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Winter diets of immature green turtles (Chelonia mydas) on a northern feeding ground:
integrating stomach contents and stable isotope analyses
Natalie C. Williams1,2*, Karen A. Bjorndal2,3, Margaret M. Lamont2,4, Raymond R. Carthy1,2,4
1
Department of Wildlife Ecology and Conservation, University of Florida, Gainesville FL 32611,
USA
2
Archie Carr Center for Sea Turtle Research University of Florida, Gainesville FL 32611, USA
3
4
Department of Biology, University of Florida, Gainesville FL 32611, USA
Florida Cooperative Fish and Wildlife Research Unit, U.S. Geological Survey, Gainesville, FL
32611, USA
Running head: immature green turtle diets
*
Email: natalie.williams@ufl.edu
Phone: 352-214-5842
This draft manuscript is distributed solely for purposes of scientific peer review. Its content is deliberative and predecisional, so it
must not be disclosed or released by reviewers. Because the manuscript has not yet been approved for publication by the U.S.
Geological Survey (USGS), it does not represent any official USGS finding or policy.
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Abstract
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The foraging ecology and diet of the green turtle, Chelonia mydas, remain understudied,
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particularly in peripheral areas of its distribution. We assessed the diet of an aggregation of
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juvenile green turtles at the northern edge of its range during winter months using two
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approaches. Stomach content analyses provide a single time sample, and stable isotope analyses
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integrate diet over a several-month period. We evaluated diet consistency by comparing the
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results of these two approaches. We examined stomach contents from 43 green turtles that died
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during cold stunning events in St. Joseph Bay, Florida, in 2008 and 2011. Stomach contents were
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evaluated for volume, dry mass, percent frequency of occurrence, and index of relative
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importance of individual diet items. Juvenile green turtles were omnivorous, feeding primarily
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on seagrass and tunicates. Diet characterizations from stomach contents differed from those
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based on stable isotope analyses, indicating the turtles are not feeding consistently during winter
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months. Evaluation of diets during warm months is needed.
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Keywords: Green turtle; Chelonia mydas; cold stun; St. Joseph Bay, Florida; carbon and
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nitrogen stable isotopes
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Introduction
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The life history of green turtles (Chelonia mydas) involves a series of ontogenetic habitat and
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resource shifts (Bolten, 2003). After completing an initial oceanic life stage, small juvenile green
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turtles (20 to 35 cm curved carapace length) in the western Atlantic become residents of neritic
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habitats, where they feed primarily on seagrasses and/or macroalgae, but may also consume
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animal matter (Bjorndal, 1997; Mortimer, 1981). Variation in feeding patterns is thought to be
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dependent on prey abundance and availability (Guebert-Bartholo et al., 2011) but may also be
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affected by feeding selectivity (Bjorndal, 1985; Fuentes et al., 2006; López-Mendilaharsu et al.,
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2008). Variation in feeding patterns has been observed in subtropical systems (Guebert-Bartholo
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et al., 2011; Nagaoka et al., 2012) where resource availability is relatively stable. In temperate
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systems, resource availability is more variable; changes in seasonal biomass of vegetation and
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animal prey may lead to greater variation and plasticity in feeding patterns. For these reasons, it
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is critical to understand how tropical/subtropical species at the latitudinal extremes of their range
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cope with environmental variation.
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Studies on the diet of West Atlantic green turtles have been primarily conducted in the core
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areas of their range using stomach content analysis. Mortimer (1981) examined the stomach
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contents of 243 adult and subadult turtles in Caribbean Nicaragua. The main diet items, in
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decreasing order of importance, comprised the seagrass Thalassia testudinum, three other species
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of seagrass, 40 different algae species, benthic substrate, and animal matter. Turtles were found
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to modify their diet opportunistically, based on the composition of available prey sources. For
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example, during migrations to breeding grounds, where turtles travel near shore, they consume
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more red algae and lignified terrestrial plant debris (Mortimer, 1981). Studies in several foraging
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grounds along the coast of Brazil have reported varied diets between locations, with some diets
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dominated by algae and others by animal matter. For example, in northern Brazil, Ferreira (1968)
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documented that juvenile and adult (31–120 cm curved carapace length) green turtle diet is
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composed of approximately 88% marine algae and less than 10% animal matter in an area where
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seagrass is not abundant. The following types of animal matter were found: ascidians, molluscs,
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sponges, bryozoans, crustaceans, and echinoderms. In a lagoon complex in southeastern Brazil,
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green turtles consumed an omnivorous diet composed of four diet categories: terrestrial plants,
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algae, invertebrates, and seagrass (Nagaoka et al., 2012).
