ELECTRONIC SUPPLEMENT
Temporal consistency and individual specialization in resource use
by green turtles in successive life stages
Oecologia
Hannah B. Vander Zanden, Karen A. Bjorndal, Alan B. Bolten
Archie Carr Center for Sea Turtle Research and Department of Biology, University of Florida, Gainesville, FL USA
Corresponding author: Hannah Vander Zanden
Oceanic vs. neritic samples
The eight juvenile green turtles classified as oceanic juveniles in our study were selected from a larger group of 26 not-previously-captured green turtles that were sampled in Union Creek, Great Inagua, Bahamas in October and November 2009 (Fig. S1). These 26 turtles were consideredto be recent recruits because they lacked flipper tags and were significantly smallerthan the resident neritic juveniles (mean CCL: 37.3 vs. 51.0 cm; p < 0.001). The isotopic patterns in therecent recruits could be separated into four categories.
1) Four juveniles hadδ13C values in all scute layers < -12‰, which are values indicative of the oceanic habitat (Reich et al. 2007). These four turtles did not appear to have been in the neritic foraging area long enough to have incorporated a neritic signal into their scute tissue and were also the smallest of the 26 turtles with CCL measurements < 33 cm. These four turtles that had likely just recruited to the coastal area were included in the oceanic juvenile life stage (Fig.S2a and b).
2) Three juveniles hadδ13C values in all scute layers > -12‰, which are values representative of the neritic habitat (Reich et al. 2007). These scute records did not extend over 1.2 years and did not appear to retain any record of the oceanic life stage (Fig.S2c and d). (See below for estimate of the time period represented in each 50-m layer.) These three turtles were not included in the oceanic juvenile life stage.
3) Fourteen juveniles had scute records that exhibited a shift in the isotope values that was assumed to represent the ontogenetic shift between the oceanic and neritic life stages (Fig. S2e and f)withδ13C values that were both > -12‰ and < -12‰. Of these 14 turtles, four individuals had four or more layers representative of the oceanic life stage that were accrued prior to the isotopic shift, and only the oceanic layers from these individuals were included in the oceanic juvenile group.
4) Five additional juveniles exhibited a change in stable isotope values over time, but showed an incomplete ontogenetic shift (Fig. S2g and h). The scute samples did not contain multiple consecutive layers with stable isotope values representative of both habitats, and did not contain a sufficient number of layers representing the oceanic habitat to be included in the oceanic life stage sample.
Therefore, a total of eight juvenile turtles contained sufficient records to assess temporal consistency and degree of individual specialization in the oceanic life stage. This selection was based on the assumption that higher δ13C values are consistent with foraging in neritic areas. It is possible, though less likely, that higher δ13C values could occur in individuals feeding in the oceanic habitat as well. We believe that the exclusion of δ13C values > -12‰ in the oceanic group is justified for three reasons. 1) A shift from low to high δ13C values was a pattern previously observed in eight green turtles in the same location and was interpreted as an ontogenetic habitat shift from the oceanic to neritic environment (Reich et al. 2007). The δ13C value of -12‰ corresponds to the approximate midpoint between the isotopic values in these two habitats. 2) This isotopic shift occurred in 14 of 26 recent recruits in this study with 5 additional turtles displaying a portion of this isotopic shift. This frequently occurring pattern in the isotope values of the turtles in our studycoincides with the previously documented ontogenetic habitat shift. 3) Finally, the juvenile turtles included in the oceanic group had five or fewer 50-m layer layers with δ13C values > -12‰, which would indicate they had been in the neritic habitat for less than one year. Because these turtles did not have flipper tags, itsuggests they had recruited to the area since the previous sampling and tagging effort that occurred approximately 15 months prior in July 2008.
Scute growth rate
A juvenile turtle in this study was originally captured in Union Creek, Inagua, in July 2008 at a size of 39 cm SCL (straight carapace length) and recaptured in November 2009 at a size of 47.4 cm, resulting in a mean growth rate of 6.3 cm yr-1 over that time period. If the turtle was assumed to have recruited at a size of 30 cm (using the size of the smallest turtles observed in the study site) and grew at the same rate prior to its capture, it would have been in a neritic zone for approximately 2.8 years. The 700-m scute sample from this turtle captures a complete oceanic-to-neritic shift (Fig. S3a and b), and assuming all the sampled scute tissue was deposited since the shift to the neritic zone, each 50-m layer represents approximately 72 days.
