Semi-enclosed bay provides favorable conditions for mariculture, however, mariculture activities can disrupt hydrological connectivity of river-coastal continuum and create a bioreactor for particulate organic matter (POM). The impacts of increasing mariculture activities on POM dynamics, including sources, transportation, and transformation, associated dissolved oxygen and nutrients in semi-enclosed bays remain unclear. This study investigated carbon and nitrogen contents, as well as the stable isotopic compositions of POM (d¹³C_POC and d¹⁵N_PN) in a semi-enclosed bay, Sansha Bay, which is the largest mariculture bay for large yellow croaker in China and is influenced by riverine input and nearshore coastal waters. Our findings revealed that, spatially, riverine POM dominated along main channel that connects river inlet and bay outlet under the influence of water mixing (C:N ratios ~5.7-15.8, d¹³C_POC ~-21‰--24‰, and d¹⁵N_PN ~13‰-17‰). On top of water mixing, autochthonous POM dominates in off-main channel (C:N ratio 5-9, d¹³C_POC -21‰--17‰, and d¹⁵N_PN 7.0‰-8.5‰). Using a three end-member isotopic mixing model, our data shows that the POM in main channel comprises ~90% of riverine POM and ~10% of phytoplankton production, while in off-main channel, ~59% of POM was contributed by autochthonous phytoplankton production and the rest ~41% was sourced from main channel. Furthermore, riverine POM degradation consumes oxygen along main channel, forming consistent with the classic Redfield stoichiometry of aerobic respiration (N:O₂, ~0.13±0.02). In contrast, in off-main channel where water exchange is slower, the degradation of fish feed regenerates dissolved inorganic nitrogen, which subsequently stimulates phytoplankton production, leading to oxygen production following the canonical Redfield stoichiometry of phytoplankton photosynthesis (N:O₂, ~0.10±0.01). Our results highlight the influence of hydrological connectivity, superimposed by mariculture activity, in shaping POM biogeochemistry and dissolved oxygen dynamics in semi-enclosed bays. These findings provide important insights for the remediation of risks of hypoxia and harmful algal blooms in coastal mariculture ecosystems.

Carbon and nitrogen stable isotopes; Organic matter; Isotopic mixing model; Oxygen consumption and production; Mariculture; Semi-enclosed coastal bay.