Spatio-temporal variation of sound speed, in seafloor geodetic precise positioning, can always be attributed to the time error. Firstly, this paper analyzes the existing error compensation model, i.e., the time ratio model, which is expressed by the recorded time multiplying a ratio coefficient. And then a time split model is proposed by expressing the acoustic ray traveling time as the recorded time pluses a perturbation time error. The theoretical differences between the proposed time bias compensation model and the time ratio model are analyzed. Under the new framework, sound speed perturbation models with optimal single-layer spatial gradient and multi-layer spatial gradients are developed to compensate for sound speed error in the complex cases. Numerical computation shows that the simple time split model keeps the same accuracy as some complicated models while considering the distribution of random error. Furthermore, multi-layer model can improve the positioning accuracy without putting the pressure on parametrization.

Merged satellite altimeter products are widely used in ocean-related fields. Currently, the altimeter merged products of archiving validation and interpretation of satellite oceanographic (AVISO) data are widely used internationally. Chinese National Satellite Ocean Application Service also released merged altimeter products (ALT MUL) in 2023. However, there are few studies on the quality assessment of ALT MUL. Based on the data of AVISO merged products, Jason3 satellite, tide gauge and drifter buoy, the quality assessment and effect analysis of ALT MUL merged products were carried out by means of error evaluation index, interpolation along rails, velocity inversion and power spectrum. The result shows that the average sea level anomaly (SLA) of ALT MUL is about 2 cm smaller than that of AVISO. And they are consistent with the large-scale characteristics and spatial distribution. These two SLA products are both in accordance with normal distribution. Results indicate a lesser congruence between ALT MUL and Jason3 satellite compared to AVISO. This difference may be attributed to the fact that AVISO products use Jason3 satellite as cross-calibrated reference satellite during the merged process. Comparing the matching effect of the two merged products with the tide gauge and drifter buoy, ALT MUL merged products are superior to AVISO in general. The energy spectral density was calculated by using Jason3 satellite data along the orbit, and the two merged products were interpolated to the data points along the orbit. The effective resolution of AVISO and ALT MUL merged products was 180 km and 210 km respectively through spectral calculation, indicating that AVISO merged products have higher effective resolution.

Macrobenthos can serve as an indicator of hypoxia in the estuarine ecosystem. This comparative study surveyed macrobenthos from hypoxic and non-hypoxic areas of the Zhujiang (Pearl) River Estuary (PRE), and explores the effects of environmental factor on the macrobenthos community structure. In July 2020, 49 macrobenthos species were collected from the hypoxic area, contrasting with 91 species found in the non-hypoxic area. July 2021 recorded 51 species in the hypoxic area and 76 in the non-hypoxic area. Analysis of similarities (ANOSIM) and non-metric multidimentional scaling (NMDS) showed no significant difference in the macrobenthos community structure between the two areas. However, Polychaeta displays higher species richness, abundance, and biomass in the hypoxic zone, negatively correlating to dissolved oxygen (DO). Canonical correspondence analysis (CCA) also showed that the abundance of Polychaeta was negatively correlated with that of Crustacea. Interestingly, despite the differences in Polychaeta, macrobenthos community structure remains stable between hypoxic and non-hypoxic samples. This study suggests Polychaeta’s potential adaptation to hypoxic conditions in the PRE’s hypoxic area. Finally, Spearman correlation analysis showed that DO have a significant negative correlation with total phosphorus (TP), total nitrogen (TN) and total organic carbon (TOC) in the PRE, indicating that water eutrophication would exacerbate the occurrence of hypoxia.

Arctic sea ice is an essential component of the climate system and plays an important role in global climate change. This study calculates the volume flux through Fram Strait (FS) and the sea ice volume in the Greenland Sea (GS) from 1979 to 2022, and analyzes trends before and after 2000. In addition, the contributions of advection and local processes to sea ice volume variations in the GS during different seasons are compared. The influence of the surface air temperature (SAT) and the sea surface temperature (SST) on sea ice volume variations is discussed, as well as the impact of atmospheric circulation on sea ice. Results indicate no significant trend in the sea ice volume flux through FS from 1979 to 2022. However, the sea ice volume in the GS exhibited a notable decreasing trend. Compared with the period of 1979–2000, the sea ice volume decreasing trend accelerated significantly during the period of 2001–2022. During winter, ice advection from the central Arctic Ocean exert a strong influence on the sea ice volume variations in the GS, whereas during summer, local processes, including the interactions with the atmosphere and ocean, as well as the dynamic process of sea ice itself, exert a considerable impact. The sea ice volume in the GS declined rapidly after 2000. Furthermore, the effects of local processes on sea ice have intensified, with the SST exerting a stronger influence on the sea ice volume variations in the GS than the SAT. The positive Arctic oscillation and dipole anomaly are important drivers for the transport of Arctic sea ice to the GS. The Winter North Atlantic oscillation intensifies ocean heat content, affecting sea ice in the GS.

