Phytoplankton size impact on export flux in the global ocean

TLDR

The efficiency of the ocean’s biological carbon pump depends on phytoplankton‑mediated export and remineralization, yet satellite studies have mainly used chlorophyll and NPP, with recent data now enabling consideration of phytoplankton size. The study aims to clarify how phytoplankton size structure influences particle export, remineralization, and transfer. Using a global dataset of sediment‑trap and 234Th‑derived POC flux, the authors constructed annual climatologies of NPP, microplankton fraction, and flux, and fitted depth‑dependent export parameters for different regions and seasons distinguished by dominant phytoplankton size. The results show that productive regions exhibit high export flux yet low transfer efficiency, whereas oligotrophic regions show the reverse, and that periods dominated by small phytoplankton have higher export flux efficiency but lower transfer efficiency compared to large‑cell dominated periods.

Abstract

Abstract Efficiency of the biological pump of carbon to the deep ocean depends largely on biologically mediated export of carbon from the surface ocean and its remineralization with depth. Global satellite studies have primarily focused on chlorophyll concentration and net primary production (NPP) to understand the role of phytoplankton in these processes. Recent satellite retrievals of phytoplankton composition now allow for the size of phytoplankton cells to be considered. Here we improve understanding of phytoplankton size structure impacts on particle export, remineralization, and transfer. A global compilation of particulate organic carbon (POC) flux estimated from sediment traps and 234 Th are utilized. Annual climatologies of NPP, percent microplankton, and POC flux at four time series locations and within biogeochemical provinces are constructed. Parameters that characterize POC flux versus depth (export flux ratio, labile fraction, and remineralization length scale) are fit for time series locations, biogeochemical provinces, and times of the year dominated by small and large phytoplankton cells where phytoplankton cell size show enough dynamic range over the annual cycle. Considering all data together, our findings support the idea of high export flux but low transfer efficiency in productive regions and vice versa for oligotrophic regions. However, when parsing by dominant size class, we find periods dominated by small cells to have both greater export flux efficiency and lower transfer efficiency than periods when large cells comprise a greater proportion of the phytoplankton community.