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Gilbert (2005) found that green turtles foraged selectively on Rhodophyta and Chlorophyta
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on Ambersand Reef, Indian River County, Florida. In contrast, Mendonça (1983) found
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immature turtles grazed exclusively on seagrasses and avoided the abundant algae species in
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Mosquito Lagoon, Florida. Juvenile green turtles off the coast of Long Island, New York, feed
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primarily on the seagrass Zostera sp. and marine algae (Burke et al. 1993). New York waters
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provide a seasonal foraging ground for several sea turtle species from June through November
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(Burke et al., 1993). Burke’s research represents one of the few available studies that examine
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green turtle diet at the edge of their range. Few reports are available on green turtle diet in the
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Gulf of Mexico. During a three year study in South Padre Island, Texas, Coyne (1994) reported
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immature green turtles feeding selectively on algae and seagrass. Similarly, in St. Joseph Bay,
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Florida, which lies on the north coast of the Gulf of Mexico, Foley et al. (2007) found that the
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primary diet constituent of stranded green turtles from the 2000/2001 cold stunning event was T.
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testudinum. Small quantities of tunicates (Styela sp.; Molgula sp.) were found in stomach
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contents from green turtles in the 2000/2001 cold stunning event in St. Joseph Bay (J.M.
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Lessmann, unpubl. report, 2002).
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In habitats with low winter temperatures, juveniles in neritic habitats can either migrate to
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warmer waters or overwinter in the neritic habitat. In temperate climates, adult turtles migrate
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long distances to overwinter in warmer waters (Meylan, 1995). However, juvenile turtles
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foraging in shallow bays with restricted entrances may face rapid decreases in water temperature
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and be unable to escape (Mendonca and Ehrhart, 1982). St. Joseph Bay, Florida, provides the
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opportunity to study foraging behavior of green turtles on the northern edge of the Gulf of
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Mexico. Despite cool winter temperatures (4 to 35°C), green turtles remain in St. Joseph Bay
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throughout the year, often remaining in the area for several years (McMichael 2005). The green
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turtles in St. Joseph Bay are susceptible to cold stunning events, with 401 turtles stranding in
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December 2000/January 2001 (Foley et al., 2007) and 1,670 turtles stranding in 2010 (Avens et
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al., 2012). Foley et al. (2007) reported a diet dominated by seagrass from turtles killed by the
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2000/2001 cold stunning event. However, seagrasses are known to experience decreases in
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abundance during the winter in St Joseph Bay (Leonard and McClintock, 1999). Diets of green
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turtles in temperate foraging grounds, especially during winter foliage reductions, are poorly
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studied.
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We investigate the diet of green turtles during winter in St. Joseph Bay using two
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approaches. First, we analyzed stomach contents from 43 green turtles that died during cold
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stunning events in St. Joseph Bay, Gulf County, Florida in 2008 (n=12) and 2011 (n=31).
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Stomach contents give a direct measure of diet, but only for a short window of time. Second, we
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used analyses of C and N stable isotopes of epidermis samples from 39 green turtles in 2011 to
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evaluate consistency of diet over a longer temporal scale because stable isotope values of
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epidermis represent an integration of diet over several months. Fig. 1 presents a conceptual
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model comparing the isotope values of individuals in a population with inconsistent and
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consistent feeding patterns. If individuals in a population have not fed consistently, their stable
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isotope values will not be associated with the stable isotope value of the major diet item from the
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stomach contents (Fig. 1a). If individuals in a population have maintained a relatively consistent
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diet over the previous months, their epidermis stable isotope values should be higher than the
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major diet item from the stomach contents, the distance representing the discrimination value
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(Fig. 1b). Evaluating feeding consistency using both stomach content and stable isotope analyses
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allows us to investigate alternative foraging strategies that may have been missed with previous
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methods (Vander Zanden et al. in press). Examination of individual resource use patterns
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through time may show significant intrapopulation differences and have important ecological
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and conservation consequences (Vander Zanden et al. 2010).
