Before the implementation of offshore oil and gas exploitation, it is essential to understand the various factors that influence the stability of submarine sediments surrounding the project. Considering the factors such as cost and operability, it is not feasible to assess the physical-mechanical properties of sediments covering the entire region by borehole sampling. In this study, the correlation between near seafloor seismic amplitude and the mean shear strength of shallow sediments was explored using seismic and core testing data from the northern continental slope area of the South China Sea. Results showed that the mean water content of sediments in the layer up to 12 m below the seafloor (mbsf) gradually increased with increasing water depth, and the mean shear strength tended to decrease rapidly near the 1 000 m depth contour. The near seafloor seismic amplitude could reflect the mean shear strength of sediments in the 12 mbsf layer under seismic frequency of 65 Hz and wave velocity of 1 600 m/s. When the mean shear strength was greater than 10 kPa or the water depth was less than 1 000 m, there was a significant linear positive correlation between mean shear strength and near seafloor seismic amplitude. Otherwise, there was a significant linear negative correlation between mean shear strength and near seafloor seismic amplitude. On the basis of these correlations, the pattern of shear strength was estimated from near seafloor seismic amplitude and mapped. The mean shear strength of sediments above 12 mbsf gradually decreased with increasing water depth in the continental slope area, whereas little change occurred in the continental shelf and the end of the canyon. Within the canyon area, the mean shear strength of sediments was characterized by larger values in both sides of the canyon walls and smaller values in the canyon bottom, which was consistent with the infinite slope stability theory. The study provides a method for using near seafloor seismic amplitude data to guide sediment sampling design, and presents a continuous dataset of sediment strength for the simulation of regional sediment stability.

The ever-increasing deepwater oil and gas development in the Qiongdongnan Basin, South China Sea has initiated the need to evaluate submarine debris-flow hazard risks to seafloor infrastructures. This paper presents a case study on evaluating the debris-flow hazard risks to the planned pipeline systems in this region. We used a numerical model to perform simulations to support this quantitative evaluation. First, one relict failure interpreted across the development site was simulated. The back-analysis modeling was used to validate the applicability of the rheological parameters. Then, this model was applied to forecast the runout behaviors of future debris flows originating from the unstable upslope regions considered to be the most critical to the pipeline systems surrounding the Manifolds A and B. The model results showed that the potential debris-flow hazard risks rely on the location of structures and the selection of rheological parameters. For the Manifold B and connected pipeline systems, because of their remote distances away from unstable canyon flanks, the potential debris flows impose few risks. However, the pipeline systems around the Manifold A are exposed to significant hazard risks from future debris flows with selected rheological parameters. These results are beneficial for the design of a more resilient pipeline route in consideration of future debris-flow hazard risks.

The back propagation (BP) neural network method is widely used in bathymetry based on multispectral satellite imagery. However, the classical BP neural network method faces a potential problem because it easily falls into a local minimum, leading to model training failure. This study confirmed that the local minimum problem of the BP neural network method exists in the bathymetry field and cannot be ignored. Furthermore, to solve the local minimum problem of the BP neural network method, a bathymetry method based on a BP neural network and ensemble learning (BPEL) is proposed. First, the remote sensing imagery and training sample were used as input datasets, and the BP method was used as the base learner to produce multiple water depth inversion results. Then, a new ensemble strategy, namely the minimum outlying degree method, was proposed and used to integrate the water depth inversion results. Finally, an ensemble bathymetric map was acquired. Anda Reef, northeastern Jiuzhang Atoll, and Pingtan coastal zone were selected as test cases to validate the proposed method. Compared with the BP neural network method, the root-mean-square error and the average relative error of the BPEL method can reduce by 0.65–2.84 m and 16%–46% in the three test cases at most. The results showed that the proposed BPEL method could solve the local minimum problem of the BP neural network method and obtain highly robust and accurate bathymetric maps.

Taking the advantage of sun glitter (SG) observed from high-resolution satellites Gaofen-1 (GF-1) and Gaofen-6 (GF-6), a new method named Shield and Vortex 1-2 (SAV1-2) for extracting spatial texture information from sun glitter was established. Sea surface texture detail information around Diaoyu Dao and its affiliated islands was extracted using SAV1-2. Meanwhile, the dynamic characteristics of the water environment were analyzed by combining the water color, temperature and dynamic data. The results show the following information. (1) Stable westward shield effect and eastward Karman vortex streets, extracted from SG, appear upstream (westward) and downstream (eastward) of the Diaoyu Dao and its affiliated islands respectively. (2) The dominant direction of the Kuroshio Current in the Diaoyu Dao and its affiliated islands area is east, the Diaoyu Dao and its affiliated islands interacting with Kuroshio, inducing western shield and eastern shelter and wake. Furthermore, wave intervals of the shield in front of the island range from hundreds to thousands of metres, and extend wider than the island itself. (3) Combining with high-resolution sea surface temperature (SST) and chlorophyll a (Chl a) concentration, the directional changes of water color elements are mutually supported and proved with the stable shield (wave packet) effect in the west and vortex street effect in the east. The change of SST upstream of the island (western shield area) is not significant, the temperature in the downstream shelter area decreases slightly, and the SST in the downstream wake area shows a general trend of obvious decrease. The wake area is dominated by Karman vortex streets in shape, supplementing by vortices. The concentration of Chl a in the west shield of island is as low as that in downstream shelter area, while it increases significantly in the downstream wake area with shape in eddy or ribbon. (4) The SAV methods of extracting sea surface texture detail using SG can be widely used in different sea areas and water quality. This paper can provide reference for the protection and development of Diaoyu Dao and its affiliated islands and its surrounding waters. It is suggested that some kind of current buffer, such as marine wind farm, should be established in the western waters of Diaoyu Dao and its affiliated islands to protect the islands, and marine pasture should be developed in the downstream of eastern waters of Diaoyu Dao and its affiliated islands.

