Wind-Driven Iron Supply by Ekman Buoyancy Flux Enhances Phytoplankton Bloom in the Antarctic Circumpolar Current

Abstract

The Southern Ocean today is a globally relevant sink for atmospheric carbon dioxide (${\text{CO}}_2$), where the biological uptake of carbon through primary productivity is largely controlled by widespread iron (Fe) limitation. We analyze observations from a submesoscale-resolving cross-section of the Antarctic Circumpolar Current (ACC) obtained around one thousand kilometers downstream of South Georgia in the Atlantic sector during austral spring 2022. Vertically integrated chlorophyll peaked in a strong ($\sim$250 mgChl-a m⁻²) phytoplankton bloom within the central portion of the jet associated with the Southern ACC Front. We infer that winds drove a northward Ekman transport of dense water across the front and that this destabilized the water column, leading to an Ekman Buoyancy Flux (EBF) and enhanced vertical mixing ($\sim$ $1\times 10$⁻² m² s⁻¹) across the base of the mixed layer. Using in situ measurements of dissolved iron, we estimate a net flux to the bloom of up to 3 $\mu$molFe m⁻²d^‒1 from a subsurface pool. This large flux can supply the same amount of Fe per unit area in one day as that supplied to the upstream Georgia Basin bloom through deep wintertime entrainment in 1 year. We calculate the bloom's daily Fe demand from in situ ⁵⁵Fe uptake measurements by phytoplankton and find it to be of a similar order of magnitude as the EBF-driven supply. We conclude that the bloom's strength and compact latitudinal extent are explained by EBF. Thus, EBF represents a previously understudied mechanism, which contributes to bloom patchiness and modulates biologically mediated ${\text{CO}}_2$ drawdown in the iron-limited Southern Ocean.