Internal solitary wave (ISW) is often accompanied by huge energy transport, which will change the pore water pressure in the seabed. Based on the two-dimensional Biot consolidation theory, the excess pore water pressure in seabed was simulated, and the spatiotemporal distribution characteristics of excess pore water pressure was studied. As the parameters of both ISW and seabed can affect the excess pore water pressure, the distribution of pore water pressure showed both dissipation and phase lag. And parametric studies were done on these two phenomena. Due to influenced by the phase lag of excess pore water pressure, the penetration depth under the site of northern South China Sea with total water depth 327 m, induced by typical internal solitary wave increased by 26.19%, 53.27% and 149.86% from T ₀ to T _0.5 in sand silt, clayey silt and fine sand seabed, respectively. That means the effect of ISW on seabed will be underestimated if we only take into accout the penetration depth under ISW trough, especially for fine sand seabed. In addition, the concept of “amplitude-depth ratio” had been introduced to describe the influence of ISW on seabed dynamic response in the actual marine environment. In present study, it is negatively correlated with the excess pore water pressure, and an ISW with smaller amplitude-depth ratio can wide the range of lateral impacts. Our study results help understand the seabed damage induced by the interaction between ISW and seabed.

Compared to single-polarization synthetic aperture radar (SAR) data, fully polarimetric SAR data can provide more detailed information of the sea surface, which is important for applications such as shallow sea topography detection. The Gaofen-3 satellite provides abundant polarimetric SAR data for ocean research. In this paper, a shallow sea topography detection method was proposed based on fully polarimetric Gaofen-3 SAR data. This method considers swell patterns and only requires SAR data and little prior knowledge of the water depth to detect shallow sea topography. Wave tracking was performed based on preprocessed fully polarimetric SAR data, and the water depth was then calculated considering the wave parameters and the linear dispersion relationships. In this paper, four study areas were selected for experiments, and the experimental results indicated that the polarimetric scattering parameter α had higher detection accuracy than quad-polarization images. The mean relative errors were 14.52%, 10.30%, 12.56%, and 12.90%, respectively, in the four study areas. In addition, this paper also analyzed the detection ability of this model for different topographies, and the experiments revealed that the topography could be well recognized when the topography gradient is small, the topography gradient direction is close to the wave propagation direction, and the isobath line is regular.

Increasing intense human activities have largely changed the coastal landscape and caused many environmental issues. However, whether human-induced activities could change the coastal land use gradient pattern, an important coastal zonal characteristic along the sea–land direction, remains unclear. Manila Bay was selected as the study area in this work. According to the distance of the land use and land cover (LULC) to the coastline, we clustered the typical coastal land use sequence patterns (CLUSPs) along the sea–land direction between 1988 and 2016 in Manila Bay and found the following. (1) Four typical CLUSPs, including the natural CLUSP dominated by forest land and grassland, the agricultural CLUSP dominated by dry farm and paddy field, the urbanised CLUSP dominated by construction land and the fishery CLUSP dominated by fishing farm, were mined in 1988. Three typical CLUSPs (a natural CLUSP, an intermediate CLUSP between the agricultural and urbanised CLUSPs, and a fishery CLUSP) were mined in 2016. (2) Affected by the dominant LULC, these typical CLUSPs showed a regular spatial pattern along the sea–land direction. For example, the typical natural CLUSP showed a landward pattern due to the long distance between the forest land and grassland and the coastline. (3) However, influenced by urban and aquaculture expansion, the land intensification of the CLUSP exhibited an obvious increase and caused the decrease of the CLUSP diversity from 1988 to 2016. The increase in the area of LULC coverage showed no obvious correlation with its distance from the coastline (DFC), but the net increase rate of LULC coverage had a significant negative correlation with the DFC. Therefore, human-induced activities have a large impact on the gradient pattern of coastal land use along the sea–land direction.

The dual isotopes (N and O) of nitrate were measured using a denitrifier bacterial method in the western South China Sea (WSCS) during September 2015 to elucidate key information during N transformation in the lower euphotic zone (LEZ)-upper mesopelagic zone (UMZ, down to 500 m in this study) continuum, which is a vital sub-environment for marine N cycle and sequestration of atmospheric CO₂ as well. The N isotopic composition (δ¹⁵N) of nitrate generally decreased from 500 m toward the base of the euphotic zone (~100 m), reaching a value of ~4.6‰ (vs. air N₂) at the base of the LEZ, suggesting the imprint of remineralization (nitrification) of isotopically light N from atmospheric source. The δ¹⁵N and δ¹⁸O of nitrate only generally conform to a 1:1 line at 50 m and 75 m, suggesting that nitrate assimilation is a dominant process to shape nitrate isotope signature in this light-limited and relatively N-replete lower part of the euphotic zone. The fractionation factors of N and O isotopes during nitrate fractionation (¹⁵ε_ASSIM, ¹⁸ε_ASSIM) using a steady-state model were estimated to be 4.0‰±0.3‰ and 5.4‰±0.3‰, respectively. The occurrence of nitrification at the base of the LEZ and most of the UMZ is corroborated by the decoupling of δ¹⁵N and the oxygen isotopic composition (δ¹⁸O) of nitrate. Our results will provide insights for better understanding N cycle in the South China Sea from a perspective of present and past.

