Isotope measurements were performed on dissolved nitrate (NO₃⁻) and ammonium (NH₄⁺) in the coastal waters of the northeastern Beibu Gulf, China, to investigate the seasonal nitrate sources and their biogeochemical processes, which are due to the rapid development of local industrialisation and urbanisation. The high N/P ratio observed in the coastal bay during both fall and spring suggests that P is a limiting nutrient, which in turn indicates that increasing P causes conditions favourable for algal blooms. Higher nutrient concentrations and δ¹⁵N-NO₃⁻ and δ¹⁵N-NH₄⁺ values were found in the nearshore area in the fall, suggesting that nutrients originated mainly from land-based pollution. A Bayesian isotope mixing model was used to calculate the contribution of potential NO₃⁻ sources and the results showed that in the nearshore area, NO₃⁻ originated mainly from manure and sewage (58%). In the spring, however, in addition to the impact of urban sewage effluents, the exchange of sediment and water was another important factor causing higher nutrient concentrations and positive NO₃⁻ isotopes in the nearshore area. There were lower concentrations of nutrients and an increase in δ¹⁵N-NO₃⁻ and δ¹⁵N-NH₄⁺ values in the offshore area in the fall, and the NO₃⁻ loss in the surface water was mainly caused by the process of assimilation. However, the exchange of sediment and water was the dominant factor causing higher nutrient concentrations (except for NO₃⁻) and positive dual nitrate isotopes but lower NO₃⁻ concentration in the offshore area during the spring. Overall, isotope analysis of NO₃⁻ and NH₄⁺ helps to illustrate the major sources of the former and their biological transformation in the northeastern Beibu Gulf.

Based on the ship-based data in the Lianzhou Bay, an advection-diffusion model of pollution is established. The sharing coefficient field (SCF) and the responding coefficient field (RCF) are simulated by this model. The environmental captivity (EC) is calculated by the simplex method according to the water quality standard. Results showed that that for seawater of Grades I to IV (according to the National Standard), the annual ECs of DIN in the Lianzhou Bay were 374.13

Nourishment grain size is a key parameter in beach nourishment projects, directly determining beach stability under extreme hydrodynamic environments. Taking Xiaoshizui Beach on Wailingding Island as the study area, this paper establishes a coupled typhoon storm surge–wave–sediment model based on the MIKE 21 HD-SW-ST coupled model. This model has been systematically verified through the measured data of tide levels, waves, and beach profiles, and the verification results are satisfactory. Four scenarios with nourishment grain sizes of 0.4, 0.6, 0.8, and 1.0 mm were established to quantify the morphological evolution patterns of the beach under strong typhoons. The results indicate that during the typhoon, the beach exhibits a cross-shore sediment transport pattern characterized by erosion of the backshore dune, accretion of the upper-middle foreshore, and erosion of the lower foreshore. The influence of nourishment grain size shows significant spatial variability: increasing grain size enhances the erosion resistance of the backshore and berm, reducing the erosion extent; however, within the breaker zone, coarse sand tends to form a steep profile, intensifying wave breaking, which increases the scour depth in this region. This study elucidates the regulatory mechanism of grain size under extreme conditions, providing scientific reference for grain size selection in beach restoration projects.

Based on the hydrodynamic background field, a Variational Assimilation (VA) Model of pollutant in the Bohai Sea is constructed. The time-varying contamination concentrations (CCs) from the pollution source (PS) are estimated using this model and a new inversion theory is proposed. Some points in the time series are selected as the independent points (IPs), while concentrations of other points are calculated through linear interpolation of these IPs. Through twin numerical experiment, all the given time-varying concentrations are successfully estimated through adjoint method. The cost function decrease greatly, and the ratio of the cost function to its initial value can even reach the magnitude of 10-5 , indicating that the adjoint technique is computationally efficient to recover the CCs from the PS.