Trace Gases Variability in the Oxygen-Depleted African Eastern Boundary
Water mass expression on surface water revealed cold and less oxygen-depleted surface waters in the more southern latitudes, as well as evidence of upwelled waters with an indication of filaments kilometres away offshore. In general, relatively moderate surface trace gas concentrations with a clear relation to sea surface temperature (i.e. SST) were revealed. The surface partial pressure of CO2 (i.e. pCO2) showed a quite uniform pattern, with the higher pCO2 in nearshore waters and the maxima around 23°S. N2O showed a comparable pattern to pCO2 with the higher inshore concentrations towards the southern transects (i.e. between 15°S and 25°S). CH4 concentrations also followed almost the same pattern to pCO2 and N2O, with gradients closer to the coast, and the maximum concentration of 250nM in inshore upwelled waters around 23°S.
N2O water column profiles revealed depleted surface waters in relation to underlying waters along all transects. The sub-surface waters to a depth of about 1000m were enriched in N2O with maxima of 50nM and became less concentrated in deeper waters. Also, a strong correlation between N2O and apparent oxygen utilization (AOU) was found during all transects. CH4 depth profiles, on the other hand, showed high concentrations through the water column in the stations closer to the shore.