The T/B ratio decreases in-shore in Kunene and Lüderitz, indicating that coastal locations display more non-thermal influences (i.e. biological imprints) on p CO2 gradients than offshore locations. In Walvis Bay however, the ratio exhibits a slight decrease seaward but remains very low. The T/B = 1 indicates an equilibrium between thermal and non-thermal processes (Takahashi et al., 2002). Therefore, our results indicate the strong dominant influence of non-thermal effects in the area (biological production and consumption of p CO2, and mixing processes), which controls the variation in p CO2 gradients.
During our seasonal campaigns, we also observed no significant relationship between CT and salinity. This lack of correlation suggests that other factors than simple dilution are likely governing the fluctuations of CT (Cai, 2003; Chou et al., 2013). Our results show a strong positive correlation between normalized AT (AT35) and normalized CT (CT35) within the hypoxic deep waters, characterized by oxygen ≤ 20.00 µmol kg-1, indicating a significant production of AT and CT in these waters. In these hypoxic-anoxic deep waters, the linear regression model produced a AT/CT ratio slightly > 1 (Figure 7), implying the influence of CaCO3 dissolution (given that CaCO3 dissolution results in a AT/CT ratio of 2:1) (Zeebe and Wolf-Gladrow, 2001). Moreover, anaerobic processes of organic matter degradation likely occur, as the AT/CTratio produced by sulfate reduction (0.9) and denitrification (0.8) closely align with the values observed in these hypoxic-anoxic waters (Figure 7). On the contrary, the well-oxygenated waters represented the lowest CT and intermediate AT, reflecting CT consumption by primary production and CO2 outgassing in the surface waters over the seasons (e.g. Zeebe and Wolf-Gladrow, 2001; Chou, et al., 2013; Cai et al., 2020). The data are predominantly situated on the left side when CT levels are low, however, there is a minor adjustment in AT accompanying this low CT range, indicating that the processes of CO2 uptake and outgassing have only a limited impact on AT (Figure 7).