Manganese (Mn) is an essential trace element for the marine ecosystem. As the transitional zone between rivers and oceans, estuaries have a significant effect on dissolved Mn and its terrigenous input. In this study, the distribution of dissolved Mn, investigated in the surface water of Changjiang River Estuary and its adjacent area during September 2019 (autumn), March 2021 (spring) and July 2021 (summer), was analyzed by automatic solid phase extraction and inductively coupled plasma mass spectrometry. The results indicated that the average concentration and estuarine behaviors of dissolved Mn showed seasonal variations between the three cruises: the maximum average concentration occurred in summer and dissolved Mn was removed firstly and then added with the increase of salinity; the medium average concentration occurred in autumn and dissolved Mn was mainly removed with the increase of salinity; the minimum average concentration occurred in spring and dissloved Mn was mainly conservative with the increase of salinity. The results of significance test indicated that only in the season when the fresh water had high concentrations, dissloved Mn carried by the Changjiang River had significant influence on the distribution of dissolved Mn in the Changjiang River Estuary and its adjacent area; the distribution of dissolved Mn was co-dominated by a variety of biogeochemical processes in the season when the concentrations were low in the fresh water. The high suspended particulate matter concentrations were the important factor of the removal of dissolved Mn in the Changjiang Estuary’s water with medium-low salinity. And the addition mechanism of dissolved Mn in water with high salinity needs further vestigation.

Based on the data from 56 acoustic survey sections in the southwestern Indian Ocean from 2011 to 2020, a total of 201 diel vertical migration were observed. The characteristics of diel vertical migration of the deep scattering layer and its spatiotemporal differences were analyzed. The research results show that the deep scattering layer in the southwestern India Ocean exhibits a stratification phenomenon, with the first scattering layer located in the shallow water layer below 200 m. The average depth of its nautical area scattering coefficient (NASC) peak is (58.66 ± 24.63) m, and there is a significant difference between summer and winter (p < 0.001); the second scattering layer is located in the water layer between 400 m and 700 m, with an average depth of (589.02 ± 66.33) m for its NASC peak. There is no significant difference between summer and winter (p = 0.51). The average time for the scattering layer to migrate upwards is 16:20, the average time for the end of migration is 18:31, and the average migration rate is (5.28 ± 1.53) cm/s; the average time for the scattering layer to start migrating downwards is 4:38, and the average time for the end of migration is 6:52. The average migration rate is (5.56 ± 2.13) cm/s. As latitude increases, the start time of downward migration become later and the migration rate slows down; as the longitude increases, the migration rate of the scattering layer also slows down, and there is a significant difference between different longitude sea areas (p < 0.001). Analysis suggests that studying the seasonal changes in the physical and chemical environment of the sea area, as well as the different life cycle stages of organisms in the scattering layer, are the main reasons for the spatiotemporal differences in the vertical structure and diurnal vertical migration characteristics of the scattering layer. They are of great significance in explaining the diurnal vertical migration behavior of tuna and indicating the distribution of tuna fishing grounds.

The semienclosed pyloric caecum of Larimichthys crocea is an ideal organ to perform the host source tracking, as it containes several local bacteria colonized during early development. The alpha diversity, relative abundance of core bacteria and network relationship of the pyloric caecum microbiota in L. crocea from Sansha Bay were analyzed using the Illumina high-throughput sequencing. Furthermore, the random forest model was used to predict the population source (the wild population or the farmed population). The results showed that the farmed fish had more unique OUT and higher alpha diversity than the wild one. The wild and farmed fish significantly differed in the relative abundance of dominant bacteria (Proteobacteria, Firmicutes, Bacteroidota, Actinobacteria and Acidobacteria) (p < 0.05). The result of network analysis showed that the wild fish had higher ratio of negative to positive edges and modularity, but fewer nodes and edges than the farmed fish. Furthermore, a random forest classification prediction model with the accuracy of 92.31%, the Kappa coefficient of 0.8452, and an area under the ROC curve of 0.952 4 was constructed. Using this prediction model, the accuracy of source identification for the wild and farmed fish was 91.67% and 92.86%, respectively. Overall, the structure of microbial communities in the pyloric caecum of L. crocea was different between the wild and farmed populations, making it a potential marker for host source tracking. Our findings provide new insights into distinguishing wild and farmed L. crocea.