Large yellow croaker (Larimichthys crocea) is an economically important fish, with the annual production ranking second among maricultured fish in China. Outbreaks of visceral white nodules disease caused by Pseudomonas plecoglossicida have led to substantial economic losses for the L. crocea aquaculture industry. However, L. crocea defense strategies against P. plecoglossicida infection, especially the role of microRNAs (miRNAs) in the defense against P. plecoglossicida, are poorly understood. Here, we analyzed changes in the mRNA and miRNA expression profiles in the spleen of L. crocea at 96 h post-infection and explored its defensive strategies. Principal component analysis (PCA) showed that P. plecoglossicida infection brought about a profound remodeling of both the miRNA and mRNA profiles. Enrichment analysis showed that the inflammatory response (IL-17 signaling pathway, chemokines and chemokine receptor pathway), ATP synthesis (TCA cycle and oxidative phosphorylation), apoptosis and necroptosis (TNF signaling pathway), and proteolysis (proteasome pathway) were enriched and upregulated by P. plecoglossicida. Thus, P. plecoglossicida infection activated the inflammatory response, stimulated ATP synthesis, and accelerated apoptosis and necroptosis, and promoted proteasome-mediated protein degradation. Additionally, integrated analysis identified 568 miRNA-mRNA pairs. KEGG enrichment analysis of the miRNA targets showed that the enriched pathways included cytokine-cytokine receptor interaction, the chemokine signaling pathway, the C-type lectin receptor signaling pathway, and apoptosis. Integrated analysis identified 14 miRNAs which targeted 44 immune-related genes. Altogether, our results revealed not only the role of the inflammatory response, energy metabolism, apoptosis and necroptosis, and the proteasome pathway in L. crocea defense against P. plecoglossicida infection, but also the regulatory networks of miRNAs associated with host defense against P. plecoglossicida.

A mobile marine seismometer (MMS) is a vertical underwater vehicle that detects ocean seismic waves. One of the critical operational requirements for an MMS is that it remains suspended at a desired depth. This article aimed to propose a fixed-depth suspension control for the MMS with a limited onboard energy supply. The research team established a kinematic model to analyze fluctuations in the vertical motion of the MMS and the delayed response of the system. We ascertained a direct one-to-one correlation between the displacement volume of the mobile ocean seismic instrument and the depth at which it reaches a state of neutral buoyancy (commonly referred to as the hover depth). A fixed-depth control algorithm was introduced, allowing a gradual approach to the necessary displacement volume to reach the desired suspension depth. The study optimized the boundary conditions to reduce unnecessary adjustments and mitigate the time delay caused by the instrument’s inertia, thereby significantly minimizing energy consumption. This method does not require calculating the hydrodynamic parameters or transfer functions of the MMS, thereby considerably reducing the implementation complexity. In the three-month sea trial in the South China Sea, the seismic instrument was set to hover at 800 m, with a permissible fluctuation of ±100 m, operating on a seven-day cycle. The experimental results show that the seismic instrument has an average hover error of 34.6 m, with a vertical drift depth of 29.6 m per cycle, and the buoyancy adjustment system made six adjustments, indicating that our proposed control method performs satisfactorily. In addition, this method provides new insights for the fixed-depth control of other ocean observation devices that rely on buoyancy adjustment.

Spartina alterniflora is now listed among the world’s 100 most dangerous invasive species, severely affecting the ecological balance of coastal wetlands. Remote sensing technologies based on deep learning enable large-scale monitoring of Spartina alterniflora, but they require large datasets and have poor interpretability. A new method is proposed to detect Spartina alterniflora from Sentinel-2 imagery. Firstly, to get the high canopy cover and dense community characteristics of Spartina alterniflora, multi-dimensional shallow features are extracted from the imagery. Secondly, to detect different objects from satellite imagery, index features are extracted, and the statistical features of the Gray-Level Co-occurrence Matrix (GLCM) are derived using principal component analysis. Then, ensemble learning methods, including random forest, extreme gradient boosting, and light gradient boosting machine models, are employed for image classification. Meanwhile, Recursive Feature Elimination with Cross-Validation (RFECV) is used to select the best feature subset. Finally, to enhance the interpretability of the models, the best features are utilized to classify multi-temporal images and SHapley Additive exPlanations (SHAP) is combined with these classifications to explain the model prediction process. The method is validated by using Sentinel-2 imageries and previous observations of Spartina alterniflora in Chongming Island, it is found that the model combining image texture features such as GLCM covariance can significantly improve the detection accuracy of Spartina alterniflora by about 8% compared with the model without image texture features. Through multiple model comparisons and feature selection via RFECV, the selected model and eight features demonstrated good classification accuracy when applied to data from different time periods, proving that feature reduction can effectively enhance model generalization. Additionally, visualizing model decisions using SHAP revealed that the image texture feature component_1_GLCMVariance is particularly important for identifying each land cover type.

