BGBiogeosciencesBGBiogeosciences1726-4189Copernicus GmbHGöttingen, Germany10.5194/bg-12-7081-2015Stable isotopes in barnacles as a tool to understand green
sea turtle (Chelonia mydas) regional movement patternsDetjenM.md2986@caa.columbia.eduSterlingE.GómezA.Department of Ecology, Evolution & Environmental Biology, Columbia
University, 1200 Amsterdam Avenue, New York, NY 10027, USACenter for Biodiversity and Conservation, American Museum of Natural
History, 200 Central Park West, New York, NY 10024, USAICF International, 1725 I St. NW, Washington, DC, 20006, USAM. Detjen (md2986@caa.columbia.edu)8December201512237081708614January201523March201518October201514November2015This work is licensed under a Creative Commons Attribution 3.0 Unported License. To view a copy of this license, visit http://creativecommons.org/licenses/by/3.0/This article is available from https://bg.copernicus.org/articles/12/7081/2015/bg-12-7081-2015.htmlThe full text article is available as a PDF file from https://bg.copernicus.org/articles/12/7081/2015/bg-12-7081-2015.pdf
Sea turtles are migratory animals that travel long distances between their
feeding and breeding grounds. Traditional methods for researching sea turtle
migratory behavior have important disadvantages, and the development of
alternatives would enhance our ability to monitor and manage these globally
endangered species. Here we report on the isotope signatures in green
sea-turtle (Chelonia mydas) barnacles (Platylepas sp.)
and discuss their potential relevance as tools with which to study green sea
turtle migration and habitat use patterns. We analyzed oxygen
(δ18O) and carbon (δ13C) isotope ratios in barnacle
calcite layers from specimens collected from green turtles captured at the
Palmyra Atoll National Wildlife Refuge (PANWR) in the central Pacific. Carbon
isotopes were not informative in this study. However, the oxygen isotope
results suggest likely regional movement patterns when mapped onto a
predictive oxygen isotope map of the Pacific. Barnacle proxies could
therefore complement other methods in understanding regional movement
patterns, informing more effective conservation policy that takes into
account connectivity between populations.
Introduction
Long-distance migratory behavior between breeding and feeding grounds, a key
component of sea turtle ecology, creates important research and conservation
challenges (Godley et al., 2010). Understanding migration and habitat use
patterns is a critical step in the design of comprehensive conservation and
management strategies aimed at protecting all of a species' range, including
the corridors connecting distant habitats. For many sea turtle populations
we lack detailed spatiotemporal knowledge about migration patterns, as well
as fine-scale understanding of habitat use. This dearth of information may
hinder conservation efforts, especially in scarcely studied areas such as
the central Pacific (Wallace et al., 2010).
Previous studies on sea turtle movement patterns have been based on
mark and recapture, satellite telemetry, or genetic analysis (Godley et al.,
2010). Although these methods have provided key insights, they also have
important shortcomings. Mark and recapture can have very low return rates
(Oosthuizen et al., 2010). Satellite telemetry is a very effective method for
tracking turtles across long distances but can be prohibitively expensive,
and loss and malfunction of transmitters is common (Hays et al., 2007;
Hebblewhite and Haydon, 2010). Genetic studies can be a very effective way of
delineating population structure and natal origin, but are uninformative
about movements after the sea turtles hatch (Bowen and Karl, 2007).
Therefore, additional methods are needed to help us map patterns of movement
and habitat use at scales useful for conservation planning (Godley et al.,
2010).
Because of their intimate connections, species that are associates of
particular hosts have been used as proxies for the study of host ecology,
demography, and evolutionary history (Nieberding and Olivieri, 2007). Recent
research has shown that studying associate species such as parasites and
commensals can be a cost-effective alternative to ecological research on the
host themselves (Byers et al., 2011; Hechinger et al., 2007). Several
barnacle species are commonly found on sea turtles, attached to the skin and
shell. Barnacles are found in the majority of green turtles observed in a
long-term study of marine turtles at Palmyra Atoll National Wildlife Refuge
(A. Gómez, personal communication, 2012), and
they have been reported widely from sea turtle populations from across the
world (Casale et al., 2004; Frick et al., 2010; Rawson et al., 2003;
Schwartz, 1960; Torres-Pratts et al., 2009; Zardus and Balazs, 2007). As
obligate commensals, these barnacles form close, presumably long-lasting
associations with their hosts, and may thus provide useful information about
turtle ecology.
Previous studies have shown that isotopes in barnacle calcite can be used to
reconstruct migratory patterns and habitat use in California gray whales
(Killingley, 1980) and loggerhead turtles (Killingley and Lutcavage, 1983).
