This study employs the MIKE 3 Flow Model, incorporating forcing conditions such as inflow from 18 major rivers along the Bohai coast, wind, precipitation, evaporation, and solar radiation, to develop a hydrodynamic and temperature-salinity model for the Bohai Sea, using a finer mesh for more detailed simulation in Laizhou Bay. The residual current in the surface layer primarily flowed eastward, exhibiting coastal transport characteristics in the southern region, leading to the formation of a large low-salinity region. The bottom salinity distribution closely mirrored that of the surface, with the isohaline shifting shoreward due to the high-salinity Bohai Sea water transported by the residual current. By grouping major runoff sources according to river outlet locations and residual current patterns, the study analyzed the impact of freshwater plumes formed by runoff from different directions on the salinity distribution in Laizhou Bay. The results indicate that the influence of freshwater inputs on both the mean salinity and the area of low-salinity zones in Laizhou Bay, ranked from greatest to least, is as follows: the Yellow River, the southwest, and the southeast. The variation in the area of low-salinity regions is closely related to factors such as runoff volume, residual currents, and the selection of boundaries for the low-salinity regions.

The Bohai Sea is the only semi-enclosed inland sea in China. With active marine economic activities, it faces a persistently high risk of oil spill accidents. This study assesses the occurrence risk and pollution risk of oil spills by considering factors such as sea state, the location of oil platform, and shipping route density in the Bohai Sea. The results show that the central part of the Bohai Sea, the southern Liaodong Peninsula, and the Bohai Strait area have a relatively high occurrence risk of oil spills due to busy maritime traffic and harsh sea conditions. In contrast, some areas in the northern, western, and southern parts of the Bohai Sea have a relatively low occurrence risk of oil spills because of weak maritime activity intensity and relatively calm sea state. In terms of the oil pollution risk, its distribution in the Bohai Sea shows significant seasonal characteristics, which are mainly comprehensively affected by multiple dynamic factors such as circulation, monsoon, and seawater exchange. Based on the oil pollution risk distribution, seasonally targeted strategies are proposed, which can provide a scientific basis for oil spill prevention and emergency management in the Bohai Sea, and help relevant departments formulate targeted prevention and control strategies.

Previous studies in the Beibu Gulf have documented the distributions and controlling factors of carbonate system parameters in surface waters; however, their dynamics in subsurface and bottom waters remain poorly constrained. Here, we present the first observations of pH and the aragonite saturation state (Ω_arag) in subsurface and bottom waters of the Beibu Gulf during spring and fall 2024. In spring, pH and Ω_arag in subsurface and bottom waters off Ledong were predominantly governed by physical mixing. In contrast, their variability off Dongfang was driven jointly by biological respiration occurring outside the Dongfang oil/gas field and by respiration within the field itself. Quantitative analysis revealed that respiration outside the field increased dissolved inorganic carbon (DIC) concentrations by ~18 ± 3 µmol kg⁻¹, a magnitude comparable to that attributable to in-field respiration (~18 ± 7 µmol kg⁻¹). Collectively, these two respiration sources induced only slightly declines in pH (0.03 ± 0.01 and 0.04 ± 0.01) but statistically significant reductions in Ω_arag (0.18 ± 0.03 and 0.21 ± 0.08, respectively). During fall, carbonate system dynamics in subsurface and bottom waters off both Dongfang and Ledong were dominated by physical mixing. The differential influence of North Pacific Tropical Water (NPTW) resulted in a decline in Ω_arag from 2.99 ± 0.10 off Dongfang to 2.75 ± 0.32 off Ledong. All observed Ω_arag values remained well above the thermodynamic saturation threshold (Ω_arag = 1), indicating no immediate risk of ocean acidification to calcifying organisms in the study area under current conditions. This study helps bridge a critical observational gap in the regional inorganic carbonate system. However, due to sampling constraints, carbonate system parameters are reported here for spring and fall only. Observations for summer and winter, as well as multi-year time-series measurements, are planned for the near future.

This study aimed to investigate the acute and chronic toxic effects of two thermodynamic inhibitors (methanol and ethylene glycol) widely used in deep-sea oil and gas operations on two typical marine fish species, the medaka (Oryzias melastigma) and the yellowstripe goby (Mugilogobius chulae), to assess their potential ecological risks in marine environments. The 96-h median lethal concentration (LC₅₀) was determined through acute toxicity tests. A 56-day chronic toxicity test was conducted to evaluate the effects on fish growth (body length) and the antioxidant defense system, specifically the activities of superoxide dismutase (SOD) and catalase (CAT). The results revealed marked species-specific differences. In terms of acute toxicity, medaka exhibited exceptionally high sensitivity to ethylene glycol (LC₅₀ 15.77 g/L), while the yellowstripe goby showed greater tolerance (LC₅₀ 22.17 g/L). Chronic exposure led to concentration-dependent growth inhibition in both species, and medaka showed significantly higher mortality than yellowstripe goby. Under methanol exposure, medaka exhibited significantly higher mortality (30–45%) than yellowstripe goby (5–20%). When exposed to ethylene glycol, medaka showed markedly high mortality (55–85%), while yellowstripe goby mortality remained below 15%. At the molecular level, both chemicals induced oxidative stress, but the response patterns of the antioxidant enzymes (SOD and CAT) were species-specific, indicating differences in toxic mechanisms and detoxification capacities. Methanol and ethylene glycol pose non-negligible ecotoxicological risks to marine fish, and the toxicity intensity is influenced by species specificity, exposure concentration, and the effectiveness of the antioxidant defense system. This study emphasizes that environmental risk assessments for such chemicals must fully account for species differences and sublethal effects, providing critical scientific evidence for formulating precise environmental safety standards for marine hydrocarbon exploitation.