Dynamics of nutrients in the Beibu Gulf: distributions, diapycnal transport, and budget during the autumn season.

Nutrients serve as the material basis for marine primary production, whose dynamics in coastal oceans are influenced by both terrestrial inputs and oceanic processes. Based on a field cruise conducted in autumn 2022, this study examines the dynamics and diapycnal transport of nutrients in the Beibu Gulf (BG). Surface nutrient concentrations exhibited an east-west gradient, with the eastern BG showing much higher concentrations, while the western basin remained oligotrophic. Four distinct water masses were identified, influencing the nutrient distributions in the BG. Stoichiometric analysis revealed that N:P < 16 prevailed in northern BG Coastal Water and South China Sea Subsurface Water, whereas the eastern BG close to Qiongzhou Strait was characterized by higher values (N:P > 16). Diapycnal nutrient fluxes were 0.0-2.0 (0.23 ± 0.12), 0.0-0.3 (0.021 ± 0.011), and 0.0-3.0 (0.29 ± 0.16) mmol m-2 d-1 for NO3- + NO2-, soluble reactive phosphorus, and silicate, respectively. These fluxes were predominantly controlled by stratification intensity, with turbulent kinetic energy dissipation rates as a secondary factor. The constructed nutrient budget revealed that lateral advection, primarily via the Western Guangdong Coastal Current, constituted the dominant external nutrient source. Notably, diapycnal nutrient transport, representing the internal nutrient supply, was comparable to the combined inputs from rivers, the atmosphere, and sediments, contributing 28.4%, 123.6%, and 105.0% of the N, P, and Si requirements of net community production in the BG, respectively.