Isotope ratios in calcite layers can be used to approximate the water
temperature throughout the life of individual barnacles because warmer waters
have reduced oxygen ratios (Killingley and Lutcavage, 1983), where the oxygen
isotopes in the barnacle calcite fractionate or change in relative proportion
during calcite formation depending on the oxygen ratios in the surrounding
water (Kendall and Caldwell, 1998). Therefore, oxygen isotope ratios obtained
from barnacles can be informative about turtle movements at large scales, as
long as those movements occurred along water temperature gradients
(Killingley and Lutcavage, 1983). These movements can be traced by comparing
barnacle oxygen isotope ratios to mapped predictions for these values.
Temporal reconstruction could potentially also be added as our understanding
of the pace at which successive barnacle calcite layers are laid down
improves. Carbon isotope ratios can be expected to vary as microhabitats
differ in the concentration of dissolved carbon, and can therefore provide
information about habitat occupancy across sites, with lagoons and the
pelagic zone assumed to have low and high carbon conditions respectively
(Killingley and Lutcavage, 1983). Here we report on oxygen (δ18O)
and carbon (δ13C) isotopes in the barnacle Platylepas sp.,
an epibiont of turtles, collected from green sea turtles (Chelonia mydas) at Palmyra Atoll National Wildlife Refuge in the central Pacific and
discuss the potential of this method as a tool with which to study sea turtle
movements.
Materials and methods
The barnacle specimens used in the experiment were collected at Palmyra Atoll
National Wildlife Refuge (PANWR; 05∘52′ N, 162∘05′ W),
central Pacific Ocean. The atoll has a wide shallow reef, extensive reef
terraces at both the eastern and western ends, and three lagoons (Collen et
al., 2009). The islets and 12 nautical miles of the surrounding ocean have
been designated a marine protected area by the US Fish and Wildlife Service
since 2001. In 2005, the Center for Biodiversity and Conservation of the
American Museum of Natural History initiated a research and conservation
program for sea turtles at PANWR. The program includes research into the
turtles' distribution and abundance, connectivity, feeding ecology, health,
and threats (McFadden et al., 2014; Sterling et al., 2013). The sea turtle
population at this site has been studied using mark and recapture, satellite
telemetry, and genetic analysis (Sterling et al., 2013).
Platylepas sp. barnacles were collected from adult green sea turtles
caught in PANWR during the summer of 2011. These barnacles were found
embedded in the turtles' soft tissue (A. Gómez, personal observation,
2011). Barnacles were removed from the turtles'
skin and stored in vials with 90 % ethanol until analysis. We analyzed a
total of 12 barnacles from four turtles. In order to assess the consistency
of recorded isotope ratios of different barnacles from a given turtle we
sampled three barnacles per turtle. The barnacles were dissected and milled
along their axis of growth using a Merchantek MicroMill (Electro Scientific
Industries, Inc., Portland, United States) to take calcite samples. The mill
was programmed to make passes on the outer facing surface of the paries
perpendicular to the axis of growth in distances 0.3–0.4 mm apart. For each
sample, a record was kept of the distance along the growth axis from
barnacles' base to where each pass had been made. Samples were taken from the
outermost part of the paries to exclude any calcite deposits that might have
been the result of aging and thickening of the individual plates. It should
be noted that nothing is known about growth rates in this species of
barnacle. The calcite samples were sent to the Keck Paleoenvironmental and
Environmental Stable Isotope Laboratory at the University of Kansas, where
they were analyzed for oxygen (δ18O) and carbon (δ13C)
stable isotope ratios. A Kiel Carbonate Device III and a Finnigan MAT253
isotope ratio mass spectrometer (Finnigan MAT, Bremen, Germany) were used to
perform the laboratory analyses.
Oxygen isotope ratios in barnacle calcite can be expected to vary
predictively as a function of the water's oxygen isotope ratios and
temperature and can be solved for using a conversion formula (Epstein et al.,
1953) with a required modification for barnacle calcite (Killingley and
Newman, 1982). We reversed the formula by rearranging variables for the
water's oxygen isotope ratio, which accounts for variations in salinity, and
temperature to create a map of predicted barnacle oxygen isotope ratios. We
used annual average sea surface temperature data from NOAA's World Ocean
Database (NOAA, 2005) for the temperature variable in the equation, and
published water oxygen isotope figures from 2006 (LeGrande and Schmidt, 2006)
as inputs in the equation. The resulting map allowed us to put the oxygen
isotope results from the barnacles into geographic context. We used this map
to create an isoscape, thereby defining the largest possible area from which
the isotope values measured from the calcite could have accumulated across
the life of the barnacles sampled. A detailed methodology is included as an
electronic supplement.
