The South Yellow Sea basin is filled with Mesozoic–Cenozoic continental sediments overlying pre-Palaeozoic and Mesozoic–Palaeozoic marine sediments. Conventional multi-channel seismic data cannot describe the velocity structure of the marine residual basin in detail, leading to the lack of a deeper understanding of the distribution and lithology owing to strong energy shielding on the top interface of marine sediments. In this study, we present seismic tomography data from ocean bottom seismographs that describe the NEE-trending velocity distributions of the basin. The results indicate that strong velocity variations occur at shallow crustal levels. Horizontal velocity bodies show good correlation with surface geological features, and multi-layer features exist in the vertical velocity framework (depth: 0–10 km). The analyses of the velocity model, gravity data, magnetic data, multi-channel seismic profiles, and drilling data showed that high-velocity anomalies (>6.5 km/s) of small (thickness: 1–2 km) and large (thickness: >5 km) scales were caused by igneous complexes in the multi-layer structure, which were active during the Palaeogene. Possible locations of good Mesozoic and Palaeozoic marine strata are limited to the Central Uplift and the western part of the Northern Depression along the wide-angle ocean bottom seismograph array. Following the Indosinian movement, a strong compression existed in the Northern Depression during the extensional phase that caused the formation of folds in the middle of the survey line. This study is useful for reconstructing the regional tectonic evolution and delineating the distribution of the marine residual basin in the South Yellow Sea basin.

Toxic harmful algal blooms (HABs) can cause deleterious effects in marine organisms, threatening the stability of marine ecosystems. It is well known that different strains, natural populations and growth conditions of the same toxic algal species may lead to different amount of phycotoxin production and the ensuing toxicity. To fully assess the ecological risk of toxic HABs, it is of great importance to investigate the toxic effects of phycotoxins in marine organisms. In this study, the short-term toxicity of 14 common phycotoxins (alone and in combination) in the marine zooplankton Artemia salina was investigated. The 48 h LC₅₀ of the 14 phycotoxins varied from 0.019 3 µg/mL to 2.415 µg/mL. The most potent phycotoxin was azaspiracids-3 (AZA3; with a LC₅₀ of 0.019 3 µg/mL), followed by azaspiracids-2 (AZA2; 0.022 6 µg/mL), pectenotoxin-2 (PTX2; 0.046 0 µg/mL) and dinophysistoxin-1 (DTX1; 0.081 8 µg/mL). For the binary exposure, okadaic acid (OA) induced potential additive effects with DTX1, probably due to their similar structure (polyether fatty acid) and mode of action (attacking the serine/threonine phosphoprotein phosphatases). On the other hand, OA showed potential antagonistic effects with PTX2, which might be accounted for by their activation on the detoxification activity of cytochrome P450 activity. In addition, DTX1 induced potential synergetic effects with saxitoxin (STX), yessotoxin (YTX) or PTX2, suggesting the hazard potency of the mixtures of DTX1 and other phycotoxins (like STX, YTX and PTX2) with regard to the ecological risk. These results provide valuable toxicological data for assessing the impact of phycotoxins on marine planktonic species and highlight the potential ecological risk of toxic HABs in marine ecosystems.

The shallow-water temperature profile is typically parameterized using a few empirical orthogonal function (EOF) coefficients. However, when the experimental area is poorly known or highly variable, the adaptability of the EOFs will be significantly reduced. In this study, a new set of basis functions, generated by combining the internal-wave eigenmodes with the average temperature gradient, is developed for characterizing the temperature perturbations. Temperature profiles recorded by a thermistor chain in the South China Sea in 2015 are processed and analyzed. Compared to the EOFs, the new set of basis functions has higher reconstruction accuracy and adaptability; it is also more stable in ocean regions that have internal waves.

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.

The estuarine tapertail anchovy (Coilia nasus) is a high-value commercial fish. Estimating the spawning site or hatchery origin and habitat is essential for its conservation. This study aimed to determine the habitat use and life history characteristics of C. nasus from the Changjiang River Estuary. We investigated the environmental signatures of strontium (Sr) and calcium (Ca) in the otoliths of the collected specimens using electron probe microanalysis; additionally, we examined their gonadal maturity stage. Our results indicate that the 31 adult C. nasus specimens used in this study could be classified into two types based on their otolith Sr:Ca concentration ratios and their gonadal maturity stage. The long freshwater early life history type (Type LF) had wider central region in the otolith with low Sr:Ca concentration ratios ranging from 1.24±0.62 to 1.92±0.78 and a bluish pattern of low Sr content level. These fish are of riverine origin and had a relatively long early life history in freshwater and low gonadal maturation when captured. The short freshwater early life history type (Type SF) had a shorter central region in the otolith with low Sr:Ca concentration ratios ranging from 1.35±0.5 to 2.82±0.97 and a correspondingly bluish pattern. These fish also had a relatively short-term early life history in freshwater and high gonadal maturation when captured. The results of the otolith microchemical analysis indicated that Type LF and Type SF originated in spawning/hatching sites far from and close to the estuary, respectively. The mature gonads of Type SF fish indicated that they may breed in areas close to the estuary, whereas the immature gonads of Type LF fish indicated that they may breed in areas far from the estuary. This study is the first to reveal that the Changjiang River Estuary contains stocks of anadromous C. nasus originating in different spawning sites during the same season. The estuarine habitat plays a critical role in the connectivity between freshwater recruitment and the marine resources available to adult spawners of this commercially valued species. From a conservation perspective, this study provides important information for identifying anadromous C. nasus stocks originating in different spawning sites in the Changjiang River Basin.

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.

The long-term spatiotemporal changes of surface biogenic elements in the Changjiang River Estuary and adjacent waters during the summer of 2008−2016 were analyzed in this study. The concentrations of dissolved inorganic nitrogen (DIN), soluble reactive phosphate (${{\rm {PO}}_4^{3-}} $) and silicate (${{\rm {SiO}}_3^{2-}} $) were generally stable, with a slight decrease of DIN and ${{\rm {PO}}_4^{3-}} $, and a slight increase of ${{\rm {SiO}}_3^{2-}} $, which mainly occurred in the estuarine waters. The grey correlation analysis was carried out between biogenic elements and chlorophyll a (Chl-a). Results showed that compared with the absolute values of biogenic elements, the correlations between the concentration ratio of nitrogen to phosphorus (N/P), ratio of silicon to nitrogen (Si/N) and Chl-a were closer, indicating the important influence on phytoplankton by the structure of biogenic elements. The study area was generally in a state of potential P limitation, and could have potential impact on the phytoplankton community, triggering the shift of red tide dominant species from diatoms to dinoflagellates.

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.

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.

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.