Numerical simulation study on the effect of aquaculture tailwater discharge on nutrient salt distribution in Zhao’an Bay, Fujian Province

Background Reasonable utilization of the bay will strongly contribute to the prosperous development of China’s marine economy.

Objective The study aims to understand the impact of the discharge of culture tail water on the distribution of nutrient salts in Zhao’an Bay.

Methods The hydrological observation data of Zhao’an Bay in September 2022 was selected, combined with the survey data of the culture discharge outlets around the bay in April 2022, to simulate the migration of major nutrient salts after discharge from the discharge outlets in the bay.

Results The results showed that: 1) The variation of \mathrmNH_4^+ -N during high and low tides: during high tide, \mathrmNH_4^+ -N accumulates in the bay, with concentrations stabilizing between 0.03 and 0.30 mg/L. During low tide, the \mathrmNH_4^+ -N concentration on the east and west sides of the bay increases significantly, generally ranging from 0.10 to 0.30 mg/L; 2) Changes in TN at high and low tides: at high tide, the concentration of TN was as high as 0.30~1.00 mg/L in the north-central part of the bay and the area west of the Bachi Gate; as the tide level increased, the concentration of TN generally decreased, but the northwestern side of the bay still remained at a high level. At low tide, TN concentration increased, mainly concentrated in the shallow water area on the northwest side of the bay; 3) Changes in TP at high and low tides: at high tide, TP reached 0.15~0.30 mg/L in the north-central part of the Bay and the area west of Bachi Gate. At low tide, the concentration increased, concentrated in the shallow water area on the northwest side of the bay, and the concentration was higher in general, especially in the nearshore waters on the east and west sides of the bay, which was in line with the situation of TN.

Conclusion The migration and diffusion of \mathrmNH_4^+ -N, TN and TP are controlled by tidal currents, and the tidal currents determine the distribution and concentration of the three nutrient salts in the bay. The high concentration of the three nutrient salts are mainly found in the near-shore waters with small water depths on the east and west sides of the bay.