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Materials and methods
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Study Area
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St. Joseph Bay, Florida (29.76º N, 85.35º W), in the northeastern Gulf of Mexico, covers
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an area just less than 30,000 hectares. St. Joseph Bay (SJB) is approximately 21 km in length,
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with a maximum width of 8 km. The maximum depth is 13.3 m in the northern end, with a
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minimum depth of 1.0 m in the southern end (McMichael, 2005). The site has a tidal range of
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approximately 0.47 m, a very low current flow and highly organic sediments. The salinity in the
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bay is similar to adjacent areas in the Gulf of Mexico (Stewart and Gorsline, 1962) and averages
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35.0 ppt (Preserves, 2008). Water temperatures range from 4 to 35°C (McMichael, 2005). Wind
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direction is usually north in the winter and south in the summer. The bay is productive due to its
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salt marsh and seagrass habitats; seagrass beds are a prominent feature in the southern end and
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cover approximately one-sixth of the bay. The abundance and distribution of seagrasses and
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macroalgae in SJB are not fully known, and future research is needed to determine seasonal
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dynamics, biomass, and productivity of these communities (Preserves, 2008). The local,
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dominant seagrass species are Thalassia testudinum, Halodule wrightii and Syringodium
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filiforme. Green turtles in St. Joseph Bay are susceptible to cold stunning events when water
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temperatures decline to less than 10°C (Morreale et al., 1992; Schwartz, 1978; Witherington and
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Ehrhart, 1989). These events are not uncommon along the Atlantic and Gulf coasts during
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autumn and winter months (Avens et al., 2012), particularly in shallow waters with restricted exit
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points (Foley et al., 2007). In exceptionally cold years (i.e. 2010 event), temperatures may
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remain below 10°C for as long as two weeks (Avens et al., 2012; Foley et al., 2007) resulting in
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high levels of cold stunning. Green turtles are present year-round, and at least some individuals
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are resident for several years in St. Joseph Bay despite the potential for cold temperatures
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(McMichael, 2005).
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Sample Collection
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In January 2008 and 2011, volunteers collected green turtles that stranded dead during cold
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stunning events in St. Joseph Bay, Florida. Curved carapace length (CCL; cm) was measured
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using a tape measure (Bolten, 1999). Body mass (kg) was measured using a hanging spring scale.
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Stomach contents were removed from the gastrointestinal (GI) tract and frozen pending analyses.
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Epidermis tissue from the dorsal surface of the neck was collected using a 6-mm biopsy punch
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and preserved in dry NaCl until analysis. For stable isotope analysis, epidermis samples were
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rinsed with distilled water to remove the NaCl and the outermost epidermis was separated from
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the dermis tissue using a scalpel blade.
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Known prey items identified from stomach contents were opportunistically collected from
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SJB in fall 2011 for stable isotope analysis. This collection period was chosen to best represent
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the season in which diet item isotope values would be incorporated and represented in epidermis
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tissue from the stranded turtles. The following known prey items were sampled: seagrasses (T.
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testudinum, H. wrightii, and S. filiforme), macroalgae (Gracilaria sp. and Enteromorpha sp.),
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and tunicates (Botrylloides sp.). The sample size was two for each species. Samples were put in a
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cool box for transport, rinsed with distilled water, and then frozen at -20C. For all plant items, a
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razor blade was used to remove epibionts from blades and stems prior to drying. A subset of
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samples (seagrasses, macroalgae, tunicates) were treated with 1N HCl using the drop-by-drop
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method of Jacob et al. (2005), and, upon seeing no release of CO2, it was concluded that
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acidification was not necessary for the samples.
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Frozen stomach contents were thawed and separated to species or the lowest identifiable
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taxon, using a dissecting scope when necessary. Each diet item was quantified using percent
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frequency of occurrence, volume, mass, and index of relative importance (IRI). Percent volume
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was evaluated by water displacement using graduated cylinders; diet items less than 0.2 ml were
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considered “trace”. Diet items were dried at 60°C for 24 hours and then weighed to calculate
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mass. IRI was modified from Hyslop (1980) by Bjorndal et al. (1997) for application to
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herbivores. Each diet category was calculated by the following equation:
IRI 
100(FiVi )
n
 (F V )
i i
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i1
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where F is percent frequency of occurrence, V is percent volume, and n is the number of diet

categories. Each of these measures (volume and frequency) in isolation can yield misleading
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interpretations (Bjorndal et al., 1997; Hyslop, 1980). For example, a diet item with a 100%
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frequency of occurrence may only be present in each stomach in trace amounts. The IRI provides
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a better interpretation for ranking the relative importance of diet categories because both
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frequency and volume are included (Bjorndal et al., 1997).
