https://doi.org/10.5061/dryad.h9w0vt4tj

Description of the data and file structure

The data contained in the file “Dryad Data ECS24-0444 (Wing et al. 2025 Ecosphere).xlsx” are derived from an extensive isotopic study of bone collagen from prehistoric, historic, and modern samples from the New Zealand sea lion (Phocarctos hookeri).  The data represent samples from three geographic regions (subantarctic islands, central New Zealand, and the mainland South Island of New Zealand) across three distinct periods early Maori, middle European expansion, and modern industrialised periods, representing the complete history of human occupation in New Zealand.

Samples from bone material were used to isolate collagen and then both bulk (d13C, d15N) as well as compound-specific analysis of d15N in amino acids was carried out.  From the resulting isotopic values we calculated both Trophic Position as well as the contribution of kelp-derived organic matter to the food web supporting New Zealand sea lions across the full range during the ca 800 yr time period over which they were extirpated and are now recolonising the region.

The attached Excel workbook contains primary data for Wing et al. 2025 published in Ecosphere. The workbook is comprised of three worksheets. 

The first worksheet entitled “SIA bulk” contains information on the values of d13C and d15N for each of the New Zealand sea lion (Phocarctos hookeri) samples that we analysed for the study. 

Archaeological collections of sea lion bones were sampled from multiple archaeological and sub-fossil sites from the Auckland Islands, Stewart Island, Southland, and Otago for early, 1250-1450 CE and middle, 1650-1850 CE time periods. Chronology of samples was determined using a combination of radiocarbon dating and stratigraphic associations.  Trophic position of modern populations was estimated from fur samples collected from live adult individuals at four sample sites along the extent of their current range (Enderby Island, Auckland Islands; Port Pegasus, Stewart Island; The Catlins, Southland and the Otago Peninsula; n = 10-20 per site for 2 years; stratified by sex).  Additional samples of fur, bone collagen, and muscle tissues were sourced from the New Zealand Department of Conservation who received deceased sea lions from the Otago and Southland regions.  

We followed standard protocols for obtaining collagen from zooarchaeological faunal material (Newsome et al. 2007a).  After cleaning using aluminum oxide in a dental sandblaster for ancient bone material, 0.2-0.3 g of bone material was weighed and cut in long thin shards running along the grain of the bone to preserve collagen fibril length.  The bone shards were demineralized using a pre-cooled solution of 0.5 M hydrochloric acid and kept at 20˚C until each sample was a soft, jelly-like fragment. The sample was then decanted and triple-rinsed with milli-Q water.

Heating, ultrafiltering, and freeze-drying

The samples were then emersed in a pH 3 solution of milli-Q water and hydrochloric acid, covered, and heated to 70 ˚ C for 48 h to denature the collagen and suspend the fibrils in solution.  The samples, now an amber or clear liquid, were filtered using an EZZE filter.  The collagen solution was then filtered using a cleaned Ultrafilter (Millipore Ultra-15 Centrifugal Filters, 30,000 MWCO) centrifuged at 2300 rpm until the liquid was between the top of the filter and the 500 µL line.  The resulting solution was pipetted into labelled, sealable, clean, plastic tubes.  The filter was rinsed twice with milli-Q water with the rinsing solution also pipetted into the plastic tube, then freeze-dried for 48 h.

Basal organic matter sources

Suspended particulate organic matter (SPOM) was collected during summertime phytoplankton bloom events and microscopically examined to confirm predominance of phytoplankton to characterize isotopic values of phytoplankton-derived organic matter in each of the study regions (e.g. Durante et al. 2021).  Samples were collected by pre-filtering water with a 300 μm mesh, followed by filtering on pre-combusted (400˚ C for 4 h) 0.7 μm GF-F filters and treatment in a sulphurous acid (H2SO3) fume for 8 h to remove inorganic carbon.  In addition, multiple samples (n=3-5) of dominant macroalgal species (n=8) were collected from sites in each of the study regions to characterize isotopic values of kelp-derived organic matter (Wing et al. 2022a). 

Stable isotope analysis of whole tissues

From each sample of fur and bone collagen (n=243), paired samples of fur, bone collagen, and muscle tissue (n =14) a 1 mg subsample was weighed and sealed into a tin capsule for stable isotope analysis.  For SPOM samples whole filters were sealed into larger tin capsules and for macroalgae samples a 3 mg subsample was weighed and sealed into a tin capsule for stable isotope analysis.  Samples were analyzed, with replicates every 8th sample, at IsoTrace, Dunedin, by combustion in an elemental analyser (Carlo Erba NA1500) coupled to a Delta Advantage isotope ratio mass spectrometer (Thermo-Finnigan, Bremen, Germany) operating in continuous flow mode. *Stable isotope ratios were expressed as parts per thousand (‰).

