Effects of sinking velocities and microbial respiration rates on the attenuation of particulate carbon fluxes through the mesopelagic zone

Abstract The attenuation of sinking particle fluxes through the mesopelagic zone is an important process that controls the sequestration of carbon and the distribution of other elements throughout the oceans. Case studies at two contrasting sites, the oligotrophic regime of the Bermuda Atlantic Time‐series Study (BATS) and the mesotrophic waters of the west Antarctic Peninsula (WAP) sector of the Southern Ocean, revealed large differences in the rates of particle‐attached microbial respiration and the average sinking velocities of marine particles, two parameters that affect the transfer efficiency of particulate matter from the base of the euphotic zone into the deep ocean. Rapid average sinking velocities of 270 ± 150 m d −1 were observed along the WAP, whereas the average velocity was 49 ± 25 m d −1 at the BATS site. Respiration rates of particle‐attached microbes were measured using novel RESPIRE (REspiration of Sinking Particles In the subsuRface ocEan) sediment traps that first intercepts sinking particles then incubates them in situ. RESPIRE experiments yielded flux‐normalized respiration rates of 0.4 ± 0.1 day −1 at BATS when excluding an outlier of 1.52 day −1 , while these rates were undetectable along the WAP (0.01 ± 0.02 day −1 ). At BATS, flux‐normalized respiration rates decreased exponentially with respect to depth below the euphotic zone with a 75% reduction between the 150 and 500 m depths. These findings provide quantitative and mechanistic insights into the processes that control the transfer efficiency of particle flux through the mesopelagic and its variability throughout the global oceans.