Dataset: Bulk tissue stable isotope analysis of zooplankton samples collected by MOCNESS from R/V Sally Ride and R/V Roger Revelle cruises in the southern California Current Ecosystem from 2020-2023

Gelatinous zooplankton were collected on four research cruises between 2020 and 2023 at seven stations representing four nearshore and escarpment, and two offshore regions within the Southern California Bight. We conducted depth‑discrete sampling of gelatinous zooplankton using a 10-square-meter (m²) Multiple Opening/Closing Net and Environmental Sensing System (MOCNESS) equipped with five depth-discrete nets (mesh sizes 5-millimeter (mm), Wiebe et al., 1985). The 10 m² MOCNESS was towed obliquely as the ship traveled at a speed between 1 to 2 knots, with depth-discrete collections occurring on the upcast. Sampling stations and depth intervals varied across stations and cruises, but there were typically two depth intervals sampled within the upper 500 meters (m) and larger depth intervals below 500 m. The maximum depth of sampling increased from 1,250 m nearshore to 3,000 m offshore, corresponding with the deepening of the water column.

Upon recovery, samples were stored in chilled seawater and kept at 5 degrees Celsius (°C) until processing. All sample processing was performed on ice to preserve body condition. Gelatinous zooplankton were identified to the most specific taxonomic level using published keys. The concentration of carbon and/or nitrogen can be low in gelatinous individuals (Lüskow et al., 2021), so we pooled multiple gelatinous individuals into a single sample, while standardizing size ranges. Individuals within a taxonomic group were split into relative size classes. The minimum, median, and maximum lengths of individuals within each size class were recorded to the nearest millimeter. Bell diameter was measured for medusae, body diameter for ctenophores, and total length for pelagic tunicates, molluscs, and chaetognaths. The number of individuals per size class was counted and then weighed as a group to the nearest 0.01 gram (g) using a motion‑compensating scale (Marel M2400), which was routinely calibrated at sea following manufacturer instructions. Samples were stored in Whirl‑Paks at ‑80°C until further processing in the laboratory.

561 samples of gelatinous zooplankton representing 13 taxonomic groups were chosen for bulk tissue stable carbon and nitrogen isotope analysis. Gelatinous zooplankton were briefly thawed to remove visible gut contents using forceps and a scalpel, which were cleaned with ethanol between samples. Both gelatinous zooplankton and mesozooplankton samples were then lyophilized and homogenized in Whirl‑Paks. To ensure sufficient sample mass for stable isotope analysis, samples often contained multiple individuals from the same net, taxonomic group, and size class. The number of individuals per sample was typically fewer than 100, with a larger number of individuals pooled for some samples of Pantachogon spp. and Hormiphora spp.

Dried, homogenized tissues were packaged into tin capsules (1.5 to 4 milligrams (mg) per sample) for bulk tissue stable isotope analyses, which were conducted at the University of Hawaii at Manoa and the University of California Merced. Briefly, samples were run on a Costech 4010 Elemental Combustion System coupled to either a ThermoScientific DELTA V Advantage, ThermoScientific DELTA V+, or a ThermoFinnigan DeltaPlus XP isotope ratio mass spectrometer through a ThermoScientific Conflo IV interface. Stable isotope values are reported in the standard per mille notation (‰), compared to the standards atmospheric N₂and Vienna Pee Dee Belemnite for nitrogen and carbon, respectively. To ensure accuracy and instrument precision, both labs used a combination of international reference materials (from the United States Geological Survey or the National Institute of Standards and Technology) and in‑house reference materials (squid or tuna) with known δ15N and δ13C values. Based on analyzed reference materials, sample reproducibility was ± 0.2‰ for samples run at the University of Hawaii at Manoa, and sample reproducibility was ± 0.4‰ for samples run at UC Merced.