The Bohai Sea is an important economic zone of China. Sea ice has been a significant threat to the human activities around the Bohai Sea. As the imaging capability of synthetic aperture radar (SAR) is independent of sun illumination and cloud condition, it is of great significance to detect the sea ice of the Bohai Sea from SAR images. Due to the limitation of the feature extraction mechanism, the accuracies of traditional sea ice detection methods need to be improved. Deep learning has a strong self-learning ability and is suitable for image detection. Here, we employ the well-known deep learning framework, U-Net, as the basic structure, and design a hybrid loss function to optimize the U-Net model, forming a hybrid loss U-Net model for sea ice detection in the Bohai Sea. The Sentinel-1 dual-polarization (VV and VH) SAR images are the inputs of the model. We compare the hybrid loss U-Net model with several traditional methods (Pulse Coupled Neural Network, Markov Random Field and Watershed Algorithm) and deep learning method based on CNN. Experiments show that the hybrid loss U-Net-based model achieves 97.567%, 98.769%, 98.767% and 98.771% in IoU, F1_Score, Precision and Recall respectively, outperforming the other methods. Compared with VV single-polarized input, the detection results of dual-polarized information input are 0.375%, 0.111%, 0.639% and 0.740% higher in F1_Score, Precision, Recall and IoU respectively. The detection results of the hybrid loss model are 1.129%, 0.947%, 1.794% and 2.231% higher than those of the non-hybrid loss function in F1_Score, Precision, Recall and IoU respectively. The model could effectively detect details such as ice water line, inter-ice water and ice gap. Our model is applied to detect the sea ice of a whole SAR image in the Bohai Sea, which can provide technical supports for sea ice monitoring, sea ice change analysis and sea ice prediction.

It is crucial to evaluate seafloor massive sulfides (SMS) in terms of their special distribution. Transient electromagnetic method (TEM) is ideal for land mineral deposits prospecting, but the complicated seafloor topography, inner structure of sulfide deposits and measuring conditions in the hydrothermal field pose a great challenge to its application in the ocean. In order to verify the application potential of TEM in deep-sea exploration, a 3D forward modeling scheme was developed with finite element method, combining with bathymetry data and drilling results. The method was then applied to the TAG hydrothermal field, Mid-Atlantic Ridge, and the result was well fitted with acquired TEM data. By comparing the forward simulation results under different instrument positions, attitudes and altitude, we found that the coincident loop system could effectively detect the active TAG mound ore body when the altitude was less than 60 meters. Complex seafloor topography and how the instrument was being towed could significantly disturb the early time response, while the instrument attitude also made an influence on the detected signal. Therefore, it was necessary to combine the bathymetry data, instrument positioning data and attitude data in the research area to better interpret the measured TEM responses.

The asymmetry of flood and ebb plays an important role in the process of sediment transport and geomorphological evolution, which is a significant feature of the flow field in estuaries. The erosion and accumulation, sediment transport and sediment characteristics in the Huanghe River Estuary based on the measured topography, sediment particle size, hydrological and sediment observation data were analyzed in this paper. The Delft 3D model was used to simulate the flow field in the Huanghe River Estuary, and the spatial distribution of magnitude differences between flood and ebb velocities under different conditions were calculated. Combining the above, the dynamic mechanism of erosion and accretion in coastal area of the Huanghe River Estuary was discussed. The results show that there are multiple siltation and erosion centers distributed between the active river mouth and the Laizhou Bay. There is a noticeable asymmetry of flood and ebb velocities in the area, and there is the flood dominant area off the active river mouth, extending the Laizhou Bay southward in a tongue shape, while the near shore and the Laizhou Bay are dominated by ebb. The erosion and deposition in the Huanghe River Estuary are largely controlled by the spatial distribution of flow velocity asymmetry and the conversion of dominant flow. The strong north wind strengthens and expands flood-dominance, and promotes siltation and coarsening of sediment in the Laizhou Bay.

