Dataset: Carbon and nitrogen isotope data from zooplankton and fish larvae collected on the SR2114 expedition in the Eastern Tropical North Pacific aboard the R/V Sally Ride from December 2021 to January 2022

An oceanographic survey was performed onboard the RV Sally Ride from Puntarenas, Costa Rica to San Diego, USA. Forty-nine CTD-rosette stations and 8 MOCNESS tows (Multiple Opening-Closing Net and Environmental Sensing System; Wiebe et al. 1985) were conducted along the cruise track (see Figure 1 of Gutiérrez-Bravo, et al. 2024). The CTD-rosette system used for water sampling and water column profiling included a SeaBird SBE9+ CTD and calibrated Seapoint-Fluorescence and SBE 43-DO sensors. A MOCNESS system was used for zooplankton and fish sampling and was equipped with 10 nets of 1 square meter (m^2) mouth opening and 333 micrometer (µm) mesh size, a SeaBird SBE9+ CTD, a SeaBird SBE 43 DO sensor, and flow meter and angle sensors.

The vertical sampling strategy followed five specific oxypleths (referred to as sampling levels) using horizontal tows. The deep level followed the 10 micromoles per kilogram (µmol/kg) oxypleth below the anoxic core (~900 meters (m) depth). The anoxic level followed the center of the anoxic core, with DO <1 µmol/kg and depth of ~450 m. The suboxic and hypoxic levels followed the 10 and 100 µmol/kg oxypleths above the anoxic core, with varying depths. The oxic level followed the >200 µmol/kg oxypleth near the surface (~20 m). Abrupt vertical oxygen gradients and the lagged depth control of the MOCNESS tow caused DO values to be less than half or more than double the desired DO values in seven out of 40 nets (M1-Oxic, M1-Hypoxic, M6-Hypoxic, M3-Suboxic, M6-Suboxic, M7-Suboxic and M8-Anoxic). These nets were considered outliers and were removed from the inter-level comparison statistics.

The zooplankton samples were preserved in ethanol 95%. Fish larvae were separated and identified to the most specific taxonomic level possible. The larval stages (preflexion, flexion, postflexion, and transformation) were defined according to Moser (1996). Juveniles and adults were separated, counted, and identified to the most specific taxonomic level possible. A more comprehensive description of the sample processing is described in Gutiérrez Bravo et al. (2024). The habitat type of each taxon was consulted in specialized literature (Moser 1996; Aceves-Medina et al. 2003; Froese and Pauly 2010).

The stable isotope ratios of carbon (13C/12C) and nitrogen (15N/14N) were measured on 39 samples of zooplankton (sample M5-Suboxic presented issues), 44 samples of fish larvae, and 8 samples of fish adults. For zooplankton, a wide-mouth 1.5 milliliter (mL) pipette was used to separate an aliquot from each of the 40 zooplankton samples. This approach was used to assess the integrated isotopic signature of the whole zooplankton community, and does not resolve species-specific differences. For fish, on the other hand, individuals of the same species, same development stage, and same net, were separated using a stereoscope and tweezer to obtain a critical weight of >2 milligrams (mg). Adjacent development stages were pooled if the sample weight was lower than 2mg.

The samples were rinsed to remove ethanol with deionized water, and then dried at 60 degrees Celsius (°C) for 24 hours. The dried samples were then ground to a fine powder using a mortar and pestle. The samples were loaded into tin cups and weighed to ~2mg using an analytical balance. Carbon and nitrogen isotope ratios were measured at the Boston University Stable Isotope Laboratory using a GV Instruments IsoPrime isotope ratio mass spectrometer coupled with an elemental analyzer.