Results
Because some of the calcite samples were not sufficiently large to be
analyzed with precision in the mass spectrometer, we obtained a complete set
of results for barnacles from two of the four sea turtles sampled and only
partial results from one other. We included nine barnacles from three turtles
in our analysis, as results from the fourth were too incomplete. The selected
barnacles on the respective turtles had the following sizes measured from the
base to the aperture: (i) 1.6, 1.3 and 1.6 mm on GD42; (ii) 1.6, 2.2, and
2.5 mm on GI41; and (iii) 2.0, 2.1 and 1.6 mm on GI43. A summary of the
stable isotope ratios are reported in Table 1. The youngest part of the
barnacle is that closest to the basal margin or bottom, as the barnacle grows
outward. These isotope ratios represent the values across the growth axis of
the barnacle shell going from the youngest to the oldest part of the
barnacle. The carbon and oxygen isotope ratios are reported versus the Vienna
Pee Dee Belemnite (VPDB) scale (Coplen, 1995), which is used as a benchmark
value. The maps predicting calcite oxygen isotope ratios in the central
Pacific showed uniform ratios along the Equator and steep gradients towards
northern and southern latitudes.
Distance from paries' base, oxygen isotope ratio and carbon isotope
ratio in Platylepas sp. barnacles collected from three green sea
turtles (GD42, GI41, and GI43) in Palmyra Atoll National Wildlife Refuge.
Rows show the average of three barnacles per turtle sampled. Distance is
given in millimeters and isotope ratios are reported versus the VPDB scale.
Distance from base δ18O concentration δ13C concentration GD42GI41GI43GD42GI41GI43GD42GI41GI430.3500.3500.350-1.359-1.343-1.3100.729-0.451-0.2990.7190.7270.743-1.283-1.220-1.4310.798-0.398-0.6191.0521.1351.107-1.414-1.168-1.2000.624-0.124-0.9141.4031.5591.451-1.500-1.097-1.1601.0900.009-0.4811.5501.9371.725-1.503-1.004-1.4761.430-0.0020.096n/a2.3542.067n/a-1.321-1.379n/a0.227-0.811
Oxygen isoscape (shaded in gray) showing the area in which
we would expect our sea turtles to have resided throughout the life of the
barnacles tested. This isoscape was calculated using an oxygen isotope ratio
of -0.951 δ18Oc. PANWR is located within this area and
depicted by the black triangle. Solid lines are contours of predicted oxygen
isotope ratios in barnacle calcite (δ18Oc).
Oxygen isotope ratios in our calcite samples did not show major fluctuations
throughout the life of the barnacle, while the carbon isotope ratios of the
barnacles spanned 3 orders of magnitude. The highest measured oxygen
isotope ratio in the collected barnacles was -0.951 δ18O. We used
this value as a contour to create an envelope in which we would expect our
sea turtles to have stayed throughout the lifetime of the barnacle (Fig. 1).
The resulting isoscape included PANWR. We also created a more conservative
isoscape that corrected for the fact that the original isoscape maps might be
overestimating the isotope ratios. The first step was to identify the
expected oxygen isotope ratio at PANWR on the map, as the isotopes in the
barnacles' youngest layer would be expected to coincide with it. The map
predicted a calcite oxygen isotope ratio of -1.075 δ18O, while
the average youngest layers of the barnacles collected were -1.337
δ18O, giving a difference of 0.262 δ18O. Adding this
difference to the original isoscape value of -0.951 δ18O gave a
corrected calcite oxygen ratio of -0.688 δ18O. This ratio was
then used to produce a larger standardized isoscape delineating the sea
turtles' movements during the barnacles' lifetime (Fig. 2).
Discussion
Our study found that oxygen isotopes in barnacles' calcite could be used to
broadly delineate the area in which the sampled sea turtles moved during the
life of the barnacles, allowing us to exclude visitation of major breeding
grounds in the Pacific. Carbon isotopes were not informative in this study,
and assessing their utility as proxies with which to explore sea turtle
habitat use requires further study. Oxygen isotope values observed in the
barnacles in this study indicated that the calcite ratios conform to sea
temperatures of 28 and 30 ∘C. Assuming that average temperatures
above 28 ∘C are found in the warmest waters of the central Pacific
that are in proximity to the Equator, our data suggest that turtles did
not venture beyond these waters during the lifespan of the barnacles
collected. This is consistent with observations from the field, which suggest
that turtles spend extended periods of time in PANWR (Sterling et al., 2013).
Adjusted oxygen isoscape (shaded in gray). PANWR is depicted
by the black triangle. Solid lines are contours of predicted oxygen isotope
ratios in barnacle calcite (δ18Oc).
