Multi-year robotic observations reveal the seasonality of downward carbon export pathways in the Southern Ocean

Abstract At high latitudes, the export of organic matter from the surface to the ocean interior, the biological carbon pump, has conventionally been attributed to the gravitational sinking of particulate organic carbon (POC). Conspicuous deficits in ocean carbon budgets have recently challenged this long-lived paradigm of a sole pathway. Multiple strands of evidence have demonstrated the importance of additional export pathways, including the particle injection pumps (PIPs). Recent model estimates revealed that PIPs have a comparable downward POC flux to the biological gravitational pump (BGP), but with potentially different seasonal signatures. To date, logistical constraints have prevented concomitant and extensive observations of these pumps, and little is known about the seasonality of their fluxes. Here, using year-round robotic observations and recent advances in optical signal analysis, we concurrently investigated the functioning of two PIPs - the mixed layer and eddy subduction pumps - and the BGP in Southern Ocean waters. By comparing three phytoplankton bloom cycles in contrasting environments, we show how physical forcing and phytoplankton phenology influence the magnitude and seasonality of these pumps, with implications for carbon sequestration efficiency.