measured whole blood stable carbon (δ 13 C) and nitrogen (δ 15 N) ratios in black ( n = 39; mean ± SD, − 16.54 ± 0.66‰ and 14.39 ± 0.77‰), yellow ( n = 13; − 15.74 ± 0.65‰ and 12.37 ± 0.55‰) and hawksbill turtles ( n = 13; − 16.23 ± 1.34‰ and 12.63 ± 0.32‰) and skin δ 13 C and δ 15 N values in black ( n = 36; − 15.32 ± 0.79‰ and 15.16 ± 0.72‰), yellow ( n = 12; − 15.38 ± 0.91‰ and 13.78 ± 0.75‰) and hawksbill turtles ( n = 10; − 14.33 ± 1.49‰ and 13.77 ± 0.29‰). Isotopic niche space revealed distinctly higher δ 15 N area in black turtles and significant overlap between yellow and hawksbill turtles, and a recent shift in diet in yellow turtles from omnivory to herbivory. In black turtles, isotopic niche suggests individual specialization during the non-upwelling season and generalization in diet during the upwelling season. Mixing model results suggest that black turtles forage at multiple trophic levels (fish: 34.8 ± 10.1% of diet and macroalgae: 51.8 ± 12.8% of diet), while yellow and hawksbill turtles primarily forage on macroalgae (85.0 ± 6.6% in yellow turtles and 85.1 ± 5.9% in hawksbill turtles). These results add to a growing understanding that diet in sea turtles is influenced by diet items present in the environment and suggest that black turtles are potential tertiary consumers.