Cruises STING I and STING II were conducted in Feb and July of 2023 along the West Florida Shelf (WFS), a coastal oligotrophic region characterized by low inorganic nutrient concentrations. Water was collected using towfish, stationary teflon pump, and ctd.

To measure ¹⁵N₂ uptake, triplicate polycarbonate bottles (0.5-4.5 L) were rinsed with whole water then filled, ensuring that no bubbles remained. ¹⁵N₂ gas (~99%, Cambridge Isotope Laboratories) was added using a gas-tight syringe (VICI Precision Sampling) through a flexible silicon septa; samples were gently rocked for 15 min as described by Selden et al. (2019). Any remaining bubble was subsequently removed to ensure constant isotopic enrichment over the incubation period, and bottles were incubated on-deck for approximately 24 hours. On-deck incubators were constructed from clear acrylic and covered with two layers of screening to reduce light to approximately surface seawater conditions and were plumbed with flowing surface seawater to maintain temperature. Triplicate initial PN and N isotope samples (0.5-4.5 L) were vacuum-filtered from whole water onto 25 mm GF/F (nominal pore size 0.7 µm) pre-combusted (450 ^oC for 2 hours) filters and frozen at -20 ^oC until pelletizing and analysis.

At the end of the incubation period, a 6 ml sample aliquot was collected from each bottle and transferred to a helium-purged Exetainer^TM to which 50 μl zinc chloride (50% w/v) was added. These samples were stored at room temperature, upside down, in 15 ml Falcon^TM tubes, submerged in ultrapure water, until analysis at the UC Davis Stable Isotope Facility using a ThermoScientific GasBench + Precon gas concentration system interfaced to a ThermoScientific Delta V Plus isotope-ratio mass spectrometer. The remaining incubation volume was then filtered as described above for initial PN samples. All PN samples were dried at 50 ^oC at least overnight and for no longer than a week. Once dry, the filters were pelletized into 30 mm-diameter tin disks (EA Consumables). Enriched and non-enriched pelletized samples were stored in separate dedicated desiccators until analysis. Initial and final incubation blank PN isotopic composition and concentration were analyzed at the University of South Florida on a Thermo Delta+XL IRMS w/ continuous-flow inlet coupled to a Carlo-Erba 1108 EA for NC isotope (15N, 13C) analysis. Final enriched PN isotopic composition and concentration samples were analyzed at the UC Davis Stable Isotope Facility using their Elementar Vario EL Cube or Micro Cube elemental analyzer interfaced with a PDZ Europa 20-20 isotope ratio mass spectrometer. Low mass samples (<10 µg N) were excluded from downstream analysis as IRMS response can become non-linear at low mass (White et al., 2020).