Marine heatwave, considered as events with prolonged anomalously high sea surface temperature in certain sea areas. The occurrence of marine heatwave could impact the function and service of marine ecosystem. In recent years, the marine heatwave in the China offshore become more frequent, and attract considerable attention. Used a range of ocean temperature data including daily satellite observations and daily in situ measurements, we analysised the characteristics of marine heatwave in the China offshore in 2019. Results show that in 2019, the frequency of marine heatwave is higher in the sea areas around Hainan Island, the Changjiang River Estuary, the Bohai Sea and the sea areas off Jiangsu Province, with the frequency of 7−12 times. The marine heatwave duration in the Beibu Gulf lasts the longest, more than 150 days. In addition, took the classical marine heatwave that last from February to April in Beibu Gulf as an example, anomaly characteristics of air temperature, sea level pressure and wind during the event period were investigated specifically and the corresponding relationships between the marine heatwave and meteorological background fields in different seasons in Beibu Gulf was further studied. On the whole, the higher air temperature, weaker East Asian winter monsoon, larger area and the northward and westward extension of subtropical high are the important factors for the occurrence and maintenance of the marine heatwave in the Beibu Gulf in the early 2019.

As a common submarine geological disaster, wave-induced seabed liquefaction seriously threatens the safety of subsea engineering facilities in the Huanghe River Delta. The structure, physical and mechanical properties of seabed soil after wave-induced liquefaction all have changed, so it has important theoretical significance and practical value to study on the evaluation of potential possibility of re-liquefaction of seabed soil after previous liquefaction. In this paper, a series of cyclic triaxial liquefaction tests were conducted on core samples collected from submarine non-liquefied and liquefied zone in the Huanghe River Delta, respectively. The differences between non-liquefied and liquefied seabed soil in the developing trends of pore pressure and axial dynamic strain with cycles were analyzed and discussed, and the corresponding liquefaction potentials were also comparatively evaluated. The test results show that compared to pore pressure, the strain standard is more suitable to evaluate the liquefaction potential of the seabed silt in the Huanghe River Delta. The pore pressure and dynamic axial strain development characteristics indicate that the re-liquefaction resistance of the liquefied seabed silt is improved to some extent compared with the non-liquefied silt. Furthermore, the correlations between the normalized pore pressure ratio u_d/σ₃ and the normalized cycle ratio N/N_f could be described quantitatively by the hyperbolic or exponential functions for liquefied and non-liquefied seabed silts. Finally, the critical cyclic stress ratio for the non-liquefied seabed silt is around 0.20 compared to 0.35 for the liquefied one in the Huanghe River Delta. The research findings will contribute to deepening the understanding of the wave-induced liquefaction mechanism of seabed silt, and also provide an example reference for the study of the mechanical properties of soil subjected to previous cyclic stress history.

Funded by the National Key Basic Research Development Program, a multi-vessel oceanography survey was conducted synoptically in the South China Sea (SCS) in August 2000, which appeared to be the unique, available synoptic survey that covered the entire SCS basin in deep summer. A number of difficulties were encountered during the survey that affected the quality of observation, even though they were resolved underway. Although some related research works were previously published, no carefully quality control of the data and a comprehensive analyses had ever appeared before. In this work, the original CTD and shipboard ADCP data were carefully calibrated in the first place, and the hydrography and circulation features were then analyzed and discussed to leave a record of the cruises as complete as possible. The results indicate that, in deep summer, the basin shows different physical characteristics on both sides of its SW–NE oriented axis of the basin. The circulation is generally anticyclonic southeast of the axis, in which the Nansha anticyclonic gyre (NSAG) is especially powerful; the area northwest of the axis, in contrast, appears to be dominated by mesoscale features of different categories, where no obvious structure of large scale circulation is found. The analyses confirm that the scales of the NSAG are over 400 km horizontally and nearly 1000 m vertically, with a maximal swirling speed close to 1 m/s, which appears to be the most remarkable circulation feature of the SCS. Moreover, its position and strength are subject to significant interannual variation. In the Mindoro Strait, the exchange flow showes a sandwich structure during the observation period. This is different from our previous understanding and suggests that the strait exchange may have different modes varying with the forcing condition.

Dimethylsulfoniopropionate (DMSP) is one of the most abundant organic sulfur molecules on earth, and plays an important role in global sulfur cycle and climate regulation. DMSP is the principal precursor of dimethyl sulfide (DMS), which can be degraded through many ways in the ocean, and microbial degradation is one of the most important ways. Coral reef is one of the major sources of marine DMS, and coral-associated DMSP degrading bacteria play an important role in the process of DMS production. In the present study, 39 strains of DMSP degrading bacteria were isolated from six reef building corals, including Acropora millepora, Acropora formosa, Acropora echinata, Acropora digitifera, Pocillopora damicornis, and Galaxea fascicularis. The phylogenetic analysis of DMSP degrading bacteria was performed based on 16S rRNA gene sequence, 39 strains of DMSP degrading bacteria belonged to 4 phyla, 6 classes, and 19 genera, the dominant genus was Bacillus. The DMS production efficiency of DMSP degrading bacteria were analyzed by GC-FPD detection of DMSP products, the results showed that 9 strains had the ability of high DMS production. The probiotic effects of bacteria with high DMS production on corals in response to climate warming need to be further studied.