Raw d X* values were normalized and reported against the international standards for carbon and nitrogen, VPDB and AIR respectively. Normalization was made by 3-point calibration with two glutamic acid international reference materials and a laboratory EDTA (Elemental Microanalysis Ltd, UK) standard for carbon (USGS-40 = -26.2‰, USGS-41 = 37.8‰, EDTA = -38.52‰) and nitrogen (USGS-40 = -4.52‰, USGS-41 = 47.57‰, EDTA = -0.73‰).  Analytical precision based on the replicate analyses of the QC standard (EDTA, n=12) was 0.2‰ for d 13C and 0.3‰ for d 15N.

The second worksheet entitled “SIAaa” contains values of d15N for selected “source”, phenylalanine (Phe), and “trophic”, glutamic acid (Glx) and proline (Pro), amino acids, as well as estimates of trophic position (TP) for the subset of samples for which we have isotopic values of amino acids.  Samples are organised by time period (ERA, Period Pre Postindustrial, Time ERA) and by Region (Region 3).  Each sample has a unique identifier that links back to data in the worksheet “SIA bulk”.

Amino acids were extracted from bone collagen (n=37), and paired samples of bone collagen and fur (n = 5) by hydrolyzing 2.5 g of each sample with 2 ml 6 M HCl at 110 oC for 24 h in a N2 atmosphere.  An internal standard, norleucine (50 μl of 1 mg/ml solution), was added to monitor the wet chemistry and amino acid stable isotope values.  Solutes were then dried under a gentle flow of N2 at 60oC and subsequently converted into N-Acetylisopropyl esters.  Stable isotopes of amino acid N (d 15NAA) were measured by gas chromatography/combustion/isotope ratio mass spectrometer (GC-IRMS), using a Thermo Trace gas chromatograph, the GC combustion III interface, and a Deltaplus XP isotope ratio mass spectrometer (Thermo Fisher Scientific). 200 µl aliquots of derivatized amino acids were injected, inlet at 270°C in splitless mode, carried by helium at 1.4 ml min−1 and separated on a VF-35ms column (30 m, 0.32 mm ID and a 1.0 μm film thickness). The oxidation rector was set at 980°C and the reduction reactor at 650°C and a liquid nitrogen cold trap employed after the reduction reactor.  Samples were analyzed in duplicates along with amino acid standards of known isotopic values (measured by EA-IRMS) and calibrated against an international standard: atmospheric N2. Precision (1SD) ranged from 0.1 to 1.0‰ with a mean of 0.5‰.

The third worksheet entitled “Mixing Model Results” contains the data obtained from mixing models for each Region and Era showing values and associated error for trophic position and for the contribution of kelp-derived organic matter in the food webs supporting New Zealand sea lions. 

Mass balance mixing model

Inputs for individually based two-source isotopic mixing models based on d 13C included isotopic values for basal organic matter source pools, an estimation of trophic level (average TPpro-phe and TPglx-phe) of sea lions based on d 15NAA and the trophic discrimination factor D13C. We used the average trophic discrimination factor (D13C) of +0.4‰ (SE 0.17).  Stable isotopic values of basal organic matter source pools in the system, phytoplankton and macroalgae, vary along environmental gradients and in time due to an anthropogenic change in d 13C values of atmospheric CO2. Accordingly, we characterized the isotopic values of phytoplankton by sampling suspended particulate organic matter during the summer bloom period at each sites detailed above (n = 20; 5 samples per site at 4 sites).  By increasing our already extensive data base of kelp isotopic values from this region we calculated a weighted aggregate isotopic value for common subtidal kelps from samples spatially stratified across the sampling range (n = 300; 3-7 species, 5 samples at 3 sub-sites at 4 sites) to calculate an aggregate macroalgal value for d 13C with variance stratified among species.  Average values of d 13C for phytoplankton (SPOM) (d 13C = -23.62 ± 0.08 ‰) and macroalgae (d 13C = -16.03 ± 0.82 ‰) were then calculated.  For all sea lion samples, averages of TPpro-phe and TPglx-phe  were used to inform an individual-based mass balance mixing model using d 13C values to calculate the relative contribution of macroalgae and phytoplankton to underlying food webs. 

Files and variables

File: Dryad_Data_ECS24-0444_(Wing_et_al._2025_Ecosphere).xlsx

Description: Excel workbook containing primary data for Wing et al. 2025 Ecosphere ECS24-0444

Variables

• d13C
• d15N
• d15N of key amino acids (Phe, Glx, Pro)
• Trophic Position
• Contribution of kelp-derived organic matter to food webs supporting New Zealand sea lions

Code/software

None

Access information

Other publicly accessible locations of the data:

• None

Data was derived from the following sources:

• Samples of prehistoric, historic and modern bone material of New Zealand sea lions