The role of particle associated microbial respiration in mesopelagic particle flux attenuation in the NE Atlantic

The oceanic uptake of carbon dioxide (CO₂) through the biological carbon pump is intrinsically linked to atmospheric levels of CO₂ and hence global climate. Rapid changes in the flux of particulate organic carbon (POC) occur in the upper mesopelagic layer of the ocean, making it a key region for understanding both the current and future balance of oceanic and atmospheric CO₂. However, the practical difficulties of making measurements in this dynamic upper region of the ocean mean that the mechanisms controlling both the magnitude and efficiency of the biologically driven carbon flux to the deep ocean are still poorly understood, in particular the relative contributions of zooplankton and microbes to POC remineralisation. Here we present the first vertical profile of particle associated microbial respiration calculated from direct measurements on individual marine snow aggregates collected in situ.

Sinking particles were collected over the upper 500 m at a site in the NE Atlantic using Marine Snow Catchers, revealing a decline in POC flux of 76 %. To determine the role of particle associated microbial respiration in flux attenuation, we carried out micro-scale measurements of oxygen gradients through marine snow aggregates collected in situ. The calculated rates of microbial respiration cannot account for the observed steep declines in POC, explaining only 4 % of POC attenuation over the upper 130 m. However, 31 % of POC attenuation in the mesopelagic zone can be explained by particle associated microbial respiration, highlighting the increasing importance of microbial respiration deeper in the water column. Our results imply that zooplankton mediated processes, such as coprorhexy, coprophagy and coprochaly are therefore important drivers of POC flux attenuation in the upper mesopelagic.