Semi-enclosed bays are ideal for mariculture, but this activity modifies dissolved inorganic nitrogen (DIN: NH₄⁺, NO₂^-, NO₃^-) pools through organic matter enrichment, remineralization, nitrification (ammonia/nitrite oxidation, AO, NiO), and ammonium, nitrite, nitrate uptake (AU, NiU, NaU), converting coastal waters into biogeochemical reactors regulating N dynamics. This study examines seasonal DIN variation in Sansha Bay, the world&rsquo;s largest <em>Larimichthys crocea</em> farming zone, influenced by river runoff and coastal water masses. A two end-member mixing model revealed persistent net DIN accumulation in the main channel in winter and summer; western mariculture areas with more prolonged water residence showed contrasting characteristics: winter DIN accumulated alongside lower particulate organic carbon (POC) concentrations and lighter isotopic signature &delta;¹³C_DIC; summer showed NH₄⁺ loss, NO₂^- generation, bidirectional NO₃^- variation, higher POC and heavier &delta;¹³C_DIC. ¹⁵N incubation assays revealed suppressed winter N uptake and nitrification, consistently high AU and AO, sustained by NH₄⁺ released via intense remineralization. High NiO converted NO₂^- to NO₃^-, while weak NaU led to net NO₃^- accumulation. Strong vertical stratification governed summer N transformations. In surface layers, dominant AU and moderate NaU partially consumed NH₄⁺ and NO₃^-, with minor NiU yielding NO₂^- surplus. At depth, nitrification outpaced weak NiU and NaU, causing low NH₄⁺ but NO₂^- and NO₃^- buildup. This study reveals the regulation of DIN dynamics coupled to C and N biogeochemistry via interactions between water mass exchange and mariculture, and clarifies the controls exerted by N uptake and nitrification on DIN conditions and eutrophication risk in intensive mariculture coastal systems.