Chlorophyll a concentration (CHL) is an important proxy of the marine ecological environment and phytoplankton production. Long-term trends in CHL of the South China Sea (SCS) reflect the changes in the ecosystem’s productivity and functionality in the regional carbon cycle. In this study, we applied a previously reconstructed 15-a (2005−2019) CHL product, which has a complete coverage at 4 km and daily resolutions, to analyze the long-term trends of CHL in the SCS. Quantile regression was used to elaborate on the long-term trends of high, median, and low CHL values, as an extended method of conventional linear regression. The results showed downward trends of the SCS CHL for the 75th, 50th, and 25th quantile in the past 15 a, which were −0.004 0 mg/(m³·a) (−1.62% per year), −0.002 3 mg/(m³·a) (−1.10% per year), and −0.001 9 mg/(m³·a) (−1.01% per year). The negative trends in winter (November to March) were more prominent than those in summer (May to September). In terms of spatial distribution, the downward trend was more significant in regions with higher CHL. These led to a reduced standard deviation of CHL over time and space. We further explored the influence of various dynamic factors on CHL trends for the entire SCS and two typical systems (winter Luzon Strait (LZ) and summer Vietnam Upwelling System (SV)) with single-variate linear regression and multivariate Random Forest analysis. The multivariate analysis suggested the CHL trend pattern can be best explained by the trends of wind speed and mixed-layer depth. The divergent importance of controlling factors for LZ and SV can explain the different CHL trends for the two systems. This study expanded our understanding of the long-term changes of CHL in the SCS and provided a reference for investigating changes in the marine ecosystem.

Mudflats play a vital role in maintaining the dynamic balance between sea and land. To understand the characteristics, sources, and pollution risks of six heavy metals (As, Cd, Cr, Cu, Hg, and Pb) in the coastal mudflats on the Leizhou Peninsula, 257 surface sediment samples were studied using mathematical statistics, correlation analysis, and factor analysis. The results show that the overall concentrations of these heavy metals are low although there are several high abnormal points in the local areas. The strong correlation between these heavy metals indicates that the sources of some of the metals are similar, yet their elemental combinations in different cities (counties) varied. According to the calculated enrichment factor (EF), anthropogenic activity-induced heavy metals were determined in order of decreasing influence: As, Cd, Pb, Cr, Cu, and Hg. The low EF values of Hg indicate that it does not present as a contaminant in the study area, while low values of Cr and Cu from the Lianjiang City suggest that these two metals were also attributed to natural sources. The presence of As, Cd, Cr, Cu, and Pb from the remaining cities (counties) should be influenced by anthropogenic activities. The overall potential ecological risk index indicates that the ecological risks posed by the six analyzed heavy metals to the Leizhou Peninsula mudflats, in order of decreasing risk, are Cd, As, Hg, Pb, Cu, and Cr. It is noteworthy that only Cd in Lianjiang City demonstrated substantial ecological risk. Other examined heavy metals in other cities of the study area showed slight ecological risk.

Mesopelagic fish, the most important daily vertically migrating community in the oceans, are characterized by high lipid content which may obscure the interpretation of stable isotopes analysis. Demersal fish, which are important consumers in the food web dominated by mesopelagic fish, also have a high lipid content. Here we collected 127 fish samples from the South China Sea and evaluated the effect of lipid contents on δ¹³C of mesopelagic and demersal fish. In lipid-extracted mesopelagic fish, the C/N content ratio (<5.5) shows a clear correlation with Δδ¹³C (the offset of bulk and lipid-extracted δ¹³C values), especially in non-migratory and semi-migratory species; these values were less correlation in demersal fish. Based on our results, we suggest that mesopelagic and demersal fish in different regions of the South China Sea should be studied separately using appropriate correction models and less fit for the traditional model. Moreover, the C/N content ratio should be used cautiously for establishing the lipid normalization model, especially for the fish in migratory mesopelagic fish and demersal fish. Our results also reveal that mesopelagic fish across nearby regions could be analyzed together. The new models described here can be applied in future studies of mesopelagic and demersal fish in the South China Sea.

