Mesopelagic Particle Layers in the Dynamic Hypoxic Northern Benguela Are Shaped by Zooplankton Activity

Abstract

Hypoxic (O₂ < 60 μmol kg⁻¹) waters are found below 27% of the ocean surface and are predicted to expand in the near future; however, the organic carbon cycle in these regions is far from understood. Here we study the origin, composition, and temporal variability of mesopelagic particle layers in the hypoxic northern Benguela. We combine ship measurements, 4 months of glider data, and a high-resolution 3D coupled model. Glider data show deep particle layers between 250 and 500 m, in the hypoxic oxygen minimum zone (OMZ), often giving rise to “deep export” events. Our data point toward biological processes generating the particle layers within the OMZ itself. Between 250 and 500 m, large particulate organic matter is fresh, small particles are more refractory, and image data show a local increase in aggregates and fecal pellets. Ship data suggest that large particles and deep export events are generated both by vertically migrating organisms importing organic carbon from the near-surface to the OMZ and by mesozooplankton constantly inhabiting the OMZ and transforming the particles there. Small suspended particles accumulate above and within the OMZ. This is likely contributed to by fragmentation via zooplankton partial feeding and reduced remineralization rates. Near-shore currents drive the temporal variability in hypoxia and the associated deep particle layers, hence modulating biological activity and export events in the mesopelagic. Our results highlight how the interplay of biological and physical processes drive the carbon dynamics of widespread hypoxic OMZs and provide insight into how to improve models in these regions.