Planktic foraminifera were collected by towing a 200-μm-mesh plankton net for 90 min at a target depth within the upper mixed layer. Approximately 90% of the tow material was preserved in a 5-10% pH-buffered formalin solution and stored at 4°C until sorting for foraminifera (Ren et al., 2012). The remaining 10% was size fractionated and frozen at −20°C for element and isotope analysis of zooplankton. During each tow, bulk (>0.3 µm) suspended PON and nitrate samples were collected from the underway intake (at 7 m depth) and frozen at −80°C and −20°C, respectively, until isotope analysis. In situ (i.e., tow-depth) nitrate samples and auxiliary hydrographic data come from CTD casts performed on station before the start of each net tow.

Foraminifera were separated by species, rinsed briefly with Milli-Q and crushed open to expose (non-shell-bound) tissue. Tissue N was converted to nitrate using the persulfate oxidation method (Nydahl, 1978; Knapp et al., 2005). Remnant shell material (having been oxidatively cleaned) was transferred, rinsed six times with Milli-Q and oven dried at 50°C. Shell samples were then dissolved with hydrochloric acid to release shell-bound organics for persulfate oxidation (Nydahl, 1978; Knapp et al., 2005). For both tissue- and shell-derived nitrate (from persulfate oxidation) as well as seawater nitrate, samples were measured for concentration by chemiluminescence (Braman & Hendrix, 1989) and N isotope composition using the denitrifier method and gas chromatography−isotope ratio mass spectrometry (GC-IRMS) (Sigman et al., 2001; Casciotti et al., 2002; Weigand et al., 2016). The N isotope composition of bulk PON and size-fractionated zooplankton were measured by elemental analyser-IRMS.

The isotopic composition of N₂O was measured either at Princeton University (USA) or the Max Planck Institute for Chemistry (MPIC; Germany) by GC-IRMS using a Thermo MAT 253 mass spectrometer with a purpose-built on-line N₂O extraction and purification system. Bulk PON and size-fractionated zooplankton samples were measured at the University of Cape Town using a Thermo Scientific FLASH 2000 elemental analyzer coupled to a Thermo Scientific Delta V Plus mass spectrometer. Hydrographic data were acquired by a Sea-Bird conductivity-temperature depth sensor during each cast.

See methods section of Smart et al. (2020) for more detail.