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Of the total individual samples (n=49) for 2011, only individual total sample volumes
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greater than 9.0 ml were included as representative samples of turtle diet (n=31). We compared
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results from all samples with those from samples >9.0 ml to test for an effect of volume on
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estimates of diet composition. Small sample volumes occurred only in 2011.
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Stable isotope analyses
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For stable isotope analysis, approximately 0.5 to 0.6 mg of each epidermis sample and 1.0
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to 15.0 mg of each prey sample was weighed and sealed in a tin capsule. Samples were analyzed
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for δ13C and δ15N by combustion in a ECS 4010 elemental analyzer (Costech) interfaced via a
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ConFlo III to a DeltaPlus XL isotope ratio mass spectrometer (ThermoFisher Scientific) in the
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Department of Geological Sciences at the University of Florida, Gainesville. Sample stable
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isotope ratios relative to the isotope standard are expressed in the conventional delta (δ) notation:
δX = [(Rsample/Rstandard) – 1] × 1000
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where δX is the relative abundance of 13C or 15N in the sample expressed in parts per thousand
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(‰) and Rsample and Rstandard are the corresponding ratios of heavy to light isotopes (13C/12C and
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N/14N) in the sample and international standard, respectively.
The standard used for 13C was Vienna Pee Dee Belemnite and atmospheric N2 for 15N. All
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analytical runs included samples of standard materials that were inserted at regular intervals to
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calibrate the system. The reference material USGS40 (L-glutamic acid) was used to normalize all
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results. The standard deviation of the reference material was 0.04‰ for δ13C and 0.08‰ for δ15N
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values (n=10). Repeated measurements of a laboratory reference material, loggerhead (Caretta
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caretta) scute, was used to examine consistency in a homogeneous sample with similar isotopic
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composition to the epidermis samples. The standard deviation of the loggerhead scute was
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0.06‰ for δ13C values and 0.10‰ for δ15N values (n=4).
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To evaluate feeding consistency, we assigned each turtle for which we had >9.0 ml
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stomach samples and isotope values (n=19) to one of three categories based on their stomach
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contents: >50% seagrasses, >50% macroalgae and >50% tunicates. We then plotted the stable
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isotope values of these turtles and the prey species. We compared the resulting graph (Fig. 2)
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with the conceptual model to determine whether turtles were feeding consistently. We did not
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adjust stable isotope values of turtle skin for diet-tissue discrimination. Such an adjustment is not
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needed for comparison with the conceptual model (Fig. 1), and discrimination values are not
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known for green turtles on these diets.
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Statistical analyses
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To test for annual differences in percent volume of diet items between 2008 and 2011,
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ANOVA was used to compare between years and diet constituents. For 2011 samples, ANOVA
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was used to compare between all samples (large+small) and only large samples and diet
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constituents. A Tukey HSD multiple comparison test for unequal sample size was used when
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significant differences were detected from the ANOVA. Volume percentages of diet items were
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arcsine square root transformed to improve normality and homogeneity of variance. All data
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were analyzed using program JMP version 9.0.2 (JMP®, 1989- 2010).
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Results
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In total, 43 stomach samples were analyzed from 2008 (n=12) and 2011 (n=31; volumes >
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9.0ml). In 2008, CCL of turtles ranged from 23.6 to 35.9 cm (n=12; mean±SD=30.4±4.34). In
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2011, CCL of turtles ranged from 22.5 to 72.7 cm (n=31; mean±SD=35.9±9.87) and body mass
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ranged from 1.2 to 40.8 kg (n=31; mean±SD=6.73±6.93). Thirteen diet categories were
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identified (Table 1): seagrass (n=3), algae (n=2), tunicate (n=2), and other materials (n=6). The
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proportion of the tunicate Botrylloides sp. in the diet of green turtles differed significantly (P=
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0.0037) between 2008 and 2011. Therefore, data from the two years were analyzed separately. In
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the 2008 samples (n=12), three species were considered major diet items (>5% volume in at least
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one sample; (Garnett et al., 1985), listed in order of importance: T. testudinum blades,
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Botrylloides sp., and Pyrosoma sp. (Table 1). In the 2011 samples (n=31), six items were
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considered major diet constituents, listed in order of importance: T. testudinum blades, Pyrosoma
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sp., Botrylloides sp., Gracilaria sp., S. filiforme, and Enteromorpha sp. There was no significant
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difference (seagrass P=0.2650; algae P=0.3637; tunicate P=0.6164; other materials P=0.4519) in
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diet composition based on percent volume between all samples (large+small) and only large
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samples in 2011 (Table 1), but the number of turtles with 100% volume of one diet species
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declined from five to one.
