Nitrogen can accumulate in the mariculture ecosystem and further enhance the risk of eutrophication, harmful algal blooms, and hypoxia within coastal bays. However, little is known about the key transport and transformation processes of nitrogen (oxidation and assimilation) and the controlling mechanisms under the pressure from mariculture. This study focuses on Sansha Bay, the largest large yellow croaker mariculture site in China and aims to investigate the oxidation rate of nitrogen (ammonium (NH₄⁺), nitrite (NO₂^-), and urea) and the assimilation rate by phytoplankton. Our results indicated that the oxidation rates of NH₄⁺, NO₂^-, and urea (AOR, NOR, and UOR) increased vertically with the increase in depth. Specifically, the AOR in the asparagus area (H₁, 208.18-2576.29 nmol L^-1 d^-1) was significantly higher than that of NOR (0-1106.99 nmol L^-1 d^-1) and UOR (76.10-552.90 nmol L^-1 d^-1), corresponding to high NO₂⁻ (15.60-15.80 μmol L^-1) and extremely low NH₄⁺ (0.10-0.17 μmol L^-1). Similarly, the AOR in large yellow croaker area (H₂) was 294.95-2669.02 nmol L^-1 d^-1, greater than the NOR (173.91-1509.15 nmol L^-1d^-1) and UOR (5.37-21.76 nmol L^-1d^-1). It is interesting to note that the UOR (27.13 nmol L^-1 d^-1) at H₂ was much lower than that at H₁ (628.99 nmol L^-1 d^-1). This can be attributed to the strong organic matter remineralization, which may produce a large amount of NH₄⁺ (as high as ~3.90 μmol L^-1) and thus inhibited the utilization of urea. Contrary to the oxidation rate distributions, the uptake rate decreased vertically with the increase in depth. Phytoplankton absorbed NH₄⁺ and urea before NO₂^- and nitrate (NO₃^-), with nitrogen preference index of NH₄⁺ 1.20-135.86 and urea 0.13-9.73. At H₁, the nitrate uptake rate (2.29-52.78 nmol L^-1 h^-1) was greater than that of nitrite uptake rate (0.55-47.62 nmol L^-1 h^-1), ammonium uptake rate (6.67-40.03 nmol, L^-1 h^-1), and urea uptake rate (2.43-19.42 nmol L^-1h^-1). The high nitrate and nitrite uptake rates at H₁ may be stimulated by high NO₃^- (23.10-24.50 μmol L^-1) and NO₂^- (15.6-15.8 μmol L^-1), and extremely low NH₄⁺ (0.10-0.17 μmol L^-1). While a high ammonium uptake rate (5.81-48.11 nmol L^-1h^-1) at H₂ was observed and was greater than that of urea (0.21-4.62 nmol L^-1 h^-1), which may be stimulated by a high NH₄⁺ (0.52-2.10 μmol L^-1). This study aims to achieve a better understanding of nitrogen dynamics in coastal bays with heavy mariculture and quantitatively evaluate the impact of mariculture on the marine environment of the entire bay.

 

Nitrogen uptake; Oxidation rate; 15N isotope labeled technique; Mariculture;