The influence of Agulhas leakage on primary production and nitrogen cycling in the Southeastern Atlantic Ocean

Abstract

The well-established importance of Agulhas Current “leakage” for Indian-Atlantic Ocean exchange of heat and salt raises the question of the relevance of Agulhas eddies for carbon and nitrogen cycling. We measured net primary production (NPP), nitrate and ammonium uptake, and nitrification along a transect of the Cape Basin (CB) in winter 2017, including across an anticyclonic Agulhas eddy sampled at high-resolution. Euphotic zone-integrated rates of NPP and nitrogen uptake were more than three-fold lower at the eddy center than at its edges, with intermediate rates measured outside the eddy. Additionally, proportionally more nitrate was consumed at the eddy edges than at its center. Mixed-layer integrated nitrification rates were highest at the eddy-center, with lower rates at the edges and in the surrounding basin. Accounting for phytoplankton consumption of newly nitrified nitrate in the eddy decreased the carbon export potential inferred from nitrate uptake (i.e., the f-ratio) by 66% ± 37% (vs. a 14% ± 22% decrease across the rest of the transect). At most stations, NPP and total measured nitrogen uptake were decoupled relative to stoichiometric expectations, with roughly a third of NPP possibly supported by regenerated dissolved organic nitrogen (DON). Accounting for DON uptake decreased the average f-ratio from 0.28 ± 0.26 to 0.22 ± 0.21, with the highest f-ratio estimated for an eddy-edge station (0.64) and the lowest for its center (0.00). While the low productivity metrics measured in the eddy-center could imply that Agulhas leakage decreases carbon export potential in the CB, enhanced nutrient supply and consumption at eddy edges may (partly) offset this reduction.