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In 2011, epidermis samples from 19 turtles were analyzed for stable isotope analyses.
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Stable isotope values in epidermis tissue ranged from -15.6 to -8.1‰ for δ13C (mean±SD: -
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12.9±2.0; Fig. 2). Epidermis δ15N values ranged from 7.4 to 11.6‰ (mean±SD: 9.1±1.2; Fig. 2).
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Body size had a significant positive relationship with δ13C (R2 = 0.66, df = 38, p = < 0.001), but
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no relationship with δ15N (p = 0.054).
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Stable carbon and nitrogen isotope values were determined for six prey species: T.
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testudinum, S. filiforme, H. wrightii, Gracilaria sp., Enteromorpha sp. and Botrylloides sp. (Fig.
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2; Table 2). These prey species were considered primary diet constituents of green turtles based
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on stomach contents. We were unable to apply mixing models to this study due to the inability to
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locate Pyrosoma sp. during searches in fall 2011 and winter 2012 (see discussion). Individuals in
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the population exhibited inconsistency in diet during the months before stomach samples were
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collected (Fig. 2). The stable isotope value of the epidermis tissue was not associated with the
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stable isotope value of the major diet item from the stomach contents.
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Discussion
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In the winters of 2008 and 2011, the diet of juvenile green turtles in St. Joseph Bay was
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considered to be predominantly omnivorous. These results are similar to other studies conducted
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in northwest Africa; Shark Bay, Australia; and San Diego Bay, California (Burkholder et al.,
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2011; Cardona et al., 2009; Lemons et al., 2011). Seagrass (T. testudinum) and tunicates
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(Pyrosoma sp. and Botrylloides sp.) presented high IRI values in both years of this study.
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The high degree of invertebrate consumption in this study highlights the importance of
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quantifying the availability of animal prey and their nutritional contribution to green turtles.
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Previous research (Davenport and Balazs, 1991) examined the nutritional content of Pyrosoma
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atlanticum in leatherback (Dermochelys coriacea) diets and found Pyrosoma bodies to be
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composed of 27% protein, 3% lipid, and 70% carbohydrate. These tunicates may be targeted for
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their ‘valuable’ stomachs, which contain digestible organic material, while the tunica may pass
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through the gut relatively undigested (Davenport and Balazs, 1991). The digestibility of tunicates
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such as Salpa and Pyrosoma spp. is not well known. It is possible that although tunicates provide
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a high concentration of protein (Dubischar et al., 2012), the protein may be bound in compounds
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not available to the turtle. Studies are needed that measure the digestibility and nutrient
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availability of the many animal species ingested by green turtles to better understand their role as
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a dietary component.
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Stomach content analysis suggests a degree of among-individual variation, but isotope data
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indicate no consistency within individuals over a few months. The wide distribution of13C
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values results from turtles feeding on mixtures of seagrasses, algae and tunicates. Although we
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were not able to collect samples of Pyrosoma sp., we believe they would have low 13C values
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because they are pelagic organisms blown in from deeper waters of the Gulf of Mexico.
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Stable isotope results indicate low consistency, which may be attributed to inconsistent
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availability of seagrass and tunicates. Seasonal variation in seagrass biomass, algae biomass and
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tunicate occurrence may lead to alternative feeding strategies. Although seagrass species are
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present in SJB year round, there is a significant dieback of T. testudinum, H. wrightii, and S.