To obtain a more concrete picture of the sea turtles' movements, we used the
predicted calcite oxygen isotope map estimating the area within which the sea
turtles may have moved. The contour delineating the isoscape of possible
movements was large (Figs. 1 and 2) as water temperatures in the central
Pacific are relatively uniform. However, some major known green turtle
grounds that are within in the potential migratory range of green turtles
from PANWR were not within this isoscape (STC, 2012). These include Ogasawara
Island (Japan), NW Australia, and Hawaii, which also remain outside of the
boundary when using the more conservative adjusted oxygen isoscape.
Importantly, recent research shows that the natal origin of sea turtles in
PANWR can almost exclusively be found to the west and south of the central
Pacific (Naro-Maciel et al., 2014). Therefore, the boundaries we delineate in
this study (1) include PANWR, (2) are consistent with ecological
observation, and (3) are consistent with new genetic evidence about the
population structure of green sea turtles at PANWR.
Because we cannot exclude the possibility that our isoscapes simply reflect
residency at Palmyra, we are unable to quantify the method's utility as an
indicator of large-scale movements. However, our data suggest that it can be
used to delineate envelopes of likely residency across the Pacific basin.
Therefore we suggest that this method has the potential to provide valuable
data to inform comprehensive management strategies, by helping identify
specific ecological and political areas within or outside a given
population's range.
A wide range in the barnacles' carbon isotopes may indicate that turtles made
use of a variety of microhabitats around the atoll, possibly moving between
areas like the lagoon and the pelagic zone, which are assumed to have low and
high carbon conditions respectively (Killingley and Lutcavage, 1983). An
alternative explanation is that the turtles are frequenting ecologically
heterogeneous areas beyond PANWR. However, any conclusions drawn from these
results need to be viewed conservatively, as a heterogeneous environment does
not necessarily explain the lack of consistency in our data, which have
marked dissimilarities in carbon isotope ratios between barnacles on the same
turtle. There could be differences in uptake or expression of carbon isotopes
in each barnacle possibly limiting the use of the carbon isotope data in this
study system. Previous studies used a larger barnacle species than the ones
found on the green turtles at PANWR (Killingley and Lutcavage, 1983).
Platylepas sp. specimens that we collected had sizes ranging between
1.3 and 2.5 mm, which is a magnitude smaller than the Chelonibia testudinaria recovered from loggerhead turtles in previous studies
(Killingley, 1980; Killingley and Lutcavage, 1983). This resulted in fewer
data points and limited statistical analysis of the results.
In summary, this limited data set suggests that inferences about green sea
turtle spatial ecology obtained from isotope analysis are broadly consistent
with field observations and genetic analyses. Isotope analysis may provide
low-resolution information about sea turtle connectivity, potentially
defining areas of interest for research and management. Therefore, we
suggest that this method can only complement but not replace other tools to
investigate turtle migration and habitat use patterns. One advantage of the
method is its low cost. The total cost of analyzing three barnacles on one
sea turtle was below USD 170 (USD 56 per barnacle in 2011). This makes using
barnacle proxies an option that could be explored further in the study of
spatial ecology and could be improved in future applications.
Future research can add critical information with which to improve this
method. We lack basic information about the natural history of many turtle
epibionts. Because of the dearth of data on baseline growth rates for
Platylepas sp., the time span between successive calcite layers is unknown, and
therefore the system cannot be attached to an absolute temporal scale. We
also lack benchmarks for isotope ratios in barnacles. Therefore, it is
difficult to draw conclusions about the significance of fluctuations that we
observed, especially for the variation in carbon isotope ratios. The utility
of barnacles as proxies of sea turtle movement at study sites such as PANWR
might not be fully realized until these key knowledge gaps are addressed.
The Supplement related to this article is available online at doi:10.5194/bg-12-7081-2015-supplement.
Acknowledgements
We are very grateful to L. Ivany for advice on sample preparation and the use
of milling equipment at Syracuse University. E. Lazo-Wasem provided guidance
on barnacle dissection and taxonomy. AMNH field staff and the staff at PANWR
provided invaluable logistical support. We would like to thank J. Drew for
his revisions and input. Two anonymous reviewers provided comments that
improved this manuscript. This material is based upon work supported by
awards NA07NMF4540185 and NA10NMF4540299 from the National Oceanic and
Atmospheric Administration's National Marine Fisheries Service, US Department
of Commerce, and a Lerner-Gray Marine Research grant from the American Museum
of Natural History. The statements, findings, conclusions, and
recommendations are those of the author(s) and do not necessarily reflect the
views of the National Oceanic and Atmospheric Administration or the US
Department of Commerce. We acknowledge the Palmyra Atoll National Wildlife
Refuge, US Fish and Wildlife Service, Department of the Interior. This is
Palmyra Atoll Research Consortium publication number PARC-0119.
Edited by: S. W. A. Naqvi
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