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.

In temperate coastal waters, phytoplankton communities often follow a classical seasonal pattern, with diatoms dominating in spring and dinoflagellates becoming more prominent during summer. However, spring-to-summer phytoplankton community reorganization in eutrophic Liaodong Bay remains poorly characterized. Here, we examined seasonal changes in phytoplankton communities and environmental conditions in Liaodong Bay in spring and summer 2025. The phytoplankton community underwent a marked spring-to-summer reorganization that differed from the classical seasonal pattern. In spring, dinoflagellates accounted for 63.51% of total community abundance, with the heterotrophic dinoflagellate N. scintillans comprising 99.56% of this fraction. By contrast, diatoms comprised 91.02% of the summer community, with Coscinodiscus granii, Skeletonema costatum, and other diatom taxa sharing dominance. This reorganization coincided with a seasonal shift in phosphorus regime and nutrient stoichiometry, from low dissolved inorganic phosphorus (DIP; 0.12 μmol P L⁻¹), dominance of dissolved organic phosphorus (DOP; 0.28 μmol P L⁻¹) and high N:P in spring to higher DIP (0.66 μmol P L⁻¹), lower DOP (0.18 μmol P L⁻¹), lower N:P, and higher Si:N in summer. Correlation and redundancy analyses indicated that the spring N. scintillans-dominated assemblage was broadly aligned with DOP-rich and high-N:P conditions, whereas summer-dominant diatoms were broadly aligned with DIP, dissolved silicate (DSi), and Si:N gradients, although not all bivariate relationships were significant. These patterns suggest that potential relative inorganic phosphorus deficiency in spring may have been less favorable for diatom dominance, whereas higher DIP, lower N:P, and higher Si:N in summer were consistent with a resource regime more favorable for multi-diatom expansion. Overall, the phytoplankton community in Liaodong Bay exhibited a non-classical seasonal reorganization from a spring N. scintillans-dominated assemblage, rather than a conventional autotrophic dinoflagellate-dominated stage, to summer multi-diatom co-dominance, coinciding with shifts in phosphorus regime and nutrient stoichiometry.

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.