Biofloat observations of a phytoplankton bloom and carbon export in the Drake Passage

Abstract

Prolonged in-situ biogeochemical observations have historically been limited in the high nitrate, low chlorophyll (HNLC) regions of the Southern Ocean (SO). Recently, large biofloat missions, including the SO Carbon and Climate Observations and Modelling (SOCCOM) program, are filling the data void. However, the standard ten-day float profiling cycle often lacks the temporal resolution required to resolve mesoscale phytoplankton bloom or export processes. This is particularly important in regions of the SO dominated by high energy mesoscale features, like the Drake Passage. In this study, we observe a naturally occurring phytoplankton bloom and subsequent carbon export event in the Drake Passage using in-situ data from an Autonomous Profiling EXplorer (APEX) biofloat that profiled the water column every two days. The fast, two-day profiling cycle meant that the biofloat's trajectory was coherent with mesoscale processes. However, because of the quasi-Lagrangian nature of the biofloat, we could not control for both spatial and temporal changes simultaneously. Therefore we explore this quasi-Lagrangian dataset as both a spatial and temporal series to understand the mesoscale processes as they relate to the observed phytoplankton bloom and its subsequent export. Our evidence suggests that eddy-driven subduction and a change in the vertical structure were important in the time period leading up to enhanced carbon export. Recent studies have shown that eddy-driven subduction may be responsible for ~20% of the total biological carbon pump in the SO. We estimate that the POC flux out of the surface ocean was 250 ± 50 mg m−2 d−1 between the peak bloom through an observed export event. In addition, we estimate that ~18 ± 10% of the particulate organic carbon observed during the peak bloom was exported out of the surface ocean via subduction and sinking.