The ocean acidification effect is increasing with the large amount of CO₂ emissions. To investigate the effects of future seawater acidification on Ruditapes philippinarum, a control group (pH = 8.1) and acidification group (pH = 7.7, 7.1 and 6.4) were set up for 42 days. The changes in tissue structure, immune and antioxidant enzyme activities of Ruditapes philippinarum under acidification conditions were measured, as well as the effects produced at the molecular level. The results show that when Ruditapes philippinarum are placed in an acidified seawater environment, gill filament spacing expands with decreasing pH, gill filament cilia adhere, and the pipes and outer epidermal folds of the mantle gradually deepen. The activities of acid phosphatase (ACP) and superoxide dismutase (SOD) in gill tissues show a pattern of decreasing followed by increasing. Alkaline phosphatase (AKP) activities exhibit different trends in each group. Total antioxidant capacity (T-AOC), catalase (CAT), and lysozyme (LZM) activities show a pattern of increasing followed by decreasing. Glutathione peroxidase (GSH-Px) activities in gill and visceral masses show a continuous increase. LZM activity in the viscera group displays varying trends, while ACP activity shows a decreasing and then increasing pattern. AKP, SOD, and CAT activities exhibited an increasing and then decreasing pattern, while T-AOC activity shows a continuous decrease. Analysis of the transcriptome reveals that the GO functions in gill tissue are mainly enriched in DNA integration, integral components of the membrane, and RNA-directed DNA polymerase activity, among others. The KEGG pathway analysis shows enrichment in the phagosome pathway and pathways related to protein processing in the endoplasmic reticulum. The acidification of seawater caused varying degrees of damage to the tissues of Ruditapes philippinarum, disrupting its internal environmental homeostasis and altering metabolic levels and immune-related gene expression, and led to an increased risk of disease and even death in Ruditapes philippinarum.

This paper investigates the characteristics of turbulent structures in combined wave-current boundary layers based on the standard $ k $-$ \varepsilon $ model. Good agreements were found between the numerical results and experimental data of the time-averaged mean velocity profiles. Periodic variations of turbulence parameters within a wave cycle (i.e. vorticity magnitudes, TKE and TKE dissipation rates etc.) were observed. The vorticity magnitudes, TKE and TKE dissipation rates all decrease during the deceleration phase, reach their minimum values during the wave trough, increase during the acceleration phase and reach their maximum values during the wave crest. The variations of turbulent structures are very high in the near-wall regions (53% for TKE dissipation rates), and are quite low in the outer regions (3% for TKE dissipation rates). The wave-current boundary layer thickness increases (decreases) during the deceleration phase (acceleration phase). The model developed in this study has solved the existing issue of low accuracy in the near-bed region by previous models based on the “high Reynolds number methods”. The present model performs well in describing the physical process of turbulence variations under the effects of wave-current interaction. This can provide some guidance for the sediment transport in coastal areas, beach erosion prediction and developments of marine renewable energy.

