Large-scale patterns of green turtle trophic ecology in the eastern Pacific Ocean

Trophic position and niche width are fundamental components of a species’ ecology, reflecting resource use, and influencing key demographic parameters such as somatic growth, maturation, and survival. The present data file contains results of stable isotope analysis (stable-carbon, δ13C; stable-nitrogen, δ15N values) that was conducted on bulk skin tissue of 718 green sea turtles (Chelonia mydas) distributed among 16 foraging areas in the eastern Pacific from the US to Chile, a range spanning ~10,000 km. These study sites were distributed across a latitudinal range from 33.736 °N to 23.098°S in the Eastern Pacific (Site Code): Long Beach, USA (LB); San Diego Bay, USA (SDB); north Gulf of Ulloa, Mexico (NGU); Magdalena Bay, Mexico (BMA); Los Angeles Bay, Mexico (BLA); Infiernillo Channel, Mexico (CIN); Navachiste Bay, Mexico (NAV); Dulce Gulf, Costa Rica (DUL); Cocos Island, Costa Rica (COC); Gorgona Island, Colombia (GOR); Punta Espinosa, Galapagos Islands, Ecuador (IGP); Bahia Elizabeth, Galapagos Islands, Ecuador (IGE); Caleta Derek, Galapagos Islands, Ecuador (IGD); oceanic waters, Peru (PPE); Pisco Paracas Bay, Peru (PAR); and Mejillones Bay, Chile (MEJ). Substantial variability in bulk tissue δ13C and δ15N values was found within and among sites. These data were also used to calculate the isotope niche space (used as a proxy for ecological niche space) using the Bayesian ellipse approach, and we found that isotope niche space varied among sites, likely influenced by the diversity of prey types and relative input of terrestrial- vs. marine-derived nutrients. In addition to providing additional spatial resolution for δ13C and δ15N isoscapes in the eastern Pacific, especially in coastal habitats, this study and resultant dataset further establish stable isotope analysis as an effective tool to study the trophic ecology of sea turtles across a variety of food webs and habitats. 

Methods
Bulk skin tissue collection from green turtles

Turtle bulk skin tissues collected during field capture efforts thoughout the eastern Pacific Ocean.  Epidermis (hereafter referred to as skin) was collected (ca. 0.10–0.25 g wet mass) from the dorsal neck or shoulder region of each turtle using a sterilized 6-mm biopsy punch or razor blade; the sampling location on the body was consistent at each study site. Samples were preserved in 2-ml cryovials filled with saturated salt solution, dry salt, or 70% ethanol solution and kept cool until transfer to the laboratory where they were stored at –20°C until analysis. 

Sample preparation for bulk tissue stable isotope analysis 

Epidermal skin was separated from underlying dermis tissue when necessary using a razor blade. Skin samples were then rinsed with deionized water, finely diced, and freeze-dried at –50°C for 12 h in a lyophilizer (BenchTop K, VirTis, SP Industries, Gardiner, NY, USA). Lipids were removed from skin samples using a Soxhlet apparatus with a 1:1 solvent mixture of petroleum ether and ethyl ether for at least two 10-h cycles, or an accelerated solvent extractor (Model ASE300, Dionex, Bannockburn, IL, USA) with petroleum ether for three consecutive 5-min cycles of heating to 100°C at 1500 PSI pressurization. Following lipid extraction, the samples were freeze dried at –50°C for 3 h to remove any residual solvent. Sub-samples of prepared homogenized tissue were weighed (0.6–1.0 mg) with a microbalance and packed in tin capsules for mass spectrometric analysis. Primary producers were also freeze-dried prior to subsampling; however, lipid extraction was not performed prior to weighing (1.0–3.0 mg) and placement in tin capsules due to the extreme low lipid content of vegetative prey types (Harwood 2012).

Bulk tissue stable isotope analysis

Bulk tissue stable isotope analyses were conducted at the University of Florida, Gainesville, Florida USA. Elemental concentrations and stable isotope ratios were measured using an on-line C-N analyzer (Carlo Erba NA1500) coupled with an isotope ratio mass spectrometer (Thermo Electron Delta V Advantage), and followed well-established procedures (see Seminoff et al. 2012). All carbon isotopic results are expressed in standard delta notation relative to VPDB. All nitrogen isotopic results are expressed in standard delta notation relative to AIR. Sample stable isotope values relative to the isotope standard are expressed in the following conventional delta (δ) notation in parts per thousand (‰): δ = ([Rsample/Rstandard]-1)*(1000), where Rsample and Rstandard are the corresponding values of heavy to light isotopes (e.g.15N/14N) in the sample and standard, respectively. All analytical runs included samples of a reference material with known δ13C and δ15N values (USGS40 and USGS41 from the USGS) inserted every 6 to 7 samples to calibrate the system and compensate for drift over time. Hundreds of replicate assays of reference materials indicated maximum measurement errors of 0.06‰ and 0.12‰ for carbon and nitrogen, respectively. The elemental concentrations of carbon (acceptable δ13C range = 25–60‰) and nitrogen ratio (acceptable δ15N range = 6–20 ‰) were used as quality assurance to assess stable isotope values before quantitative analyses; samples were excluded from analyses if they did not meet these criteria. The mean % C and mean % N for the retained samples was 41.1 ± 6.0 % and 13.0 ± 2.2 %, respectively (n = 718). The mean C:N ratio (mol/mol) for turtles at each study site was from 2.8 to 4.0.