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. Key Points Different export systems were dominated by fecal material in the subantarctic and phyto-aggregates in polar waters Sevenfold range in microbial remineralization rate was likely driven by the confounding effect of temperature and particle characteristics Multi-faceted observational, laboratory and modeling approaches reveal a depth-varying role of microbes in setting POC flux attenuation Plain Language Summary In the surface ocean, plankton fix dissolved atmospheric CO2 into particulate organic carbon (POC) that sinks into the ocean's interior, that is the POC flux. On its way down, POC is consumed by microbes and zooplankton, resulting in a depth-decrease of POC flux—the POC flux attenuation. The magnitude of the attenuation is set by the sinking rate and consumption rate of POC which varies widely throughout the ocean. Using multiple observational and modeling approaches at contrasting sites of the Southern Ocean, we specifically explore the role of microbes in the flux attenuation. We found that the relative role of microbes becomes increasingly important with depth (from ∼120 to 500 m depth), to become the predominant actor deeper than 200 m depth. Our work supports the need to visualize the POC flux attenuation as a depth-varying interplay of zooplankton and microbes to understand what drives its magnitude. This would improve computer simulations that predict how much carbon is stored in the ocean's interior.