The influence of zooplankton and oxygen on the particulate organic carbon flux in the Benguela Upwelling System

Abstract. We conducted sediment trap experiments in the Benguela Upwelling System (BUS) in the southeastern Atlantic Ocean to study the influence of zooplankton on the flux of particulate organic carbon (POC) through the water column and its sedimentation. A total of 2 long-term moored and 16 short-term free-floating sediment trap systems (drifter systems) were deployed. The mooring experiments were conducted over more than a decade (2009–2022), and the 16 drifters were deployed on three different research cruises between 2019 and 2021. Zooplankton was separated from the trapped material and divided into eight different zooplankton groups. In contrast to zooplankton which actively carries POC into the traps in the form of biomass (active POC flux), the remaining fraction of the trapped material was assumed to fall passively into the traps along with sinking particles (passive POC flux). Our results show, in line with other studies, that copepods dominate the active POC flux, with the active POC flux in the southern BUS (sBUS) being about 3 times higher than in the northern BUS (nBUS). In contrast, the differences between the passive POC fluxes in the nBUS and sBUS were small. Despite large variations, which reflected the variability within the two subsystems, the mean passive POC fluxes from the drifters and the moored traps could be described using a common POC flux attenuation equation. However, the almost equal passive POC flux, on the one hand, and the high POC concentration in the surface sediments of the nBUS in comparison to the sBUS, on the other hand, imply that the intensity of the near-bottom oxygen minimum zone (OMZ), which is more pronounced in the nBUS than in the sBUS, controls the preservation of POC in sediments significantly. This highlights the contrasting effects of the globally observed expansion of OMZs, which on the one hand mitigates the accumulation of CO2 in the atmosphere and the ocean by increasing POC storage in sediments and on the other hand poses a threat to established ecosystems and fisheries.