Date

Spring 2023

Document Type

Master's Thesis (Open Access)

Degree Name

Master of Science (M.S.)

Department

Moss Landing Marine Laboratories

Abstract

Quantitative and taxonomic resolution of the particles exported into the mesopelagic is necessary to provide the missing mechanistic connections that determine the strength of the oceanic Biological Carbon Pump (biological pump) and more specifically the fate of sinking carbon through the region of highest remineralization, the upper twilight zone (~95-500 m deep). Through the investigation of phytoplankton cells that individually settled into polyacrylamide gel layers (gels) in two oceanic basins, we were able to quantify the export of single phytoplankton cells in the upper mesopelagic. Solitary sinking phytoplankton cell (SSPC) fluxes were inventoried extensively in three successive deployments of sediment traps containing gels during the EXport Processes in the Ocean from RemoTe Sensing (EXPORTS) campaigns in the North Pacific Ocean Station Papa (OSP) in August-September 2018 and again at the North Atlantic Porcupine Abyssal Plain (PAP) in May 2021. We characterize this distinct export pathway as highly variable in space and time, exhibiting up to 7.5-fold magnitude change over a three-week period, and also decoupled from other biogeochemical flux proxies. We suggest that SSPCs in both ocean basins were primarily delivered to mesopelagic depths by particle disaggregation. We observed that cells continued to sink after being ejected from particles at depth, at rates of 5-17 m d-1, and that many deeply injected cells were still alive, providing a labile, high-value food source and potentially seeding future phytoplankton blooms. Although SSPCs do not account for a large percentage of total POC fluxes, their presence provides a proxy for important but transient, and often fine-scale, export processes in the mesopelagic that have continuously eluded direct observation.

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