Figure 2. The seasonal pattern of sea surface temperature (SST) along the survey track during winter (left) and summer (right) surveys. No surface monitoring was conducted during the autumn season.
4.2 Seasonal Dynamics of pCO2, CH4 and N2O

4.2.1. Variability in Water Column Distribution

Our first multi-seasonal investigation, encompassing both surface seawater and depth-cross profile assessments ofp CO2, CH4, and N2O in the nBUS represents the seasonal variability of water column distribution of these trace gases. The vertical profiling of p CO2 unveils significant seasonal variations, especially within the inner shelf. Enhanced coastal upwelling in winter amplifies mixing layers, contributing to the dilution of carbon mineralization imprints. Conversely, the stratification process in summer decouples surface layers from deeper strata, favoring faster surface warming and primary production. This leads to surfacep CO2 depletion attributed to intensified biological production, concomitant with enhanced mineralization over the shelf. The strong imprint of enhanced near-coast mineralization is particularly pronounced in the inner shelf of Lüderitz and Walvis Bay (Figures 4 and 5), while not observed in Kunene (Figure 3). An increase in organic carbon burial is associated with factors such as enhanced primary production, shallower water depths, and amplified oxygen depletion in bottom waters during the autumn and summer seasons, in contrast to the winter season was also underscored previously (e.g. Santana-Casiano, et al., 2009; Siddiqui, et al., 2023).
The offshore segment of the CT section reflects the open ocean conditions, with the lowest CT observed at the sea surface waters attributed to primary production. There is an enrichment in the mid-water layers due to mineralization. Notably, higher and maximum CT levels in the waters predominantly overlaying the shelf, regardless of the season and region, indicate the enhanced remineralization of organic carbon. This process results in the release of CT, including the flux of inorganic carbon from the sediments.
CH4 concentrations in slope and open ocean waters off Kunene and Walvis Bay exhibited a comparatively low seasonal variability likely associated to lateral transport (Figures 3 and 4). In contrast, there is a noticeable increase in CH4 concentrations on the shelf. This enhancement is more pronounced off Walvis Bay than off Kunene, likely attributable to the extent of the shelf and the presence of organic-rich mud belt (e.g. Van Der Plas, et al, 2007; Sabbaghzadeh et al., 2021). In the winter season in Kunene, two sources of CH4 within the water column are proposed, including a sedimentary source and an upper source involving in-situ biogenic production of CH4 by bacteria within the oxygenated layer (Sabbaghzadeh et al., 2021). This has a significant impact on sea-air CH4 fluxes (see Figure 6). Off Lüderitz, both slope and shelf waters tend to show elevated CH4concentrations during the transition from winter to summer (Figure 5).
In overall, N2O concentrations within slope and open ocean waters off all studied regions did not show significant seasonal variability. During the autumn and summer months in the near-shore regions of Lüderitz and Walvis bay, a notable depletion of oxygen was observed. This observation suggests intensified biological activity, including microbial respiration or the decomposition of organic matter (Emeis, et al., 2018). Also, the low oxygen content and strong increase in CT indicate the pronounced microbial degradation of organic matter over the shelf (Figures 5 and 4).