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filiforme in shallow areas of the bay during winter. Previous investigations have found seagrass
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production in SJB to be highly seasonal, with shoot biomass and density peaking during the
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summer months (Iverson and Bittaker, 1986). Additionally, it appears that red algae coverage
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increases in the fall months, then decreases during winter (pers. obs.). Pyrosoma are pelagic
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species, known to form extensive, dense colonies that occur in irregular swarms (Andersen and
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Sardou, 1994) throughout temperate waters. We believe green turtles in SJB fed on Pyrosoma
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when they were carried into SJB by currents. Mass benthic deposition events of pyrosomids have
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been recorded off the African coast (Lebrato and Jones, 2009). Previous studies have found
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tunicates to be patchily distributed in the Gulf of Mexico (Graham, 2001). The other tunicate
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species found in this study (Botrylloides sp.) is a benthic tunicate that grows on blades of T.
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testudinum. These tunicates appear when water temperatures decrease (Sheri Johnson, pers.
310
comm.; pers. obs.). Green turtle digestive efficiency decreases with a decrease in water
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temperature (Bjorndal, 1980), so during winter green turtles may select animal diet items which
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are of higher digestibility.
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This study also indicates that setting a minimum stomach sample size for gut content
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analyses should be considered, especially with gastric lavage where samples tend to be small
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(Nagaoka et al. 2012). Of the stomach samples in 2011 (n=49), three individuals had stomach
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contents composed of 100% seagrass, two were 100% tunicates, and the remaining samples were
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mixed diets of seagrass, tunicates and macroalgae. Although there were no significant
318
differences between all samples and only large samples in 2011, the number of turtles with 100%
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seagrass or tunicates was greatly reduced from five samples to one sample after excluding small
320
sample volumes (<9.0 ml). Small, homogenous sample volumes may represent one feeding event
321
and give an unrealistic picture of diet.
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In the present study, green turtles revealed flexible foraging strategies in the northern
fringe of their year-round range. Prior to the 2008 and 2011 SJB cold stunning events, green
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turtles in SJB were feeding as omnivores; however, prior to the cold stunning event in SJB in
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2001, green turtles exhibited a herbivorous diet (Foley et al., 2007). Questions of feeding
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consistency should be further examined by measuring seasonal and annual variation in prey
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availability and selection. Future studies should address the relationship between diet selection,
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digestive efficiency, and the nutritional value of animal matter in green turtle diets.
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Understanding the temporal and geographic variation in sea turtle foraging ecology in peripheral
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habitats, where they may be more susceptible to environmental changes, is necessary for
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implementing effective, long-term conservation and management plans.
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Acknowledgements
We would like to express sincere thanks and appreciation to Dr. Jane Brockmann for her
335
creative assistance, support, and advice throughout the research and writing process. We thank L.
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Avens, B. Stacy, A. Bolten, P. Eliazar, M. Frick, M. Lopez-Castro, M. Pajuelo, J. Pfaller, L.
337
Soares, H. Vander Zanden, and P. Zarate for project and creative assistance. J. Curtis of the
338
Stable Isotope Lab at the University of Florida assisted with stable isotope analyses. We thank S.
339
Farris, M. Pajuelo, and B. Stephens for the collection of stomach contents. We also thank the
340
many volunteers who assisted during the cold stunning events. The Sea Turtle Grants Program
341
(funded from proceeds from the sale of the Florida sea turtle license plate), Knight Vision
342
Foundation, and Jennings Scholarship funded this research. All project work was performed
343
under MTP # 094 and MTP # 016. Samples were collected and processed in compliance with
344
the Institutional Animal Care and Use Committee at the University of Florida. Any use of trade,
345
product, or firm names is for descriptive purposes only and does not imply endorsement by the
346
U.S. Government.
