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The hydrodynamics and water quality evolutions in response to the land reclamations in Bohai Bay were investigated by developing a coupled hydrodynamics-water quality model. Simulated results indicate reclamations considerably change the residual current fields and decrease the tidal prisms. After 1990–2006 reclamations, salinity isolines in the west of the bay obviously move towards open sea, and cause the decrease of salinity value. Compared with the 2006 reclamation scenario, the 2022 reclamation scenario generates larger salinity concentration, thereby intensifying salinity intrusion. Besides, a large amount of dissolved inorganic nitrogen (DIN) is retained in the bay and increases its concentration value due to 1990–2006 reclamations, which almost presents opposite variation in the 2022 reclamation scenario. 1990–2006 reclamations increase phytoplankton carbon concentrations in the most bay areas, with a maximum increased value of 53.37 μg/L. With the further construction of reclamations (2006–2022), phytoplankton carbon concentrations increase within several estuarine areas, nevertheless the decreases appear in the large areas from estuarine mouths to bay's center. Statistically, the averaged salinity concentration slightly changes in the whole bay, while the averaged DIN and phytoplankton carbon concentrations significantly increase by 14.92% and 13.33% during 1990–2006, then they decrease by 16.02% and 20.68% during 2006–2022, respectively. [Display omitted] • A coupled hydrodynamics-water quality model is established in Bobai Bay. • Land reclamations change residual current fields and decreases tidal prisms. • Land reclamations affect spatial distributions of water quality parameters. • Averaged dissolved inorganic nitrogen and phytoplankton carbon increase by 14.92% and 13.33% after 1990–2006 reclamations. • Nevertheless they decrease by 16.02% and 20.68% due to 2006–2022 reclamations. [ABSTRACT FROM AUTHOR]

The mechanism that underlies the evolution of the seasonal environment and the effects of reclamation projects in the Pearl River Estuary (PRE) were studied by modeling the hydrodynamics, suspended sediment, and water quality. Due to the seasonal variation of the freshwater discharge, the residual currents in the estuary are generally larger during the summer than during the winter. Reclamation projects significantly affect the residual current fields and decrease tidal prisms across the PRE, resulting in the variation of water exchange and material transport process. During the summer, waters high in chlorophyll -a move downstream, with increased seaward residual currents, and they more widely range from the middle estuary to the estuarine mouth than during the winter; moreover, the chlorophyll -a value peaks in these areas. Reclamation projects significantly promote the expansion of waters high in chlorophyll -a towards the estuarine mouth. The distribution of suspended sediment, nitrate, ammonium, and silicate also obviously varies with altered water exchange and chlorophyll -a distribution. Average chlorophyll -a , suspended sediment, and silicate concentrations in the whole computational domain increase by 12.71%, 15.19%, and 1.91%, respectively, while the growth of algae consumes a large amount of nitrate and ammonium. • Reclamations in the Pearl River Estuary significantly affect the water quality. • Chlorophyll -a , suspended sediment, and silicate concentrations increase after reclamations. • Nitrate and ammonium concentrations decrease by 28.82% and 19.09%, respectively. [ABSTRACT FROM AUTHOR]

The time required for water renewal in an estuary is closely related to its self-purification ability and environment degree. A FVCOM-based water age (WA) model was conducted to investigate the renewal timescale in the Pearl River Estuary (PRE) under various hydrodynamic scenarios. Model sensitivities revealed the depth-averaged WA during the summer was significantly smaller than that during the winter; meanwhile, the region with vertically significant variations in the WA varied from almost the whole estuary to the downstream. It can be explained that the seaward residual currents became stronger and narrowed the region of vertical circulations by impeding salinity invasion and stratification. During typhoon Hagupit, the depth-averaged WA was apparently increased in the most of the estuary because the horizontal water circulations between the estuary and open sea were significantly decreased by the strong up-estuary surge-waves. Moreover, the vertical WA distribution almost became a single-layer structure with the severely weakened vertical circulations. After reclamations, the variable shorelines in the PRE significantly affected the hydrodynamic characteristics and water circulations, thus the depth-averaged WA complicatedly changed, and the vertical difference of the WA values slightly increased in narrowed waterways and their downstream due to the limited effects on the vertical circulations. • A 3D water age (WA) model is built to quantify water renewal in the Pearl River Estuary (PRE). • The WA in the PRE was characterized by significantly spatial distributions and seasonal variations. • The WA was significantly increased in the most areas and less stratified in vertical direction during typhoon Hagupit. • After reclamations, the WA complicatedly changed in horizontal direction and became more stratified in vertical direction. [ABSTRACT FROM AUTHOR]