A Circum‐Antarctic Plankton Isoscape: Carbon Export Potential Across the Summertime Southern Ocean.

The Southern Ocean accounts for ∼30% of the ocean's CO2 sink, partly due to its biological pump that transfers surface‐produced organic carbon to deeper waters. To estimate large‐scale Southern Ocean carbon export potential and characterize its drivers, we measured the carbon and nitrogen isotope ratios of surface suspended particulate matter (δ13CSPM, δ15NSPM) for samples collected in summer 2016/2017 during the Antarctic Circumnavigation Expedition (364 stations). Concurrent measurements of phytoplankton community composition revealed the dominance of large diatoms in the Antarctic and nano‐phytoplankton (mainly haptophytes) in open Subantarctic waters. As expected, δ13CSPM was strongly dependent on pCO2, with local deviations in this relationship explained by phytoplankton community dynamics. δ15NSPM reflected the nitrogen sources consumed by phytoplankton, with higher inferred nitrate (versus recycled ammonium) dependence generally coinciding with higher micro‐phytoplankton abundances. Using δ15NSPM and a two‐endmember isotope mixing model, we quantified the extent of nitrate‐ versus ammonium‐supported growth, which yields a measure of carbon export potential. We estimate that across the Southern Ocean, 41 ± 29% of the surface‐produced organic carbon was potentially exported below the seasonal mixed layer during the growth season, with maximum export potential (50%–99%) occurring in the Antarctic Circumpolar Current's southern Boundary Zone and near the (Sub)Antarctic islands, reaching a minimum in the Subtropical Zone (<33%). Alongside iron, phytoplankton community composition emerged as an important driver of the Southern Ocean's biological pump, with large diatoms dominating regions characterized by high nitrate dependence and elevated carbon export potential and smaller, mainly non‐diatom taxa proliferating in waters where recycled ammonium supported most productivity. Plain Language Summary: The Southern Ocean plays a crucial role in regulating climate by transferring atmospheric CO2 to the deep ocean. This transfer is partly facilitated by the biological pump, which involves organic matter production by phytoplankton in surface waters and the subsequent sinking of some of this material into the deep ocean. To estimate the fraction of newly produced organic carbon potentially exported to depth (i.e., biological pump strength) and determine the drivers thereof, we measured the carbon and nitrogen isotope ratios of surface organic matter and characterized the phytoplankton community at high resolution around Antarctica in summer. We found that nearly half of the carbon produced by Southern Ocean phytoplankton during their growth season was potentially exported. The biological pump was strongest in the southernmost waters of the Antarctic Ocean and near the (Sub)Antarctic islands where local processes supplied iron to phytoplankton and large diatoms dominated the community. Export potential was weakest in open Southern Ocean waters where iron limitation was pervasive and smaller phytoplankton were dominant. While it is well known that iron exerts a strong control on the Southern Ocean's biological pump, our study highlights the complementary role of phytoplankton community composition in modulating carbon export. Key Points: Phytoplankton community composition influences suspended particulate matter carbon and nitrogen isotope ratios across the Southern OceanThe nitrogen isotopes of particulate matter and a two‐endmember isotope mixing model can be used to estimate carbon export potential40% of summertime primary production is potentially exported, with a higher fraction exported near (Sub)Antarctic islands and melting ice [ABSTRACT FROM AUTHOR]