In this study, the nitrogen and oxygen isotope compositions of nitrite in the upper 150 m water column of the Amundsen Sea in the summer of 2019 and 2020 were measured to reveal the distribution and transformation of nitrite in the euphotic zone of the Southern Ocean. We found that primary nitrite maxima (PNMs) are widely present in the Amundsen Sea, where the depth of occurrence deepens from east to west and nitrite concentrations increases. Evidence from dual isotopes suggests that the formation of PNMs in all regions of the Amundsen Sea is dominated by ammonia oxidation. More importantly, the nitrogen and oxygen isotope compositions of nitrite in the Amundsen Sea mixed layer are abnormal, and their depth profiles are mirror symmetrical. Isotopic anomalies exhibit spatial variations, with central surface water having the lowest nitrogen isotope composition (−89.9‰±0.2‰) and western surface water having the highest oxygen isotope composition (63.3‰±0.3‰). Isotopic exchange reaction between nitrate and nitrite is responsible for these isotope anomalies, as both nitrogen and oxygen isotopes have large isotopic fractionation and opposite enrichment effects. This proves that isotopic exchange reaction operates extensively in different regions of the Amundsen Sea. Our study highlights the unique role of dual isotopes of nitrite in deepening the understanding of nitrogen cycle. Further studies on ammonia oxidation and isotopic exchange between nitrate and nitrite are warranted in the future to understand their roles in the nitrogen cycle in the Southern Ocean.

Mangrove forests are under the stress of sea level rise (SLR) which would affect mangrove soil biogeochemistry. Mangrove soils are important sources of soil-atmosphere greenhouse gas (GHG) emissions, including carbon dioxide (CO₂), methane (CH₄) and nitrous oxide (N₂O). Understanding how SLR influences GHG emissions is critical for evaluating mangrove blue carbon capability. In this study, potential effects of SLR on the GHG emissions were quantified through static closed chamber technique among three sites under different intertidal elevations, representing tidal flooding situation of SLR values of 0 cm, 40 cm and 80 cm, respectively. Compared with Site SLR 0 cm, annual CO₂ and N₂O fluxes decreased by approximately 75.0% and 27.3% due to higher soil water content, lower salinity and soil nutrient environments at Site SLR 80 cm. However, CH₄ fluxes increased by approximately 13.7% at Site SLR 40 cm and 8.8% at Site SLR 80 cm because of lower salinity, higher soil water content and soil pH. CO₂-equivalent fluxes were 396.61 g/(m²·a), 1423.29 g/(m²·a) and 1420.21 g/(m²·a) at Sites SLR 80 cm, SLR 40 cm and SLR 0 cm, respectively. From Site SLR 0 cm to Site SLR 80 cm, contribution rate of N₂O and CH₄ increased by approximately 7.42% and 3.02%, while contribution rate of CO₂ decreased by approximately 10.44%. The results indicated that warming potential of trace CH₄ and N₂O was non-negligible with SLR. Potential effects of SLR on the mangrove blue carbon capability should warrant attention due to changes of all three greenhouse gas fluxes with SLR.

The migration and transformation of nitrogen (N) in sediments play an important role in regulating the N concentration and nutrient structures in shallow seas. However, studies of sedimentary N dynamics are rarely focused on carbonate sediments, although these account for about 40% of the continental shelf area. Thus, the regulation mechanisms of the N dynamics in the carbonate sands of coral reefs are not clear. Taking the coral reef area of Weizhou Island, which has a relatively high N concentration, as the research object, we conducted a series of flow-through reactor experiments to investigate the fluxes of different N forms at the interface of sediment and seawater and their regulation mechanism by environmental factors. The fluxes of dissolved inorganic and organic N (DIN and DON) at different stations were −0.39–0.12 mmol/(m²·h) and −0.18–0.39 mmol/(m²·h), respectively. Denitrification (0.11–0.25 mmol/(m²·h)) was closely coupled to nitrification, which was limited by the availability of organic matter and its degradation product (i.e., ${\rm{NH}}_4^+ $). Thus, the excessive ${\rm{NO}}_3^– $ might be reduced to ${\rm{NH}}_4^+ $ by dissimilatory nitrate reduction to ammonium, rather than to N₂ by denitrification. ${\rm{NO}}_3^– $ reduction peaked at intermediate advection rates (96 L/(m²·h)) and flow path lengths (10 cm), but the release of DON also peaked at the same condition. In addition, climate warming would significantly affect sedimentary N dynamics at Weizhou Island. These results may help address the broader issue of the N cycle in coral reef ecosystems under the dual pressure of climate warming and anthropogenic activities, and these results are beneficial to coral reef protection and local ecological management.