There is little information about the relationship between aerobic anoxygenic photoheterotrophic bacteria (AAPB) and upwelling. In this work, the response of AAPB to upwelling in the southern Taiwan Strait was examined, using the “Time-series observation-based cyanobacteria-calibrated InfraRed Epifluorescence Microscopy, TIREM”. The results showed that in the initial stage of upwelling, the abundances of AAPB and total heterotrophic bacteria were low; with the development, their abundances both increased and reached the highest value at the mature stage of upwelling; notably, when the upwelling declined, they began to decrease. During the developmental process of upwelling, AAPB abundance was positively correlated with chlorophyll a concentration in a certain range, and limited by low phosphorus concentration, while total heterotrophic bacteria abundance was positively correlated with nitrogen, phosphorus and silicon nutrients concentration, suggesting that dissolved organic carbon released by phytoplankton and phosphorus limitation might play a more direct and important role in AAPB, while nutrients probably acted as an important role in the response of total heterotrophic bacteria to upwelling. This study will help to better understand the unique role of AAPB in the biogeochemical cycle of carbon and other biogenic elements.

Based on the on-site observation of the topography, a generalized three-dimensional physical model of coral reef-lagoon-channel system was established in the wave basin. Wave gauges, velocity meters, and surface velocity measurement system were used to analyze wave and current field characteristics at different locations under regular wave condition. The experimental results indicate that over the reef flat, the wave height gradually decreases by 86.7% cross-shore, and wave-induced setup first increases and then decreases by 65.9% along the reef. The mean current direction is mainly cross-shore, and there is a tendency of increasing first and then decreasing. In the lagoon, the wave height is larger near the channel, where wave-induced setup is the smallest. The maximum wave height is about 2.8 times of the minimum value, and the wave-induced setup is 25.5% lower than on both sides. The mean current is mainly a longshore one that points symmetrically to the rip channel. The velocity increases from the two sides to the rip channel first and then decreases. The wave height in the channel does not change much, and largest wave-induced setup is 47.6% than that on the reef. The mean current flows offshore, and increases first and then decreases. Using the results measured by the wave gauges, the spatial changes of the radiative stress and wave surface pressure gradient which drove the circulation were quantitatively analyzed. The current change on the reefs is the result of the interaction between wave surface pressure gradient and the radiant stress. The driving force of the offshore flow in the rip channel is mainly the radiant stress, while the change of the longshore current in the lagoon is determined by the pressure gradient of the mean water level.

Salinity mixing and stratification in the river-dominated estuaries are important dynamic mechanisms for controlling transport and diffusion of suspended sediment. Based on the synchronous field investigation with three surveying vessels in the flood season in 2017, covering the spring and neap tidal cycles, the influence mechanism of salinity stratification on suspended sediment distribution in the Modaomen Estuary was analyzed in this study. Vertical distribution of salinity in the estuary also displayed spatial differences under the influence of interaction between riverine and tidal dynamics. The salinity at M1 Station (Guading Jiao), dominated by runoff, was mixed well vertically over the tidal cycles; salinity stratification occurred at the M2 Station (outlet location) and at M3 Station (outside the mouth), which were influenced by interaction between runoff and tide over the tidal cycles. Spatial distribution of suspended sediment was closely related to spatial distribution of salinity. In general, salinity mixing promoted the vertical mixing of suspended sediment, while salinity stratification constrained the suspended sediment to be concentrated in the bottom water layer, and high suspended sediment concentration (SSC) tended to appear at the layer where salinity stratified occur. The vertical distribution curve of SSC was L-shape or paracurve shape, while high SSC always concentrated in the front of the salinity wedge in the longitudinal direction, indicating a significant sediment trapping effect caused by salinity stratification. Comparing the stratification ratio and vertical diffusion coefficient at three gauging stations, there was a negative relationship between them, the larger stratification ratio is, the smaller vertical diffusion coefficient is, indicating the suppression effect of stratification on vertical diffusion. Furthermore, the higher stratification is, the larger suppression effect is. Such mechanism contributes to the sediment trapping caused by stratification. This study is helpful to reveal the mechanism of fine sediment movement and the mechanism of evolution of mouth bar in a complicated estuary, and provide scientific basis for regulation of mouth bar in the Modaomen Estuary.