Description: The high surface productivity triggered by nutrient-rich Benguela upwelled waters results in significant enrichment of organic carbon in the sub-surface waters due to enhanced mineralization in the water column and benthic fluxes. Hence, microbial oxygen demand processes are promoted, driving oxygen depletion that favors trace gas production at relatively shallow depths. During upwelling, gas-rich subsurface waters are transported towards the surface waters, enhancing trace gas sea-air fluxes. We investigate the variability of these fluxes on seasonal and shorter timescales to understand the intensity of the Benguela upwelling system in gas emissions. The data might serve as a base for projections under a changing climate. The fieldwork took place during the cruise MSM105 (January 11th – February 23rd, 2022) onboard the R/V MARIA S. MERIAN, which encompassed close-coastal and open ocean regions between Mindelo (Cape Verde) and Walvis Bay. The working area of the cruise MSM105 was the Namibian shelf between 18°S and 27°S which is suggested to represent some regional hotspots of trace gas emissions to the atmosphere. The partial pressures of CH4, N2O, and CO2 in the sea surface and atmosphere were determined using IOW's self-built Mobile Equilibrator Sensor System (MESS). The system was described in detail elsewhere (Sabbaghzadeh et al., 2021) but in brief, it consists of a custom-built equilibrator (combined shower-head/bubble type) with a water flow rate of about 5 l min-1 and an airflow rate of ~ 4.00 - 5.00 L min-1. The system is linked to two off-axis integrated cavity output laser spectrometers (oa-ICOS, Los Gatos Instruments) for the detection of CH4 / CO2 and N2O / CO. To operate the system, seawater was supplied by a deep-well pump (CAPRARI Desert E4XP30-4 with CAPRARI XPBM1 control unit, ~ 100 L min-1, Italy), installed in the moon-pool at ∼ 6.00 m water depth on board of the R/V MARIA S. MERIAN. All gas analyzers were calibrated against three standard gasses at the beginning and end of each survey daily for data recalibration and drift correction. In addition, one "zero" gas (i.e. Nitrogen 5.00, LINDE) was measured infrequently throughout the survey to check any system deficiency like leakage detection. To quantify sea-air gas fluxes, the atmospheric concentration of studied trace gas was measured at several positions during the cruise using a tube with the inlet positioned at the front of the bow to minimize ship contamination. All other ancillary parameters were synchronized with D-ship data with a simultaneous data reduction to one-minute intervals.

Description: Upwelling systems are significant sources of atmospheric nitrous oxide (N₂O). The Benguela Upwelling System is one of the most productive regions worldwide and a temporally variable source of N₂O. Strong O₂ depletions above the shelf are favoring periodically OMZ formations. We aimed to assess underlying N₂O production and consumption processes on different temporal and spatial scales during austral winter in the Benguela Upwelling System, when O₂⁻deficiency in the water column is relatively low. The fieldwork took place during the cruise M157 (August 4ᵗʰ – September 16ᵗʰ 2019) onboard the R/V METEOR. This expedition included four close-coastal regions around Walvis Bay at 23°S, which presented the lowest O₂ concentrations near the seafloor and thus may provide hotspots of N₂O production. Seawater was collected in 10 L free-flow bottles by using a rosette system equipped with conductivity-temperature-depth (CTD) sensors (SBE 911plus, Seabird-electronics, USA).Seawater samples were collected from 10 L free-flow bottles bubble-free, filled into 200 mL serum bottles and immediately fixed with saturated mercury chloride (HgCl₂). Concentrations of dissolved N₂O were measured by a purge and trap system using a dynamic headspace (Sabbaghzadeh et al., 2021). The N₂O gas saturation (N₂Oₛₐₜ in %) was calculated from the concentration ratio between the seawater sample and seawater equilibrated with the atmosphere. ∆N₂O (N₂O saturation disequilibrium in nmol L⁻¹) was calculated as the difference between the measured N₂O concentration and the atmospheric equilibrium N₂O concentration using Bunsen solubility coefficient (Weiss and Price, 1980). AOU (apparent oxygen utilization in µmol L⁻¹) expresses the O₂ consumption by microbial respiration and was calculated as the difference between the equilibrated O₂ and observed O₂ concentration with the same physico-chemical properties (Weiss and Price, 1980).