The intraseasonal variability (ISV) associated with mesoscale eddies in the northern South China Sea has been significant. Comparing the intraseasonal variability of flow at different times helps to elucidate the influence of mesoscale eddies with various dynamic instability, therefore, this study analyzed the dynamic instability of mesoscale eddies in the spring of 2009 and 2020 in the northern South China Sea, to figure out the characteristics of intraseasonal variability. Based on mooring velocity data, it conducted kinetic spectral analysis, and the results demonstrated that the ISV with period of 10‒60 days in 2009 and the ISV with period of 30‒90 days in 2020 displayed a similar vertical feature, with strong intraseasonal signals primarily occurring in the upper layer above 200 m. Moreover, the ISV with period of 30‒90 days was the main intraseasonal component during the corresponding observation period. Lag-regression analysis and calculation of dynamic instability showed that the intraseasonal variation of spring 2009 was affected by fast-moving but weak surface mesoscale eddies, and the dynamic instability was modulated by baroclinic instability and barotropic instability. On the other hand, the intraseasonal variability in the spring of 2020 was influenced by strong baroclinic mesoscale eddies, which might trigger the occurrence of intraseasonal variability in the flow fields more rapidly through enhanced vertical shear of velocity. The findings of this study contribute to a deeper understanding of the impact mechanisms of mesoscale eddies on intraseasonal activities in the northern South China Sea, providing important references and a theoretical basis for ocean dynamics and climate research.

Previous research has conducted extensive work on sediment grain size testing, data analysis, and information extraction. However, studies on whether the collected sediment samples are representative are still relatively rare. Based on indicators such as the mode, median grain size, particle size distribution, sediment type, and grain size parameters, this study compared and analyzed the quantitative and qualitative results of the grain size distribution parameters of 30 parallel sediment samples collected from beaches in the southern part of Leizhou Peninsula, China. The study examined the differences, correlations, and consistencies of these parameters. The results showed that: (1) the mode, median grain size, and particle size distribution of parallel samples exhibited a certain degree of variability; (2) the skewness and kurtosis of parallel samples showed significant differences, with average values of P1 ranging from –2.90 to –1.53 and 11.23 to 21.59, P2 ranging from –2.55 to –1.52 and 11.23 to 21.59, and P3 ranging from –2.81 to –1.86 and 13.41 to 27.69; (3) about two-thirds of the combinations of grain size parameters had no correlation (R² ≤ 0.29); (4) qualitative results of sorting coefficients, skewness, and kurtosis showed differences in 1/3, 1/2, and 4/15 of the cases, respectively. The spatial heterogeneity of sediment grain size distribution information and the randomness of sample collection are the main reasons for these differences. To minimize the randomness of sample collection and characterize the spatiotemporal heterogeneity of grain size distribution information, parallel samples are recommended for future sample collection. This study provides a typical case of comparing the results of parallel sample grain size parameters for beach sediment, which can improve our understanding of effective sediment sample collection strategies.

Research on the reconstruction of underwater three-dimensional temperature and salinity fields and the acquisition of acoustic field characteristics based on satellite sea surface observations has significant practical value in military oceans and other fields. However, its effectiveness not only depends on the reconstruction method but also changes with different sea surface observations used. Although there are few reports on related research, it has significant guiding value for the design of satellite sea surface observation schemes. In this study, based on the latest variational method applied by the US Navy, the influence of the vertical gradient of temperature and salinity and the sea surface height, sea surface temperature, and their joint use on the reconstruction of three-dimensional temperature and salinity and acoustic field characteristics were investigated. The results showed that the reconstruction scheme incorporating the three constraint items had the highest accuracy, with average reconstruction errors of 1.08℃ for temperature field and 0.11 for salinity field, and could better capture the spatial features of the temperature and salinity fields. By analyzing the spatial characteristics of different schemes, the sea surface temperature mainly plays a role in capturing the temperature and salinity characteristics of the shallow region of the mixing layer, which has a great influence on the Sound Layer Depth (SLD). Both the sea surface height and the vertical gradient of warm salt field can improve the inversion accuracy of mixed layer and deep area, which can affect the accuracy of the whole sound velocity profile. According to the analysis of acoustic characteristics, when SST, SSH, and the gradient were constrained simultaneously, the SLD had the smallest difference from HYCOM in the shallow sound speed, which was about 1 m/s. When there was no gradient constraint, the SLD differed significantly from HYCOM and failed to reflect the surface duct characteristics. It can be seen that the surface duct is more sensitive to sea surface temperature and gradient constraints.

