Dataset: Nutrient, CTD Profile, and Nitrous oxide data from the Southern Benguela Upwelling System (Integrated Environmental Programme, February 2017)

Seawater samples were collected during a South African Department of Forestry, Fisheries and Environment (DFFE) Integrated Ecosystem Programme cruise aboard the FRV Mirabilis in February 2017 along 14 stations across the St Helena Bay Monitoring Line (~32.5°S) and the Columbine upwelling cell. Hydrographic profiles were obtained using a Sea-Bird SBE 911 CTD with oxygen sensors and 12 x 6-liter (L) Niskin bottles. Oxygen sensors were calibrated against discrete Winkler titrations. Seawater for nutrient analysis (NO₃⁻+NO₂⁻, NO₂⁻, PO₄³⁻, Si(OH)₄) was collected following GO-SHIP protocols, frozen at -20 degrees Celsius (°C), and analyzed using flow injection analysis (FIA) and manual colorimetric methods. NH₄⁺ was determined fluorometrically using OPA derivatization immediately after collection.

N₂O samples were collected in 125-milliliter (mL) gas-tight glass bottles, preserved with saturated HgCl₂, and stored inverted at room temperature. Concentrations and isotopic compositions (δ¹⁵N-N₂O and δ¹⁸O-N₂O) were determined using a continuous-flow isotope ratio mass spectrometer (CF-MC-IRMS) following Bourbonnais et al. (2017) and McIlvin & Casciotti (2011). Calibration included internal seawater standards and N₂O reference gases spanning a wide isotopic range. Derived variables include apparent oxygen utilization (AOU), N-deficit (based on AOU–NO₃⁻ stoichiometry), and N₂O saturation disequilibrium (ΔN₂O), with equilibrium concentrations estimated from Weiss & Price (1980) using atmospheric N₂O histories from Liu & Tanhua (2024).