Nitrogen and Carbon stable isotope values and body size data of megafauna collected from the Porcupine Abyssal Plain Sustained Observatory (PAP-SO) on cruises JC165, DY103, JC247 and JC263.

Stable isotope analyses of nitrogen and carbon are used to identify trophic size structuring in a deep-sea benthic ecosystem. The data includes δ15N, δ13C and body size data of 459 specimens spanning 23 taxonomic groups of invertebrates and fishes. Megafauna were collected from the Porcupine Abyssal Plain Sustained Observatory (PAP-SO; 48°50’N 16°30’W; 4850m water depth) using a semi-balloon otter trawl (13mm minimum mesh size) on the following research expeditions: RRS James Cook 165, RRS Discovery 103, RRS James Cook 247, and RRS James Cook 263 (2018 - 2024). Specimens were selected from the catch to span the body sizes gathered by the trawl and all feeding types therein and further specimens were selected from abundant morphotypes to gather replicates across the size range for population analyses. The body size of each selected specimen was determined by blotting dry before measuring fresh wet weight using a motion-compensated balance (0.2g resolution). To prepare for stable isotope analyses, tissue samples were dissected from each specimen using a scalpel and cleaned of sediment/debris with ultrapure water, then frozen at -80°C. Tissue type varied between taxonomic groups and were targeted for low lipids and carbonates. Stable isotope analysis of the tissue samples followed standard protocols (Peterson and Fry 1987). Each tissue sample was freeze-dried, homogenised, and 600-700 μg was placed in tin capsules. Body wall tissue of the holothurians were weighed separately into silver capsules (700-800 μg) and decarbonated with an acid vapour treatment (HCl, 12 h) to produce carbon isotopic values without carbonate influence. Isotopic ratios 15N/14N and 13C/12C were measured using a Delta V Isotope Ratio Mass Spectrometer (ThermoFisher) coupled to an ECS8020 Elemental Analyser (NC Technologies) via a Conflo IV interface (ThermoFisher) at the Liverpool Isotopes For Environmental Research laboratory and Elemtex Ltd. Isotope ratios are reported in standard 𝛿-notation (‰) relative to atmospheric nitrogen (N2) and Vienna PeeDee Belemnite carbon (V-PDB) scales and were corrected using internationally calibrated reference materials USGS40 (δ13C = -26.4 ± 0.1‰; δ15N = -4.1 ± 0.3‰) and USGS41a (δ13C =36.5 ± 0.2‰; δ15N = 46.9± 0.1‰; Reston Stable Isotope Laboratory). δ13C values were arithmetically lipid-corrected according to Fischer‐Rush et al. (2021) for invertebrates and Hoffman and Sutton (2010) for fishes to avoid introducing size bias.