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457
20
458
459
460
Figure legends
461
representing (a) inconsistent resource use through time and (b) consistent resource use through
462
time. Closed symbols represent prey items; open symbols represent individual turtles. See text
463
for discussion of model
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464
465
Fig. 2 Chelonia mydas. Plot of stable isotope ratios of nitrogen and carbon from epidermis
466
samples and major prey items of juvenile green turtles. Closed symbols represent prey items;
467
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468
shown as mean (standard deviation)
21
Table 1 Chelonia mydas. Diet composition of juvenile green turtles in St. Joseph Bay, Florida
Prey item
% Volume (ml) mean (SD)
% Dry Mass (g) mean (SD)
IRI
%F
2008
2011
2011 ALL
2008
2011
2011 ALL
2008
2011
2011 ALL
2008
2011
2011 ALL
n=12
n=31
n=49
n=12
n=31
n=49
n=12
n=31
n =49
n=12
n =31
n=49
T. t. blades
22.8 (11.9)
12.2 (10.7)
7.8 (9.8)
2.30 (1.28)
1.14 (1.02)
0.7 (0.94)
74.40
70.14
70.33
100
100
93.88
T. t. rhizome
0.4 (1.2)
0.1 (0.5)
0.1 (0.4)
0.03 (0.09)
0.02 (0.06)
0.01 (0.05)
0.21
0.13
0.11
16.67
16.13
12.24
H. wrightii
0.1 (0.4)
0.0
0.1 (0.3)
0.01 (0.03)
0.01 (0.01)
0 (0.02)
0.03
0.02
0.05
8.33
12.9
10.20
S. filiforme
0.0
1.8 (9.0)
1.2 (6.9)
0.00
0.08 (0.37)
0.05 (0.28)
0.00
0.65
0.66
0.00
6.45
6.12
Gracilaria sp.
0.0
0.6 (3.1)
0.6 (2.5)
0.00
0.03 (0.12)
0.02 (0.10)
0.01
0.22
0.66
0.00
6.45
12.25
Enteromorpha
0.0
0.9 (3.3)
0.5 (2.6)
0.00
0.01 (0.07)
0.01 (0.05)
0.00
0.49
0.30
0.00
9.68
6.12
Pyrosoma sp.
3.5 (2.7)
5.1 (4.9)
3.2 (4.3)
0.23 (0.39)
0.19 (0.21)
0.13 (0.19)
11.40
26.70
26.70
100
90.32
83.67
Botrylloides sp.
5.7 (7.0)
0.6 (1.1)
0.3 (0.9)
0.27 (0.35)
0.04 (0.07)
0.02 (0.05)
14.00
1.66
1.20
75
45.16
32.65
feather
0.0
tr
tr
tr
tr
tr
0.00
tr
tr
0.00
tr
tr
UM
0.0
tr
tr
0.00
tr
tr
0.00
tr
tr
0.00
tr
tr
plastic
0.0
tr
tr
0.00
tr
tr
0.00
tr
tr
0.00
3.23
2.04
shell
0.0
tr
tr
tr
tr
tr
0.00
tr
tr
0.00
tr
tr
UPM
0.0
tr
tr
0.00
tr
tr
0.00
tr
tr
0.00
tr
tr
Seagrasses
Macroalgae
sp.
Tunicates
Other materials
Percent volume, percent dry mass, index of relative importance (IRI), and frequency of occurrence (% F) for green turtles from cold stunning events in St. Joseph Bay,
Florida, in January 2008 and 2011. “ALL” refers to both small and large volumes of stomach contents from 2011; 2011 includes only large volumes ( > 9.0 ml). “tr”
refers to trace. Values are presented as mean (SD). UM: unidentified material; UPM: unidentified plant material; T.t.: Thalassia testudinum.
22
Table 2 Chelonia mydas. Mean stable isotope values of prey items and green turtle epidermis. Prey values are presented as mean (range) and
epidermis values as mean (standard deviation)
Prey item
N
d13C (‰)
d15N (‰)
Seagrasses
H. wrightii
2
-11.4 (-11.38 to -11.40)
1.4 (1.22 to 1.53)
S. filiforme
2
-6.9 (-6.88 to -6.97)
4.1 (4.01 to 4.16)
T. testudinum
2
-6.8 (-6.75 to -6.86)
5.6 (5.56 to 5.57)
Macroalgae
Gracilaria sp.
2
-13.0 (-12.9 to 13.14)
6.6 (6.05 to 6.76)
Enteromorpha sp. 2
-12.8 (-12.72 to -12.78)
5.5 (5.51 to 5.56)
Tunicates
Botrylloides sp.
2
-16.1 (-16.11 to -16.20)
5.1 (5.04 to 5.22)
Green turtles
19
-12.9 (2.0)
9.1 (1.2)
23
Macroalgae
(a)
Seagrasses
Tunicates
15
 N
(‰)
13
 C (‰)
(b)
15
 N
(‰)
13
 C (‰)
24
14
Tunicates
Macroalgae
Seagrasses
> 50 % tunicates
> 50 % macroalgae
> 50 % seagrasses
12
15N (‰)
10
8
6
4
2
0
-18
-16
-14
-12
-10
13C (‰)
25
-8
-6
-4