The sea area of Changhai County in Dalian City is a typical floating raft aquaculture area, located in Liaoning Province, China, where a key issue in determining the scale and spatial layout of the floating raft aquaculture is the assessment of the impact of aquaculture activities on the hydrodynamic environment. To address this issue, we established depth-averaged two-dimensional shallow water equations and three-dimensional incompressible Reynolds-averaged Navier–Stokes equations for the open sea area described in this paper. The impact of floating rafts for aquaculture on hydrodynamic force was reflected in the numerical model by changing the Manning number, where scenarios with different aquaculture densities were taken into account. Finally, the water exchange rate of the floating raft aquaculture area in the study area was calculated. It was found, through a comparison between the simulated value and the measured value obtained via layered observation, that the two values were in good agreement with each other, indicating that the model exhibits great accuracy. In addition, the calculation results for scenarios before and after aquaculture were compared and analyzed, showing that from low-density to high-density aquaculture zones, the variation in flow rate was greater than 80% at the peak of a flood tide. The water exchange rates of the water body after 1 day, 4 days, and 8 days of water exchange were also calculated, and the results show that they had been reduced by 17.92%, 13.59%, and 1.63%, respectively, indicating that the existence of floating rafts for aquaculture indeed reduced the water exchange capacity of the water body. The model described in this paper can serve as a foundation for other studies on aquaculture in open sea areas, and it provides a theoretical basis for the scientific formulation of marine aquaculture plans and the rational optimization of the spatial layout.

A monthly survey of the feeding selectivity of Ruditapes philippinarum in the Yalu River Estuary in 2020–2021 was conducted using high-throughput sequencing identification and visual grading technology. The results showed that the most-dominant species in the water of the shellfish culture area and in the stomachs of R. philippinarum was Karlodinium veneficum in those years. The selectivity index (E) indicated that R. philippinarum avoided consuming Bacillariophyta, Chrysophyta and Cryptophyta throughout the year and preferentially consumed Dinophyta and Chlorophyta. In 2020, the annual average biomass of Dinophyta, Bacillariophyta, Chlorophyta, Dictyochophyta, Cryptophyta and Chrysophyta in the stomach contents of R. philippinarum was 54:14:16:1:10:4; it was 41:12:28:0:1:17 in 2021. The annual average biomass ratio of picophytoplankton, nanophytoplankton and microphytoplankton in the stomachs of R. philippinarum was 13:48:39 in 2020; it was 14:66:20 in 2021. R. philippinarum actively fed on nanophytoplankton and avoided picophytoplankton. Among the phytoplankton of different sizes and groups that R. philippinarum prefer to feed, chemical oxygen demand (COD) and organic phosphorus (DOP) have a significant negative effect on the nanophytoplankton community, pH has a positive effect on the Dictyochophyta community and COD and the inorganic nitrogen to phosphorus ratio (DI-N/P) have a significant positive effect on the Chlorophyta community.

Floating raft aquaculture has gradually become a mainstream aquaculture model in the waters of Changhai County, Dalian. To quantitatively describe the impact of floating raft aquaculture facilities on the hydrodynamic environment of nearby sea areas, in this study, we took a single floating raft aquaculture structure as the research object and built a numerical prediction model for water flows passing through the floating raft aquaculture structure using a six-degree-of-freedom VOF (volume of fluid) multiphase flow simulation method based on an overset moving mesh system. Then, we verified the numerical model by utilizing oblique hydraulic jumps and water flows passing through a submerged bar. As shown by the findings, the simulated values are in good agreement with the theoretical solutions and measured values, indicating that the model features high precision and great stability. The impact of the raft area on the hydrodynamic force was introduced into the source term of an equation for consideration. In order to further determine the hindering effect of the raft body on the water body, transport equations and the tracer method were used to simulate the impact of floating raft aquaculture facilities on the water exchange performance of nearby sea areas. This study shows that the VOF multiphase flow model can be easily and accurately applied to studies on floating raft aquaculture, which can greatly reduce the limitations of experiments that utilize pure hydraulic models, wherein the impacts of floating raft aquaculture facilities on hydrodynamic force are generally considered simply based on observations, water roughness or the secondary drag force coefficient, thereby effectively improving the scientific understanding of the physical mechanism involved in floating raft aquaculture.

