Dataset: CSIA of gelatinous zooplankton

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 from the same taxonomic groups into a single sample, while standardizing size ranges. 561 samples of gelatinous zooplankton representing 13 taxonomic groups were chosen for bulk tissue stable carbon and nitrogen isotope analysis. A subset of twenty samples from seven gelatinous genera were chosen for nitrogen compound-specific isotope analysis of amino acids (CSIA-AA): Pyrosoma atlanticum, Beroe cucumis, Hormiphora spp., Periphylla periphylla, Atolla vanhoeffeni, Aegina spp., and Pantachogon spp. Taxa were selected for CSIA-AA because they represented a range in hypothesized feeding guilds, were found across a range of depth habitats (0 to 1,025 m), and were abundant in our sampling region. To constrain possible spatiotemporal variability in baseline δ15N values, all the samples selected for CSIA-AA were limited to a daytime collection event at the escarpment in August 2021.

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 N₂ and 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 the University of California Merced.

Samples were processed for CSIA-AA at the Laboratory for Marine Organic and Isotope Geochemistry at the University of Miami following the methods of Popp et al. (2007), Hannides et al. (2013), and Wojtal et al. (2023). Briefly, ∼50 mg of each dried sample was hydrolyzed, purified, and derivatized. Derivatives were then injected into a Thermo Trace 1310 gas chromatograph with a BPX5 column (50 m x 0.32 mm, 1.0 micrometer (μm) film thickness), fed into a combined oxidation/reduction reactor (Thermo Isolink II, 1000°C), and passed through a liquid nitrogen cold trap and into a Thermo Conflo IV and MAT 253 isotope ratio mass spectrometer. Three separate laboratory mixtures with known isotope ratios were used to correct δ15N values of gelatinous zooplankton samples and to ensure instrument accuracy and precision. Reliable data for each sample in comparison to standards were obtained from triplicate injections where possible but replicate injections for eight samples.

We used five approaches to estimate the trophic position of samples run for CSIA‑AA. The first approach, TP_diet, was based on published diet studies, including in situ observations and gut content analyses. The second approach used δ15N values of bulk tissues to estimate the consumer trophic position (TP_bulk) following the methods outlined in Post (2002). This estimate is based on the δ15N_bulk values of gelatinous zooplankton, the trophic discrimination factor (TDF), and the δ15N value and trophic position of the isotopic baseline. Limited studies suggest that the TDF of gelatinous zooplankton may be lower than 3.4‰ (e.g., Schaub et al., 2021; Stukel et al., 2024; Tilves et al., 2018). We thus used a range in TDF (2.4 – 4.4‰; TP_bulk-TDF-2.4 and TP_bulk-TDF-4.4) based on the reported standard deviation in TDF of 1‰ (Post, 2002). The third approach utilized published diet data along with δ15N values of bulk tissues to estimate trophic position (TP_diet-bulk). We used known information about animal diet to designate a more accurate TDF. The fourth and fifth approaches for estimating trophic position relied on CSIA-AA following methods outlined in Chikaraishi et al. (2009) (TP_AA). Briefly, phenylalanine was designated as a 'source' amino acid (after Chikaraishi et al., 2009; McMahon & McCarthy 2016; Nielsen et al., 2015). We estimated TP_AA two different ways by considering two 'trophic' amino acids, glutamic acid (TP_Glu) and alanine (TP_Ala). Glutamic acid is commonly used as the 'trophic' amino acid in food web studies (e.g., Nielsen et al., 2015; McMahon & McCarthy 2016). However, we expected microbial cycling to be prominent within southern California Current Ecosystem food webs and subsequently designated alanine as another 'trophic' amino acid to capture these microbial contributions (after Décima et al., 2017).