Nutrient Budget and Biogeochemical Dynamics in Sansha Bay, China: A Coastal Bay Affected by Intensive Mariculture

AbstractThis study examines the spatiotemporal distribution of dissolved and particulate nutrients in Sansha Bay, a coastal bay characterized by complex hydrodynamics (monsoon‐driven river inputs and alongshore coastal waters), and intensive mariculture. However, nutrient stoichiometry and underlying biological responses in this ecosystem remain unclear. High nutrient (39.6–87.5 μmol L−1 dissolved inorganic nitrogen (DIN), 1.79–3.77 μmol L−1 dissolved inorganic phosphorus (DIP)) in winter suggested entrainment of the nutrient‐enriched China Coastal Current superimposed with mariculture inputs; lower nutrient concentrations (9.6–47.1 μmol L−1 DIN, 0.31–1.57 μmol L−1 DIP) in summer, and high Chlorophyll a (0.8–17.0 mg m−3) and dissolved oxygen (4.48–8.78 mg L−1), were driven by the exchange of nutrient‐poor coastal waters and enhanced biological consumption. Using the Land‐Ocean Interaction Coastal Zone (LOICZ) mass balance model, field observations of nutrient sources/sinks allowed construction of a robust nutrient budget, in which Δ indicated non‐conservative dissolved nutrient fluxes. In both winter and summer, the external DIN and DIP additions ratio ΔDIN:ΔDIP was ∼21.8–20.4:1, while Si(OH)4 addition/removal occurred seasonally. Potential N biogeochemical processes were also explored. Using a three end‐member mixing model, nutrient biogeochemistry (regeneration/uptake, denoted by δ) within the aquaculture ecosystem was estimated separately from complex physical mixing effects. In winter, nutrient regeneration dominated (δDIN:δDIP ∼20:1; δSi(OH)4:δDIN <1:2), likely due to remineralization of particles (phytoplankton with a 16:1 Redfield ratio and/or fish feed/feces with a non‐Redfield ratio). In contrast, nutrient consumption dominated in summer (δDIN:δDIP ∼13.5:1; δSi(OH)4:δDIN ∼2.4:1). Results of this study are relevant to other coastal bays affected by intensive aquaculture, riverine inputs and coastal waters.