DOI: 10.5061/dryad.bvq83bkf6
title: Fine-scale intraspecific niche partitioning in a highly mobile, marine megafauna species: implications for ecology and conservation

description: A species may partition its realized ecological niche along bionomic
 and/or scenopoetic axes due to intraspecific competition for limited
 resources. How partitioning manifests depends on resource needs and
 availability by and for the partitioning groups. Here we demonstrate the
 utility of analyzing short- and long-term stable carbon and nitrogen
 isotope ratios from imperiled marine megafauna to elucidate realized niche
 partitioning in these species. We captured 113 loggerhead (Caretta
 caretta) sea turtles at a high-use area near Crystal River, Florida,
 between 2016–2022, comprising 53 subadults, 10 adult males, and 50 adult
 females. Isotopic analyses indicated that loggerheads partition their
 realized ecological niche by life stage, along a scenopoetic axis. Most
 adults likely forage and reside in shallow areas of the study site, with
 most subadults displaying habitat switching between deeper locations to
 forage and shallower locations to rest. Some subadults, like adults, also
 forage and reside in shallow areas, and may display behaviors to avoid
 intraspecific competition. Analysis of stable isotopes with different
 turnover rates was critical for this characterization of intraspecific
 niche partitioning in loggerhead turtles, which has direct implications
 for ongoing research and conservation efforts for this and other imperiled
 species.
description: Capture surveys for sea turtles were conducted in the coastal
 waters of the Florida Big Bend between the mouths of the Crystal and
 Homosassa Rivers in Citrus County, Florida. Surveys were conducted
 approximately bi-monthly between January 2016 and February 2022 dependent
 on logistics and boating conditions (see Table S1). Sea turtles were
 spotted opportunistically from a vessel and captured using the “rodeo”
 method. Sighting and capture GPS locations were recorded using a Garmin
 GPSMAP. Captured turtles were brought on board the vessel and checked for
 Inconel flipper tags and passive integrated transponders (PIT). Tag
 numbers were recorded when already present and new flipper (National Band
 and Tag Company, Style 681) and PIT tags (Biomark, GPT12) were applied in
 the absence of old tags. Standard (nuchal notch to caudal tip) and minimum
 (nuchal notch to caudal notch) lengths were measured from each carapace
 using a tape measure (curved carapace length, CCL) and calipers (straight
 carapace length, SCL). Turtles sampled in this study
 were classified as either subadults (CCLmin &lt; 80 cm) or adults
 (CCLmin &gt; 80 cm). Turtles that were classified as adults with tails
 that extended past their carapace were further classified as males. All
 other turtles classified as adults were further classified as females.
 Subadults were not further classified by sex. Epidermis
 and scute were sampled for isotopic analysis using 5mm and 6mm biopsy
 punches, respectively. Epidermis was collected from the shoulder of
 captured turtles after sterilization with a 95% ethanol pad and stored in
 dry salt (NaCl) until downstream preparation and isotopic analysis. One
 costal scute on each carapace was selected for keratin sampling. The
 selected scute was scrubbed with a Scour Pad (Scotch-Brite) and rinsed to
 remove small epibionts and algae. One central and one distal circular
 punch were made in each scute to ensure adequate sampling. Samples of all
 scute layers were collected, as determined by visual observation of the
 white carapace epithelium after removing each circular punch. Scute
 samples were wrapped in foil with the outer surface marked, placed in
 individual vials, and frozen until downstream preparation and isotopic
 analysis. Turtles were released within 200 m of their capture
 locations. Epidermis and scute samples were sent to the
 Fish and Wildlife Research Institute of the Florida Fish and Wildlife
 Conservation Commission and the Marine Environmental Chemistry Laboratory
 at the University of South Florida College of Marine Science for
 preparation and isotopic analysis. Epidermis samples were brushed and
 rinsed with deionized water to remove salt, separated from any underlying
 fat, dried in an oven for 2 hours at 60°C, and homogenized. Lipids were
 extracted from epidermis and scute samples using an accelerated solvent
 extractor (Model 200, Dionex) with petroleum ether (3 cycles of 5 minutes
 heating followed by 5 minutes of static purging). 0.5–0.7 mg of each
 epidermis sample were weighed using a Mettler Toledo microbalance and
 placed into 3mm x 5mm Costech tin cups. Central and distal scute samples
 from each turtle were measured, and the thickest scute (representing the
 longest time period) from each turtle was selected for analysis. The
 outside layer of each scute sample was glued to a glass slide and sampled
 in 50 µm intervals using a carbide end mill. Each of these powdered scute
 layers was collected using forceps with a small piece of quartz wool, to
 which the scute powder is electrostatically attracted. The quartz wool and
 associated scute powder were then wrapped in Costech aluminum foil. Skin
 and scute layer samples were converted to N2 and CO2 using a Carlo – Erba
 EA1108 Elemental Analyzer (Thermoquest Italia). Isotope ratios and
 percentages-by-mass were measured in a continuous flow mass spectrometer
 (Delta PlusXP, Thermofinnigan, Bremen). Sample ratios are expressed in per
 mil (‰) as calculated using the equation: δX =
 [(R<sub>SAMPLE</sub>/R<sub>STANDARD</sub>)-1]*1000<br> where X is <sup>15</sup>N or <sup>13</sup>C, and R is the ratio of <sup>15</sup>N:<sup>14</sup>N or <sup>13</sup>C:<sup>12</sup>C. Standards for <sup>15</sup>N and <sup>13</sup>C were AT-Air and VPDB, respectively. Secondary reference materials (NIST 8574 δ<sup>13</sup>C = +37.63 ± 0.10 ‰, δ<sup>15</sup>N = +47.57 ± 0.22 ‰, %N = 9.52 %, %C = 40.81 %, C:Nmolar = 5.0; NIST 8573 δ<sup>13</sup>C = -26.39 ± 0.09 ‰, δ<sup>15</sup>N = -4.52 ± 0.12 ‰, %N = 9.52 %, %C = 40.81 %, C:Nmolar = 5.0) were used to normalize raw measurements to standards. Measurements of analytical uncertainty (reflecting ±1 SD) were obtained by replicate measurements (n=308) of an internal laboratory reference material (NIST1577b Bovine liver, δ<sup>13</sup>C = -21.69 ± 0.14 ‰, δ<sup>15</sup>N = 7.83 ± 0.16 ‰, %N = 9.95 ± 0.48 %, %C =48.04 ± 0.71 %, C:Nmolar = 5.63 ± 0.27), and were ≤ 0.25 ‰ (δ<sup>13</sup>C) and ≤ 0.31 ‰ (δ<sup>15</sup>N).