High‐Resolution Particle Imaging Reveals the Importance of Phytodetrital Aggregate Flux and Mesopelagic Grazing for Carbon Storage in the Southern Ocean

Abstract

The biological, chemical, and physical processes in the mesopelagic zone that impact carbon flux to the deep ocean are poorly understood, particularly in the Southern Ocean, despite their importance to deep ocean carbon storage. Using a CTD‐mounted underwater imaging system, the Underwater Vision Profiler 5, we observed distributions of large marine particles across two basin‐scale global ocean ship‐based hydrographic investigations program repeat hydrography transects in the Pacific and African sectors of the Southern Ocean. Utilizing deep learning features and unsupervised clustering, 2.6 million images of particles were classified by type into 46 detrital categories within four supercategories (e.g., fluffy aggregates, dense aggregates, fibers, feces) and 61 living categories within four supercategories (e.g., crustaceans, rhizarians, gelatinous zooplankton, Trichodesmium ). Different frontal zones generally have distinct patterns in particles, which illuminate different flux pathways. We observed a “Fluffy Aggregate Flux Pathway”, where high fluffy aggregate abundance was observed from the surface to 3,000 m at the same time as elevated surface chlorophyll a . We also observed a “Zooplankton Mediated Flux Pathway,” where grazers were located primarily in the lower mesopelagic zone, and feces and dense aggregates were in high abundance in the abyssopelagic zone. These results highlight two export pathways in the Southern Ocean, both of which contribute to long‐term carbon storage at depth.