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 m...