Comparing respiration-based vertical carbon flux with sinking POC in the Namibian Upwelling system

Abstract

Both loss of material from the water column and gain to the sediment are important processes in marine ecology and biogeochemistry, as the transport balance between water and sediment controls quantity and quality of marine life on local and global scales. Upwelling systems, as hotspot of marine productivity, are particularly interesting in this concern. Here, we have studied the short-term variability of vertical carbon flux (C-flux) by two alternative approaches at two stations on the Namibian shelf (20ºS) during August/September 2011. The first approach was based on modeling the respiratory organic carbon consumption in the water column, by means of the integration of vertical profiles of the respiratory electron transport system (ETS). Temporal variability in the respiration (RO2) profiles proved to be stronger than the effect of distance between the sediment traps. Thus, two of the three vertical samplings were characterized by water arising from the upwelling filament, with the highest respiration rates due to microplankton and small zooplankton. The other profile, in contrast, was associated with a later temporal stage of the upwelled waters (in essence, more oligotrophic waters), where the respiration of large zoo- plankton dominated the water column. Modeled vertical C-fluxes (Fc) were determined by converting the RO2 into carbon units and integrating them from below the euphotic zone to the seafloor. Fc at the sediment-trap depths were 2.57 mmol C m−2 d−1 in the more oligotrophic water and 3.67±0.33 mmol C m−2 d−1 in the upwelling filament influenced waters. In the second approach, automatic sediment traps with daily sampling periods were moored successively for 13 days and 10 days. Fluxes of all variables (C, N, P, Si and dry mass) showed high temporal variations, which could be identified with periods of high and low particle export. Differences in qualitative properties of the sinking material like species composition of microalgae (diatoms/coccolithophorides), C/N, C/Si in bulk material or the opal/carbonate ratios of single particles coincided with these high/low flux situations, which were like- wise attributed to transit periods of either filaments of upwelling water or oligotrophic waters. Mean daily C-flux in both traps was 1.43±0.35 mmol C m−2 d−1 in oligotrophic water and 7.85±1.83 mmol C m−2 d−1 in upwelling filaments. Although both approaches yielded similar values during the two basic situations, differences between them were mainly attributed to changes in the composition of the sinking particles and consequently, in their settling velocities. When the productivity and the diatoms abundance were high, the fast sinking particles predominated in the water column and the Fc approach underestimated the total flux. In the low productivity scenario, however, the sediment traps values did not balance the estimated respiratory carbon demands. In combination, both approaches served the requirements for understanding the quantitative and qualitative aspects of the northern Benguela upwelling system functioning at short timescale. The Fc approach was further applied to zooplankton samples collected on a cross-shelf transect during the same cruise, in order to quantify their importance in the particulate organic carbon attenuation.