Winter ice-cover periods pose substantial risks to Apostichopus japonicus aquaculture in northern China. We combined field monitoring of three representative aquaculture ponds between 2014 and 2017 with controlled single-factor laboratory exposures to low temperature, hypoxia, and hyposalinity in adult and juvenile sea cucumbers. Field observations revealed pronounced temporal, vertical, spatial, and interannual variability in water temperature, salinity, and dissolved oxygen (DO) during freezing and ice-melting periods. All three stressors caused clear deterioration in physiological condition, with juveniles generally exhibiting greater short-term sensitivity than adults under low-temperature and hypoxic exposure, whereas severe hyposalinity caused pronounced deterioration in both life stages. Lactate, malondialdehyde (MDA), and glutathione (GSH) showed distinct treatment- and time-dependent responses, consistent with stress-associated changes in anaerobic metabolism, lipid peroxidation, and glutathione-associated antioxidant status. Integration of field observations with laboratory responses indicated that water temperatures approaching 0 °C and salinities approaching approximately 20‰ represent environmentally relevant conditions associated with elevated overwintering risk. DO concentrations approaching approximately 3 mg/L may warrant intensified monitoring and management intervention, whereas 2 mg/L represents a more severe experimental hypoxia condition. The more extreme treatments of −2 °C and 15‰ should similarly be regarded as severe experimental scenarios rather than commonly occurring field conditions. These environmental values should therefore be interpreted as preliminary management-oriented risk references rather than definitive physiological thresholds. Overall, the study provides an empirical field-to-laboratory basis for biologically informed winter environmental monitoring and risk-based overwintering management of A. japonicus aquaculture.

In this study, a numerical simulation method was used to explore the impact of changes in riverine runoff input flow variations on the marine environmental dynamics in the northern Yellow Sea of China. Based on the depth-averaged two-dimensional shallow water equations, a hydrodynamic and water quality coupling model was established to simulate the changes of the water environmental indicators under five conditions. Validation against filed-measured data confirmed the model’s great accuracy and stability. The findings show that interception activities had a relatively small impact on hydrodynamic conditions, and the changes in velocity did not exceed 10 cm/s; however, the salinity changed significantly. As the interception rate increased, the moving distance of the isohaline with a value of 5 towards the estuary gradually increased, with a maximum distance of 3420 m. Meanwhile, the amount of reduction of the area of the envelope curve with a salinity of 26.8 gradually increased, with a peak areal reduction rate of 10.7%. The amount of changes in nutrient concentration was related to the interception rate and the distance of a station from the estuary. The maximum percentages of changes in inorganic nitrogen and inorganic phosphorus contents were 5.39% and 6.34%, respectively. This study provides a technical methodology for evaluating the impacts of analogous riverine runoff variations on estuarine and adjacent ecosystems.

Coastal reclamation projects have drastically altered hydrodynamic conditions, sediment transport, and tidal exchange in semi-enclosed bays. However, quantitative research on the ecological restoration effects achieved through reclamation removal remains limited for high-latitude cold coastal bays, such as the northern Liaodong Bay. In this study, a well-calibrated two-dimensional depth-averaged hydrodynamic–sediment fully coupled numerical model was developed based on the finite-volume method, employing an unstructured triangular mesh for flexible discretization and local refinement of complex coastal boundaries. The Smagorinsky subgrid-scale turbulence parameterization scheme was incorporated to close the momentum equations, and a multi-component sediment transport module was coupled with a multi-layer bed erosion–deposition module to achieve two-way feedback between hydrodynamic and sediment processes. Four restoration test scenarios were designed, including the baseline status, full reclamation removal, partial reclamation removal, and full removal combined with tidal creek dredging, to quantitatively analyze the spatiotemporal responses of flow field, suspended-sediment concentration, bed topography evolution, and tidal prism. A total of five numerical simulations were conducted: one for model validation (covering water level, current velocity, and suspended-sediment data) and four for simulating the four restoration test scenarios. The results show that reclamation removal hardly changes the overall tidal flow structure and spatial distribution of suspended sediment in the bay. Variations in flow and sediment are confined to a narrow zone within 500 m around the project area, exerting negligible impacts on the entire Liaodong Bay and the Liaohe Estuary. Natural tidal creek networks, as vital geomorphic units, can buffer large-scale hydrodynamic disturbances. After full reclamation removal, the average tidal prism of the bay increased from 2.76 × 10⁷ m³ under the baseline condition to 2.79 × 10⁷ m³ (a 1.27% increase); under the combined scheme of full removal and tidal creek dredging, it further increased to 2.82 × 10⁷ m³ (a 2.17% increase). This improvement is more pronounced during spring tides. This study confirms that reclamation removal avoiding major tidal creeks is a low-disturbance ecological restoration approach suitable for semi-enclosed estuarine bays in northern China. A combined restoration scheme of full removal and tidal creek dredging is recommended to enhance tidal connectivity. These findings can provide quantitative references for coastal reclamation management and tidal-flat ecological restoration in temperate semi-enclosed bays.