Report and preliminary results of R/V POSEIDON cruise POS531, Las Palmas (Canary Islands) – Mindelo (Cape Verde), 18.01.2019 – 01.02.2019.

During the RV POSEIDON research cruise, POS531, from Las Palmas (Canary Islands) to Mindelo (Cape Verde), we studied organic matter transport in the Cape Blanc upwelling region. Cape Blanc upwelling is one of the coastal upwelling regimes of the Eastern Boundary Current Systems (EBUEs), which are the most productive ocean areas and key players in the global carbon cycle and climate change. Pelagic primary production drives the biological pump which, in coastal upwelling regions, is largely controlled by the sinking behaviour and lateral transport of marine snow aggregates and faecal pellets. We still lack a quantitative understanding about the role that zooplankton grazing, sediment processes, and lateral transport play in the transformation and export of particulate organic matter. Further gaps in knowledge exist about the distribution of particles, their size and density and the particle sinking velocities of various biogenic and lithogenic components. Even less is known about what happens when particles reach the sandy seafloor and the role that benthic primary production plays in shallow shelf systems. The sandy seabed is regularly reworked and resuspended and therefore biogeochemical processes in the sediment likely play an important role in nutrient and organic matter transformation. To study how the sandy shelf area is connected to open ocean carbon export and carbon sequestration, we deployed free-drifting sediment traps to study short term variability in mass fluxes and the decay and transformation of large, organic-rich marine snow particles. In addition, the water column was sampled and studied using in situ-pumps, Marine Snow Catchers, particle cameras, and vertical multi-net hauls. To link the pelagic processes with the benthic remineralization and production, we deployed benthic boundary layer samplers and benthic lander systems. This allowed us to study the physical environment near the seabed and elucidate its effects on the biogeochemistry of the sediment, which was quantified through in situ and on-board laboratory rate measurements. The major aim of the research cruise was to better quantify the carbon and nitrogen cycling in the sandy shelf region and to quantify the impact from this cycling on lateral transport to the open ocean.