Two Decades of Increase in Southern Ocean Net Community Production Revealed by BGC-Argo Floats

Abstract

Nitrate is an essential nutrient for phytoplankton growth and is a primary component of ocean carbon cycling. In this study, we developed a neural network constrained by the high spatial and temporal coverage of BGC-Argo floats to predict nitrate in a consistent way throughout space and time in the Southern Ocean, a key area for ocean carbon uptake and controlling global ocean nutrient distributions. After correcting for physical and sampling biases using the Biogeochemical Southern Ocean State Estimate model, we show that annual net community production (ANCP), originally calculated from seasonal nitrate drawdown, reveals the greatest production around the 45–55°S meridional band, and an average basin-wide ANCP of 3.91 ± 0.13 PgC y⁻¹ with a significant increase of 0.67% y⁻¹ from 2004 to 2022. We also highlight that using the common nitrate seasonal drawdown method to derive ANCP might underestimate the true carbon export at depth by about one third. Our findings align with previous studies, which indicate an increase in surface satellite chlorophyll-a and model export fluxes. Our results demonstrate the potential of leveraging machine learning constrained by BGC-Argo observations to study long-term changes of biogeochemical processes in the ocean.