This study analyzed the horizontal and vertical distribution characteristics of temperature and salinity in the central and eastern regions of the Beibu Gulf, based on conductivity measurements in summer 2022, temperature, and depth (CTD) measurement data from the Sino–Vietnamese cooperative project “Demonstration Study on Ecological Protection and Management in Typical Bays: Seasonal Survey of the Beibu Gulf”. Furthermore, the study utilized the computational results from the numerical Finite-Volume Coastal Ocean Model (FVCOM) to elucidate the intrinsic patterns that formed the temperature and salinity distribution characteristics in August 2022 from both thermodynamic and dynamic perspectives. The circulation in the Beibu Gulf drives external seawater to move northward from the bay mouth. During this movement, numerous upwelling areas are created by lateral Ekman transport. The formation of different scales of cyclonic and anticyclonic vortices and current convergence zones is influenced by topography, runoff, and the water flux from the Qiongzhou Strait, which are key factors in the formation of upwelling and downwelling. The surface circulation in August 2022 significantly differed from the 20-year average surface circulation, with an influx of 1.15 × 10⁴ m³/s more water entering the Beibu Gulf compared to the multi-year average. The water flux from the Qiongzhou Strait is a critical factor affecting the circulation patterns in the Beibu Gulf. The northeastern waters of the Beibu Gulf are characterized by current convergence zones, where extensive upwelling occurs. The rich nutrient salts in these areas promote the reproduction and growth of phytoplankton and zooplankton, making this the most favorable ecological environment in the Beibu Gulf and serving as a natural reserve for fisheries, coral reefs, dugongs, and Bryde’s whales.

The diffusion characteristics of thermal discharge from coastal power plants were studied by analyzing the Ningde Nuclear Power Plant and Kemen Power Plant, which are located in different geographical regions in China. The former is in the open sea, and the latter is in a well-sheltered bay. In the vicinity of the outfall areas of the two power plants, large-area surface temperature observations and tidal current observations were conducted. The results indicate that the thermal discharge diffusion characteristics of coastal power plants located in different geographical environments are significantly different. In the well-sheltered sea area of the Kemen Power Plant, the water temperature diffuses faster along the coast, in line with the direction of tidal movement, and slower in the offshore direction under the influence of rectilinear tidal currents within the bay, resulting in a significantly greater longitudinal diffusion distance of thermal discharge along the shore than the transverse diffusion distance offshore. In the area surrounding the Ningde Nuclear Power Plant, rotational currents diffuse the thermal discharge in various directions, causing the range of temperature rise to expand toward the outer sea. Dominant tidal currents within the tidal cycle in the sea area can influence the distribution of high-temperature rise zones near outfalls. The distribution of high-temperature rise zones predominantly occurs on the side with the higher average tidal velocity, either the ebb tide side if its velocity is greater than that of the flood tide or the flood tide side otherwise.

Utilizing data from the four survey voyages in the Beibu Gulf from 2021 to 2022, this study investigates the variations in water temperature and salinity at different survey periods across its central and eastern regions, analyzing their horizontal and vertical distribution patterns. The research further explores the relationship between these distributions and the gulf’s circulation dynamics, elucidating their underlying formation mechanisms. The results indicate that the variations of water temperature and salinity are obviously different in different periods. Horizontally, spring and summer survey voyages exhibit a transition from high-temperature (max 31.90 °C), low-salinity (min 29.54 PSU) waters in the north to low-temperature (min 21.10 °C), high-salinity (max 34.19 PSU) waters in the south, while autumn and winter survey voyages show low-temperature (min 18.57 °C), low-salinity (min 29.54 PSU) waters in the north shifting to medium-to-low-temperature (21.52–28.20 °C), high-salinity (max 34.19 PSU) waters in the south. Vertically, the spring survey period displays distinct stratification with multiple thermoclines and haloclines in different regions. The summer survey period exhibits the strongest thermohaline gradients, with intensified thermoclines and haloclines accompanied by regional up- and downwelling. In the autumn survey period, vertical mixing homogenizes temperature and salinity, while the winter survey period shows near-complete mixing, effectively eliminating stratification. These patterns are driven by a combination of oceanic dynamics and regional climatic factors, including monsoon forcing, solar radiation, coastal runoff, horizontal circulation, and up-/downwelling processes.

The runoff of the Jiulong River (JLR) is a key parameter that affects the estimation of pollutant flux into Xiamen Bay (XMB). The precise runoff estimation of the JLR can be used to determine the accuracy of the pollutant flux estimation flowing into XMB. In this study, to analyze the hydrological dynamic characteristics and identify the correlation between fixed-site real-time ocean current observations and cross-sectional navigation flow observations, we conducted six navigation observations on two cross-sections of the JLR estuary during the spring tide and neap tide in the normal season, wet season, and dry season in 2020. Simultaneously, we measured hydrological observation data by a fixed-site buoy located in the JLR estuary and collected runoff data that were measured upstream of the JLR. The results showed that the average correlation coefficient between the average velocity of the fixed-point buoy and average velocity of the section was more than 0.90, higher than expected, the minimum average deviation was 4%, and the minimum sample standard error was 5.7%, which was a good result. In this study, we constructed a model for estimating the runoff of the JLR into the sea. The findings demonstrated that Acoustic Doppler Current Profiler (ADCP) online monitoring data were useful to estimate runoff of the JLR with high accuracy, could promote the accuracy of estimated pollutant flux of the JLR’s discharge into XMB, and could provide more scientific and reliable basic data for future load flux estimation research.

The coupling of physical transport and phase-transfer processes represents a fundamental mechanism governing metal cycling in estuarine systems under tidal oscillations. Taking Quanzhou Bay as a model system, we conducted continuous observations and sample collection at the river channel (Q1), the turbidity maximum zone (Q2), and the outer bay channel (Q3). The metals (Al, Ti, Ba, Cu, Mn, and Zn) were measured by ICP-MS to systematically investigate the distribution, transport, and inter-media transfer across multiple water layers under varying estuarine processes. Our findings demonstrate that particulate metal concentrations in Quanzhou Bay exhibit strong synchrony with suspended sediment concentrations (SSC) over tidal cycles, displaying a distinct sediment-following pattern controlled by alternating end members. Particulate metal fluxes during flood and ebb-tides generally followed the hierarchy Q1 > Q2 >> Q3. Notably, stations Q1 and Q2 were dominated by flood-tide fluxes with net transport directed landward, whereas Q3 was characterized by ebb tide dominance with net flux directed seaward—revealing a spatial division of labor between “inner bay retention/reallocation” and “outer bay channel export”. In contrast, dissolved metals exhibited marked element-specific responses to tidal forcing: Al and Ti increased during flood tides at stations Q1 and Q2, while Ba and Cu showed opposite trends, and Mn and Zn displayed more conservative behavior. Concurrently, solid/liquid partition coefficient (logKd) values for Al, Ti and Ba, Cu exhibited inverse patterns over tidal cycles, suggesting divergent adsorption–desorption regulation under identical hydrodynamic conditions that drives differential phase-transfer dynamics. These disparities likely reflect intrinsic chemical properties and source variations among the elements. This study elucidates, at the tidal timescale, the coupled processes of “alternating end-member control—estuarine filter modulation—concurrent channelized export and inner bay retention” in Quanzhou Bay, providing critical process-level insights for metal flux quantification and bay pollution remediation initiatives in an ecological restoration project.