Exploring nitrification's impact on nitrate assimilation: Dual isotope insights from the Amundsen Sea, Antarctica

Abstract

Summer nitrification's role in nitrate assimilation remains poorly quantified in the high‐latitude Southern Ocean. This study presents the first nitrate δ 15 N and δ 18 O data from the rapidly melting Amundsen Sea, showing that nitrification is active throughout the continental shelf mixed layer, while nitrate assimilation dominates in adjacent open ocean waters. Box modeling estimates that nitrification supports 20 ± 14% of mixed‐layer nitrate assimilation, with no continental shelf and open ocean difference. Redundancy analysis indicates that nitrification covaries with multiple factors, with particulate nitrogen being the strongest environmental predictor, suggesting a potential role for ammonium availability in shaping nitrification patterns. The isotope effect of nitrate assimilation varies spatially, with lower values on the continental shelf, potentially related to increased nitrification and iron availability. These findings fill a critical gap by quantifying the role of mixed‐layer nitrification in nitrate assimilation and identifying key environmental correlates of nitrification variability in the high‐latitude Southern Ocean.

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