Subduction process is a dynamical bridge for the exchanges of heat between the atmosphere and subsurface ocean water, which is regarded as a central proxy for the ocean climate studies. Given its key indicator in climate signals, it is of importance to examine the ability of a model to simulate the global subduction rate before investigating the climate dynamics. In this paper, we evaluated the ability of 21 climate models from Coupled Model Intercomparison Project Phase 6 (CMIP6) in simulating the subduction rate. In general, the simulation ability of the models to the subduction climatology is better than that to the long-term variation trend. Based on the comprehensive analysis of climatology distribution and long-term trend of the subduction rate, GISS-E2-1-G performs better in reproducing the subduction rate climatology and IPSL-CM6A-LR can simulate positive long-term trend for both the global mean subduction rate and the lateral induction term in the Antarctic Circumpolar Current (ACC) region. However, it is still challenging to capture both the distribution characteristics of the subduction climatology and the long-term temporal trend for the 21 CMIP6 models. In addition, the model results demonstrate that, the ACC area is the major region contributing to the long-term trend of the global mean subduction rate. The analysis in this paper indicates that the poor simulation ability of reproducing the long-term trend of global mean subduction rate might be attributed to the ocean dynamics, for example, the zonal velocity at the bottom mixed layer and zonal gradient of mixed layer depth.

Sediment collapse and subsequent lateral downslope migration play important roles in shaping the habitats and regulating sedimentary organic carbon (SOC) cycling in hadal trenches. In this study, three sediment cores were collected using a human-occupied vehicle across the axis of the southern Yap Trench (SYT). The total organic carbon (TOC) and total nitrogen (TN) contents, δ¹³C, radiocarbon ages, specific surface areas, and grain size compositions of sediments from three cores were measured. We explored the influence of the lateral downslope transport on the dispersal of the sediments and established a tentative box model for the SOC balance. In the SYT, the surface TOC content decreased with water depth and was decoupled by the funneling effect of the V-shaped hadal trench. However, the sedimentation (0.002 5 cm/a) and SOC accumulation rates (~0.038 g/(m²·a) (in terms of OC)) were approximately 50% higher in the deeper hadal region than in the abyssal region (0.001 6 cm/a and ~0.026 g/(m²·a) (in terms of OC), respectively), indicating the occurrence of lateral downslope transport. The fluctuating variations in the prokaryotic abundances and the SOC accumulation rate suggest the periodic input of surficial sediments from the shallow region. The similar average TOC (0.31%–0.38%), TN (0.06%–0.07%) contents, and SOC compositions (terrestrial OC (11%–18%), marine phytoplanktonic OC (45%–53%), and microbial OC (32%–44%)) of the three sites indicate that the lateral downslope transport has a significant mixing effect on the SOC composition. The output fluxes of the laterally transported SOC (0.44–0.56 g/(m²·a) (in terms of OC)) contributed approximately (47%–73%) of the total SOC input, and this proportion increased with water depth. The results of this study demonstrate the importance of lateral downslope transport in the spatial distribution and development of biomes.

Nitrogen fixation is one of the most important sources of new nitrogen in the ocean and thus profoundly affects the nitrogen and carbon biogeochemical processes. The distribution, controlling factors, and flux of N₂ fixation in the global ocean remain uncertain, partly because of the lack of methodological uniformity. The ¹⁵N₂ tracer assay (the original bubble method → the ¹⁵N₂-enriched seawater method → the modified bubble method) is the mainstream method for field measurements of N₂ fixation rates (NFRs), among which the original bubble method is the most frequently used. However, accumulating evidence has suggested an underestimation of NFRs when using this method. To improve the availability of previous data, we compared NFRs measured by three ¹⁵N₂ tracer assays in the South China Sea. Our results indicate that the relationship between NFRs measured by the original bubble method and the ¹⁵N₂-enriched seawater method varies obviously with area and season, which may be influenced by incubation time, diazotrophic composition, and environmental factors. In comparison, the relationship between NFRs measured by the original bubble method and the modified bubble method is more stable, indicating that the N₂ fixation rates based on the original bubble methods may be underestimated by approximately 50%. Based on this result, we revised the flux of N₂ fixation in the South China Sea to 40 mmol/(m²·a). Our results improve the availability and comparability of literature NFR data in the South China Sea. The comparison of the ¹⁵N₂ tracer assay for NFRs measurements on a larger scale is urgently necessary over the global ocean for a more robust understanding of the role of N₂ fixation in the marine nitrogen cycle.