Molecular and optical signatures of labile dissolved organic matter during bloom senescence in the Amundsen Sea Polynya

Abstract The Amundsen Sea Polynya (ASP) in the Southern Ocean serves as a potentially significant carbon sink, where phytoplankton blooms drive intense primary production and dissolved organic matter (DOM) cycling. This study focuses on DOM dynamics during the late‐summer bloom phase dominated by Phaeocystis antarctica . Despite the ASP's high productivity, DOM characteristics and their response to bloom senescence remain poorly resolved, limiting insights into carbon sequestration efficiency. We integrated absorbance and fluorescence spectroscopy with Fourier transform ion cyclotron resonance mass spectrometry to analyze seawater samples collected along transects in the ASP during austral summer 2022. Key findings include non‐exponential chromophoric DOM spectra with distinct shoulders at 270–290 nm (aromatic amino acids) and 300–350 nm (mycosporine‐like amino acids), elevated protein‐like fluorescent DOM components linked to P. antarctica degradation, and enrichment of labile, low‐molecular‐weight compounds (< 350 Da) with increased sulfur‐containing molecules from dimethylsulfoniopropionate oxidation. Bloom decline featured high dissolved organic carbon (0.85 ± 0.14 mg·L −1 ) amid reduced chlorophyll a (3.81 ± 0.77 mg·m −3 ), indicating DOM release under iron‐replete, nitrate‐limited conditions. This work advances knowledge by identifying unique phytoplankton‐derived DOM signatures that challenge conventional optical metrics like spectral slopes in polar systems, highlighting autochthonous production over terrestrial inputs. Broader implications include improved predictions of carbon cycling in expanding polynyas under climate change, emphasizing reduced sequestration due to labile DOM remineralization and its vulnerability to warming‐induced shifts in Antarctic ecosystems.