Microbial Remineralization Is a Depth-Varying Contributor to Particle Flux Attenuation in the Southern Ocean

The biological carbon pump contributes to set the magnitude of carbon sequestration in the oceans' interior. Estimating the relative contribution of microbial versus zooplankton-mediated processes to particulate organic carbon (POC) flux attenuation provides insights into how this pump functions. Our study took place during the high productivity summer period in the Subantarctic and Polar Front Zone. In the upper mesopelagic (i.e., 180–300 m depth), we concurrently measured the downward POC flux, particle size and morphology, microbial remineralization rates and estimated size-specific sinking velocities. These concomitant measurements revealed two different export systems, dominated by fecal material in the Subantarctic, and phyto-aggregates in polar waters. These two systems were characterized by similar low particle sinking velocities (∼10 m d⁻¹), while microbial remineralization rates differed by an order of magnitude. Higher microbial remineralization rates in the Subantarctic (0.11 d⁻¹), compared to polar waters (0.04 d⁻¹), were likely driven by the confounding effect of temperature and particle characteristics. Despite this difference in microbial remineralization rates, these two export systems were characterized by relatively similar transfer efficiencies, suggesting that microbes had differing influences. A comparison of microbially mediated (i.e., scaled using observed remineralization rates) with total POC flux attenuation (i.e., driven by the dual impact of microbes and flux-feeders) revealed a higher microbial contribution to the flux attenuation in the upper mesopelagic of the subantarctic compared to the polar region. This deconstruction of the flux attenuation revealed an increasing influence of microbes on POC degradation with depth to become the predominant actor in the lower mesopelagic.