Contribution of mesozooplankton to the vertical flux of organic carbon in the northern Benguela Upwelling System off Namibia

The prevailing vertical transport of carbon produced by phytoplankton in the upper water layers of the ocean to deeper zones takes place by sinking of particulate organic carbon (POC). Several processes mediated by the zooplankton community can either enhance or limit the vertical carbon flux and regulate the efficiency of the biological carbon pump. Main biological pathways by which carbon is exported to deeper water layers are the active transport of carbon by vertical migration of pelagic animals and the passive transport of faecal pellets. Vertically migrating zooplankton may in this way significantly contribute to the dissolved inorganic carbon (DIC) pool by respiration and the dissolved organic carbon (DOC) pool by excretion. In the northern Benguela Upwelling System (BUS) jellyfish, krill, copepods, decapods and occasionally salps dominate the mesozooplankton community in terms of biomass. We will present a conceptual model, combining diel vertical migration (DVM) patterns of dominant krill species, with environmental parameters such as temperature, food availability and with physiological constraints such as species-specific respiration rates that may explain seasonal adaptations of DVM to changes in water temperature, oxygen and food availability. Additionally, we will show how the different taxa contribute to the downward transport of organic carbon by vertical migration (Krill, copepods and decapods) or through high amounts of fast sinking faeces (salps). Variable spatial and temporal distributions, such as a high patchiness of krill, decapods and salps and a more homogenous horizontal distribution of copepods will lead to seasonal and regional differences in the export of organic carbon. Furthermore, high primary production in the upper water layers related to upwelling intensity enhances the carbon export due to phytoplankton sinking after a bloom and/or faecal pellet production and grazing by zooplankton. Overall, we will estimate the role of zooplankton in the downward transport of carbon and assess its importance relative to other vertical carbon fluxes in the BUS. Keywords: biological carbon pump, zooplankton, Benguela Curren