Upper Mesopelagic Bacterial Remineralization, an In Situ Perspective Across Diverse Ocean Provinces

Matthieu Bressac1, Alyson Santoro2, Emmanuel Christian Laurenceau-Cornec1 and Philip W Boyd3, (1)University of Tasmania, Institute for Marine and Antarctic Studies, Ecology and Biodiversity, Hobart, TAS, Australia, (2)University of California Santa Barbara, Ecology, Evolution, and Marine Biology, Santa Barbara, United States, (3)University of Tasmania, Institute for Marine and Antarctic Studies, Antarctic Climate Ecosystems Cooperative Research Centre, Hobart, Australia
Bacterial remineralization within the mesopelagic is a key particle transformation controlling the transfer efficiency of organic carbon into the deep ocean. Despite its importance, this mesopelagic process is one of the most poorly understood aspects of the biological carbon pump due to technical issues involved in studying this zone. Estimates of bacterial rates of particle remineralization in the mesopelagic zone are still scarce, and based on in vitro laboratory assessments using oxygen consumption and/or radio-tracers. Changes in pressure and temperature, when transferring samples from the mesopelagic zone to the laboratory, likely introduce a range of unknown biases to remineralization measurements. A novel approach called RESPIRE (REspiration of Sinking Particles In the subsuRface ocEan), provides rates of particle remineralization (predominately by particle-attached bacteria) under in situ temperature and pressure conditions. In recent years, this dual particle interceptor/incubator has been deployed within the upper mesopelagic (100-250 m depth) of contrasting oceanic regions, such as the subantarctic and polar Southern Ocean (GEOTRACES, COMICS), the low oxygen subsurface waters of the South Atlantic (COMICS), the oligotrophic Mediterranean Sea (GEOTRACES), and the low iron North-East Pacific Ocean (EXPORTS). These provinces, characterized by contrasting pelagic ecosystems, differing subsurface temperature and oxygen regimes offer a wide range of downward flux magnitude and composition. This diversity enables investigation of how these flux characteristics and particle remineralization rates are related. Here, we present a synthesis of these fluxes and discuss observed particle remineralization rates along with estimates of other processes potentially influencing the particle flux attenuation length-scale (e.g. diurnal vertical migration) to assess how and why the fate of the downward particle flux differs across these biogeochemical provinces.