Interactive dynamics of particulate and dissolved organic matter under sustained hypoxia conditions in a semi-enclosed bay (Jinhae Bay)

We investigated the isotopic variation of particulate and dissolved organic matter (OM; δ 13 C POC , δ 15 N PN , δ 13 C DOC ) associated with the frequent occurrence of hypoxia in a semi-enclosed bay (Jinhae Bay, South Korea). During the hypoxic period (August–October 2024), surface-water physical properties (temperature: 25.8 ± 3.0 °C; salinity: 30.5 ± 1.9 psu; dissolved oxygen: 7.0 ± 2.0 mg/L) coincided with significantly elevated concentrations of particulate and dissolved organic components (POC: 0.5 ± 0.2 mg/L; PN: <0.1 mg/L; DOC: 1.3 ± 0.3 mg/L; DTN: 0.3 ± 0.1 mg/L) throughout the water column. Under these conditions, the isotopic signatures of particulate and dissolved OM displayed distinct ranges (δ 13 C POC : −20.7 ± 1.8 ‰; δ 15 N PN : 4.8 ± 1.7 ‰; δ 13 C DOC : −22.1 ± 0.9 ‰), reflecting in situ remineralization of autochthonous sources such as phytoplankton. By integrating apparent oxygen utilization with a Bayesian end-member mixing model (C₃ terrestrial plants, marine phytoplankton, and tidal-derived coastal water), we suggest potential seasonal shifts in dominant mode of OM production from new production to regeneration, under progressively declining oxygen conditions. This transition may be linked to the utilization of reduced nitrogen compounds. In particular, autumn isotopic variations (Δδ 13 C POC : 2.4 ± 1.8 ‰; Δδ 15 N PN : −2.4 ± 2.4 ‰; Δδ 13 C DOC : 1.0 ± 1.4 ‰) may be consistent with a potential shift in nitrogen assimilation strategies, possibly involving the incorporation of regenerated dissolved nitrogen (i.e., DIN and DON) by phytoplankton under persistent hypoxia. Consequently, the isotopic signatures characterizing OM dynamics in this semi-enclosed bay provide a framework for interpreting OM interactions within ecological models.