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).