Estuarine environmental capacity is the foundation for coastal biological diversity and self-purification capacity. Hence, studies on the marine environmental capacity (MEC) are the foundation for the total discharge control and water quality improvement of land-based pollutants. In the article, A calibrated two-dimensional hydrodynamic model was used to study the environmental characteristics of Sanmen Bay, including the tides, the residual currents, the tidal prism, and water exchange abilities. The model results were used to estimate the environmental capacity of the bay. Taking the pollution problem in Sanmen Bay as an example, the method of response factor, the sub-unit control method, and the phased control method were used to estimate the environmental capacity, pollutant amounts, and the pollutant reduction in the bay. The concentrations of COD, inorganic nitrogen, and acid salt in Sanmen Bay are spatially varied, with higher values occurring in the western part and in the inner bay. The half exchange time of the whole bay is about 23 days, and the exchange time of 95% water body is about 60 days. The evaluation of MEC cannot only provide technical support for the offshore aquaculture industries but also provide a scientific basis for the total control of terrigenous pollutants in coastal cities in Southern Zhejiang Province.

The water environment in estuaries is a crucial factor affecting the biodiversity and self-purification capacity of coastal zones. This study focuses on Sanmen Bay as an example to study the characteristics and temporal variations of the water environment in the turbid coastal waters on the East China Sea coast. The field data of hydrodynamics and water environment from 2018 to 2023 including different seasons in the bay were collected and analyzed. We analyzed the correlation between water environmental factors and sediment and explored the impact of sediment mixing layers on the water environment. Field data indicate that water temperature, dissolved oxygen content, and suspended sediment concentration (SSC) vary seasonally. In summer, the water temperature and SSC are the highest; in autumn, the dissolved oxygen content is the highest. Salinity and pH values showed little variation from 2018 to 2023. The concentration of oils in sediments across the entire area within Sanmen Bay varied from 2018 to 2023, which decreased from (30.6–92.2) × 10⁻⁶ mg/L to below 10⁻⁶ mg/L. Correlational analysis indicates that dissolved oxygen concentration and heavy metal content were correlated with sediment in 2018, with correlation coefficients of approximately 0.5. Sediments impact the water environment through changing stratification and mixing due to suspended particulate matter and through changing water environment parameters (e.g., heavy metal) due to bed sediment erosion. The bulk Richardson number in most areas is larger than 0.25. These results indicate that sediment impacts heavy metals in Sanmen Bay. In highly turbid waters, sediments are more likely to adsorb heavy metals and other pollutants, thereby impacting water quality.

The turbidity maximum zone (TMZ) plays an important role in the morphology and ecosystems of estuaries. The distributions of TMZ in Hangzhou Bay (HZB) and mechanisms of sediment fluxes in TMZ have been studied by applying a fully calibrated three-dimensional baroclinic model that considers salinity, high turbidity, sediment flocculation, and density coupling between water and sediment. Based on the method of SSC thresholds, the TMZ is mainly distributed from YG (Yanguan, toponymy in HZB) to ZP (Zhapu, toponymy in HZB). S1 and S2 are the cross-sections within the TMZ. Along the estuary, there is an ascending-to-descending trend observed in the monthly averaged bed current shear stress, while the monthly averaged density displays a descending-to-ascending pattern. The orientation of the monthly averaged surface fluxes of suspended sediment within the TMZ shows a clockwise circular pattern. Moreover, the net flux of suspended sediment along the estuary at section S1 is seaward, while at the section S2, it is landward. The high bottom stress and the sediment fluxes facilitate the retention, deposition, and enrichment of sediment within the TMZ. The suspended sediment fluxes at the two cross-sections was primarily influenced by Eulerian transport, Stokes transport, and tidal pumping transport. At cross-section S1, the sediment transport is dominated by tidal pumping. At cross-section S2, advective transport controls the sediment transportation near the deep northern bank; while near the southern bank of cross-section S2, tidal pumping is still the dominant factor. The outcome of this study provides a foundation for TMZ research in similar macro-tidal turbid estuaries around the world.