The adult scute turnover time is slower than that in juveniles because of a difference in body mass, and isotopic turnover rates have been shown to scale with body mass to the -0.25 power (Carleton and Martínez del Rio 2005; Bauchinger and McWilliams 2009). At the midpoint of the size range in the neritic zone (SCL =38.7 cm), the body mass of the juvenile turtle used in this example would have been approximately 7.2 kg using a previously published conversion (Bjorndal and Bolten 1988). The average body mass of an adult female nesting at Tortuguero is 128 kg (Bjorndal and Carr 1989). Therefore, by scaling the scute growth rate in juveniles to the appropriate adult body size, the estimated time period represented in each 50-m layer of adult scute is nearly twice that of the juveniles, or 148 days.
The time period represented in 50-m scute layer was determined through growth rates, as there are no published measures of isotopic incorporation rates for green turtle scute. The rate of carbon and nitrogen isotopic incorporation into scute has been measured in juvenile loggerheads, (Reich et al. 2008) and can provide a rough comparison to our calculations. However, we cannot make direct comparisons due to the following differences. First, the juvenile loggerheads were much smaller (9.0-13.1 cm SCL) than the juvenile green turtles in our study. We would expect larger juveniles and adults to have lower isotopic turnover rates than small juveniles. Second, the loggerhead scute was analyzed by using the entire depth of the sample, rather than using 50-m layers as in our study. Reich et al. (2008) reported the average residence time was 51 and 16 d for δ13C and δ15N, respectively. This translates to a turnover rate of 213 and 43 d for δ13C and δ15N, respectively, using 4 half-lives (in whichone half-life is calculated using ln(2)/kdtwherekdt is the fractional rate of incorporation). Despite the caveats of comparing the rates, the estimated time period that is represented in each 50-m layer of juvenile green turtle scute(72 d)falls within the range of isotopic turnover times of small juvenile loggerhead scute.
References
Bauchinger U, McWilliams S (2009) Carbon turnover in tissues of a passerine bird: allometry, isotopic clocks, and phenotypic flexibility in organ size. PhysiolBiochemZool 85:541–548.doi:10.1086/605548
Bjorndal KA, Bolten AB (1988) Growth rates of immature green turtles, Cheloniamydas, on feeding grounds in the southern Bahamas. Copeia 1988:555–564
Bjorndal KA, Carr A (1989) Variation in clutch size and egg size in the green turtle nesting population at Tortuguero, Costa Rica. Herpetologica 45:181–189
Carleton SA, Martínez del Rio C (2005) The effect of cold-induced increased metabolic rate on the rate of 13C and 15N incorporation in house sparrows (Passer domesticus).Oecologia 144:226–232.doi:10.1007/s00442-005-0066-8
Reich KJ, Bjorndal KA, Bolten AB (2007) The “lost years” of green turtles: using stable isotopes to study cryptic lifestages. BiolLett 3:712–714 doi:10.1098/rsbl.200.0394
Reich KJ, Bjorndal KA, Martínez del Rio C (2008) Effects of growth and tissue type on the kinetics of 13C and 15N incorporation in a rapidly growing ectotherm. Oecologia 155:651–663.doi:10.1007/s00442-007-0949-y
Table S1: Within-individual contribution (WIC) and total niche width (TNW) approximated through the ANOVA framework for carbon and nitrogen stable isotope values among three life stages
Life stage / 13C WIC / 13C TNW / 15N WIC / 15N TNWOceanic juveniles / 0.95 / 10.68 / 0.34 / 6.09
Neritic juveniles / 0.60 / 4.89 / 0.71 / 10.89
Adults / 0.32 / 13.57 / 0.17 / 28.54
Figure S1 Map of study locations. Scute samples were collected from 8 oceanic and 14 neritic juvenile green turtles (Cheloniamydas) at Union Creek, Great Inagua, Bahamas. Samples were collected from 21 nesting females at Tortuguero, Costa Rica
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Figure S2 Carbon (left column) and nitrogen (right column) stable isotope values in successive 50 m subsections of scute in 26 juvenile green turtles from Inagua, Bahamas, that were untagged when sampled. The individuals were categorized by their isotopic patterns and classified as (a, b) recent recruits to the habitat (n = 4), (c, d) residents (n = 3), (e, f) recruits that retain a history of the ontogenetic shift from the oceanic habitat (n = 14), or (g, h) recruits that show an incomplete ontogenetic shift (n = 5). Increasing distance from the lower surface of the scute sample corresponds to older time periods in the turtle’s foraging history. Individuals are represented by unique symbols
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Figure S3Carbon (a) and nitrogen (b) stable isotope values in successive 50 m subsections of scute in a single neritic juvenile green turtle from Inagua, Bahamas. The scute sample from this turtle captures a complete oceanic-to-neritic shift and was used to determine the time period represented in each 50 m scute subsection
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