Seasonal Re-Entrainment of Respired Organic Matter Decouples Southern Ocean Surface Production and Annual Net Community Production

Abstract

The long-term storage of carbon by the biological carbon pump depends on the amount of organic carbon produced in sunlit waters, the depth at which that organic carbon is respired, and the time until the respired carbon is again in contact with the atmosphere. In the Southern Ocean, estimates of organic carbon export, net community production (NCP), and export efficiency vary in their magnitude and variability. Results from different methods are often difficult to compare due to inconsistent time and depth scales. To address these discrepancies, we leveraged biogeochemical Argo float observations to explore the relationship between organic carbon production during seasonal phytoplankton blooms, estimated from a mixed layer nitrate budget, and the annual carbon removal from the mixed layer, estimated from oxygen-derived respiration rates calculated on isopycnals. We compared these estimates to a POC flux data set. Our findings reveal a median of 47 ± 21% of seasonal organic carbon production is respired and re-entrained into the mixed layer in the subsequent winter, decoupling surface seasonal NCP from annual NCP (ANCP). Due to this decoupling, satellite-derived estimates of export production are correlated with NCP during seasonal blooms but not with ANCP. The average fraction of organic carbon respired and re-entrained is similar across distinct regions due to compensation between respiration rates beneath the seasonal mixed layer and the depth of the winter mixed layer. These results quantify the importance of accounting for winter entrainment when estimating ANCP and assessing the strength and efficiency of the biological carbon pump.