Total dissolved nitrogen (TDN) is an important parameter for assessing the nutrient cycling and status of natural waters. The accurate determination of TDN in natural waters is essential for assessing its contents and distinguishing different forms of nitrogen in the water. The TDN in various systems has been largely documented, and the concentrations of TDN are usually obtained using high-temperature catalytic (HTC) or persulfate oxidation (PO). However, the accuracy of these methods and their suitability for all types of natural waters are still unclear. To explore both methods in-depth, assorted samples were tested, including eight solutions composed of nitrogen-containing compounds (3 dissolved inorganic nitrogen fractions: ${{\rm {NO}}_3^-}$, ${{\rm {NO}}_2^-} $ and ${{\rm {NH}}_4^+} $; 5 organic compounds: EDTA-2Na, vitamin B1, vitamin B12, amino acids, and urea) and 105 natural waters which were collected from an open ocean (Northwest Pacific Ocean, 28), a marginal sea (Yellow Sea, 34), an estuary (Huanghe River mouth, 31), rivers (Huanghe River, 4; Licun River, 4), and precipitations (4 samples). The results showed that heterocycles and molecular dimensions had certain effects on the oxidation efficiency of the PO method but had little effect on HTC. There was no significant difference between the two methods for natural waters, but HTC was more suitable for deep-sea samples with low TDN concentrations (less than 10 μmol/L) and low organic activity. Overall, HTC has a relatively simple measurement process, a high degree of automation, and low error. Therefore, HTC can be recommended to determine the TDN of samples in freshwater and seawater.

The quantification of transparent exopolymer particles (TEP) by colorimetric method is of large error and low repeatability, one major reason of which is related to the absence of clear definition and evaluation for part steps of the original method. It is obscure that the 80% sulfuric acid solution, acted as the extraction solution in the determination of TEP, is prepared based on a volume ratio or mass ratio. Furthermore, the change of solubility of recently available Gum Xanthan (GX) from the market means that the original protocol is no longer applicable, and the grinding of GX stock solution with a tissue grinder is replaced by shaking with a rotating shaker in the study to prevent the excessive dissolution of GX. We found that different preparation techniques could result in the varied concentrations of 80% H₂SO₄. The duration of shaking during the preparation of standard solution significantly affected the slope of the calibration curve, which caused different correction results of TEP. The impacts of different extraction solution concentrations and shaking time of GX solution on the quantification of TEP were investigated based on the field sampling and laboratory analysis. The extraction capacities of H₂SO₄ with different concentrations for Alcian Blue were distinct, but had limited effect on the final measuring result of TEP. The change of the standard curve slope came along with the variation of shaking time, which markedly altered the detection limit and calibration result, and the extended shaking time was in favor of the determination of low-concentration TEP. It was suggested that the extraction solution concentration, shaking time and filtration volume of standard solution are required to be well controlled and selected to obtain more accurate results for TEP with different concentrations.

Climate change has affected and will continue to affect the spatial distribution patterns of marine organisms. To understand the impact of climate change on the distribution patterns and species richness of the Sciaenidae in China’s coastal waters, the maximum entropy model was used to combine six environmental factors and predict the potential distribution of 12 major species of Sciaenidae by 2050s under Representative Concentration Pathways (RCPs) 2.6 and 8.5. The results showed that the average area under the receiver operating characteristic curve of the model was 0.917, indicating that the model predictions were accurate and reliable. The main driving factors affecting the potential distribution of these fishes were dissolved oxygen, salinity, and sea surface temperature (SST). There was an overall northward shift in the potential habitat areas of these fishes under the two climate scenarios. The total potential habitat areas of Larimichthys polyactis, Pennahia argentata, and Pennahia pawak decreased under both climate scenarios, while the total habitat area of Johnius belengerii, Pennahia anea, Miichthys miiuy, Collichthys lucidus, and Collichthys niveatus increased, suggesting that these might be loser and winner species, respectively. The expansion rate, contraction rate, degree of centroid change, and species richness in the potential habitats were generally more significant under RCP8.5 than RCP2.6. The mean shift rates of the potential distribution were 41.50 km/(10 a) and 29.20 km/(10 a) under RCP8.5 and RCP2.6, respectively. The changes in Sciaenidae species richness under climate change were bounded by the Changjiang River Estuary waters, with obvious north-south differences. Some waters with increased species richness may become refuges for Sciaenidae fishes under climate change. The richness and habitat area change rate of some aquatic germplasm resources will decrease, meanings that these reserves are more sensitive to climate change, and more attention should be paid to the potential challenges and opportunities for fishery managers. This study may provide a scientific basis for the management and conservation of Sciaenidae in China under climate change.