The tectonic evolution of the northwestern continental margin of the South China Sea were controlled by many factors, such as the Red River Fault, the Hainan mantle plume and the formation and evolution of the South China Sea. The Yinggehai Basin is located in the northwest of the South China Sea, where thick Cenozoic sediments were deposited. The Cenozoic evolution history of the northwestern continental margin of the South China Sea was recorded in detail in the Yinggehai Basin. However, which factor has mainly controlled the evolution of the Yinggehai Basin since the Cenozoic is still ambiguous. In this paper, 7 drilling wells and 23 simulated wells were selected in the Yinggehai Basin, and the sedimentation rate and subsidence rate of the Yinggehai Basin were reconstructed by empty basin tectonic subsidence analysis method. The deep structure of the Yinggehai Basin was simulated by gravity inversion method based on previous study. The results show that during the rift period, the subsidence rate of the Yinggehai Basin in the north and middle sections is larger than in the south section. There are two stages of “step acceleration subsidence” in the north and middle sections, which are 23–11.7 Ma BP and 11.7 Ma BP–present, respectively. The maximum tectonic subsidence rate can be up to 80 m/Ma in the post rift stage in the north section and about 110 m/Ma in the middle section. The maximum tectonic subsidence rate of the Southern Graben and Uplift is both close to 70 m/Ma during the periods of 11.7–5.7 Ma BP and 15.9–11.7 Ma BP, respectively. The Cenozoic subsidence of the Yinggehai Basin is consistent with the change of sedimentary rate, indicates that tectonic subsidence plays a significant role on sedimentary evolution. According to the results of gravity inversion, it may be lower crust high density anomaly intrusion underlying the Yinggehai Basin. We suggested it may be basic rock according to the drilling basalts in the sediments. By comparing with the subsidence rate of surrounding basins in the South China Sea, the acceleration subsidence in the 15.9–11.7 Ma BP may be related to the disappearance of secondary mantle convection due to the ceasing of the seafloor spreading of the South China Sea. The 5.7 Ma BP–present accelerated subsidence of the Yinggehai Basin may be related to the dextral strike-slip activity of the Red River Fault.

The Zhujiang River Mouth Basin is located in the northern shelf of the South China Sea, and its tectonic evolution is very complicated. At the same time, as one of the hydrocarbon-rich basins in offshore China, Zhujiang River Mouth Basin ranks the top in oil and gas production among the offshore basins. With the deepening of exploration, the boundary of the Zhujiang River Mouth Basin has been changing, and the southern boundary of the basin is disputed. In this paper, based on satellite gravity and magnetic anomaly data, the fault distribution of Zhujiang River Mouth Basin is re-studied and the plane location of igneous rocks is identified. The Cenozoic thickness of the study area is retrieved with high-precision seismic data, and the tectonic units of the study area are adjusted according to the processing results. The results show that the fault distribution in the study area is mainly NEE, NE and NW trending. In this study, four new faults were identified based on the satellite gravity data. The overall strike of the igneous rocks in the plane is NEE strike, but there are differences among different regional strikes. There are nearly EW trending igneous rocks in the western basin, NWW trending igneous rocks in the central basin, and no other strike in the eastern basin. According to gravity data and Cenozoic characteristics, Yangjiang Sag is divided into Yangjiang West Sag and Yangjiang East Sag, and Wenchang D Sag and Wenchang E Sag are deleted. The Beiweitan Fault is the same as the Yangjiang-Yitong Fault, which divides the basin into three parts: the west, the middle and the east. The fault strike, the plane position of igneous rock, the thickness of Cenozoic and the tectonic units of the three parts are different. The study of faults, igneous rocks and tectonic units in Zhujiang River Mouth Basin aims to provide geophysical support for subsequent basin research and oil and gas breakthrough.