Preprints
https://doi.org/10.5194/egusphere-2024-700
https://doi.org/10.5194/egusphere-2024-700
27 Mar 2024
 | 27 Mar 2024
Status: this preprint is open for discussion.

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

Luisa Chiara Meiritz, Tim Rixen, Anja K. van der Plas, Tarron Lamont, and Niko Lahajnar

Abstract. We conducted extensive sediment trap experiments in the Benguela Upwelling System (BUS) in the south-eastern Atlantic Ocean to study the influence of zooplankton on the flux of particulate organic carbon (POC) through the water column and its sedimentation. Two long term moored and sixteen short term free-floating sediment trap systems were deployed. The mooring experiments were conducted for several years and the sixteen drifters were deployed on three different research cruises between 2019 and 2021. Zooplankton was separated from the trapped material and divided into 8 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). The 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 three 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 large variations in the POC concentration in the surface sediments between the nBUS and sBUS, on the other hand, imply that factors others than the POC supply exert the main control on POC sedimentation in the BUS. The varying intensity of the near-bottom oxygen minimum zone (OMZ), which is more pronounced in the nBUS than in the sBUS, could in turn explain the differences in the sediments, as the lack of oxygen reduces the POC degradation. Hence, globally expanding OMZs might favour POC sedimentation in regions formerly exposed to oxygenated bottom water but bear the risk of increasing the frequency of anoxic events in the oxygen-poor upwelling systems. Apart from associated release of CH4, which is a much more potent greenhouse gas than CO2, such events pose a major threat to the pelagic ecosystem and fisheries.

Luisa Chiara Meiritz, Tim Rixen, Anja K. van der Plas, Tarron Lamont, and Niko Lahajnar

Status: open (until 12 May 2024)

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Luisa Chiara Meiritz, Tim Rixen, Anja K. van der Plas, Tarron Lamont, and Niko Lahajnar
Luisa Chiara Meiritz, Tim Rixen, Anja K. van der Plas, Tarron Lamont, and Niko Lahajnar

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Short summary
The transport of particles through the water column and their subsequent burial on the seafloor is an important process for carbon storage and the mediation of carbon dioxide in the oceans. Our results from the Benguela Upwelling System distinguish between the northern and southern parts of the study area and between passive (gravitational) and active (zooplankton) transport processes. The decomposition of organic matter is doubtlessly an important factor for the size of oxygen minimum zones.