Coral gardening has become a promising technique for restoring reefs worldwide in the Anthropocene era. The microbiome plays an important role in enhancing adaptive resilience in situ nursery propagation of corals. However, little is known about the response patterns of bacterial community dynamics, co-occurrence networks and assembly processes of different species in coral restoration nurseries over time. Here, we collected two Acropora coral samples from transplanted fragments and source colonies at 1-month and 3-month post-transplantation (May and July 2022) in an upwelling-affected fragmented reef. Full-length 16S rRNA gene sequencing revealed that bacterial communities of coral fragments in nurseries exhibited consistent temporal shifts compared to those of the source colonies. High host specificity was observed in the bacterial community and network structure associated with source colonies. In contrast, for the two coral species within nurseries, there were no differences in bacterial diversity, composition and core microbiome. Stochastic assembly processes were identified as the primary drivers of bacterial communities in all May samples, whereas deterministic processes played a more prominent role in July. Seawater properties (e.g., temperature and ammonium concentration) partially explained the compositional changes in the bacterial communities of these coral samples. Our findings suggested that coral nurseries contributed to the homogenization of bacterial communities in different Acropora corals, despite the apparent temporal dynamics of bacteria. These results enhance our understanding of the coral microbiome, ecological interactions and assembly principles in different host within in situ nurseries.

Swells are critical concerns regarding safety, marine transportation, and coastal engineering construction of coastal countries along the Gulf of Guinea and have been scientific problems due to the lack of systematic theoretical, numerical, and observational research. In this study, a double nesting numerical model was constructed and validated from the Atlantic Ocean to the Gulf of Guinea based on simulating waves nearshore (SWAN) to explore the swell characteristics and source tracing in the Gulf of Guinea in winter and summer seasons from 2020 to 2021. Simulation results reveal that swells are stronger and deflect more to the west in winter than summer, even though they dominate in both seasons in the Gulf of Guinea in the S-SW directional range. Simulated two-dimensional (2D) wave spectral patterns not only clarify wave composition, variation, and propagation properties from the central South Atlantic Ocean to the Gulf of Guinea, but also distinguish swell strength and directional range in winter and summer. The NW wind events induce swells which spread toward the SSE-ESE direction from the North Atlantic Ocean, big wind source generates sustained and stable S-SW swells from the South Atlantic Ocean, and corresponding swell-influenced areas are discussed. The strongest swell event in the Gulf of Guinea during the simulation was used as a case study to trace its source. A strong clockwise wind vortex within the Roaring Forties induced these large swells in the Gulf of Guinea approximately 5.5 days later, and swell propagation formed a regular isoline of peak period distribution from the South Atlantic Ocean to the Gulf of Guinea in the SSW-SW direction.

The Diaoyu Island (Diaoyu Dao) and its affiliated islands (DAA) have abundant fishery resources, and chlorophyll a (Chl a) concentration is an important marine color element and an important indicator of primary productivity in the ocean. Therefore, it is meaningful to understand the distribution and variation characteristics of Chl a concentration in the waters near the DAA. The distribution details of Chl a concentration in the adjacent waters of DAA were revealed by high-resolution satellite GF-1 Wide Field View (WFV) data with 16 m spatial resolution. The results indicate that: (1) The Chl a concentration is between 0.06 μg/L and 0.38 μg/L throughout the year and the concentration of Chl a in the northeast and east directions of the island (downstream) is significantly higher than that in the west (upstream), and there are observed vortexes with high Chl a concentration in the downstream of DAA. This phenomenon continues to persist in the waters surrounding the DAA all year. (2) The vortex induced by the interaction between the current and island results in the vorticity change of water, inducing the replenishment of a large number of nutrients to the surface, thereby promoting the growth of plankton in the downstream of DAA on the east. In addition, the DAA also plays a significant role in regulating the downstream mix layer depth (MLD). The MLD in the northeast downstream is considerably deeper than the upstream in winter and spring; it is deeper in the east downstream in summer and autumn, and the range of influence for the MLD is greatly larger than the size of the island itself. This shows that the disruption of the DAA on the current is substantial. In addition, the combined effects of SST and wind also play a significant role in modifying Chl a distribution. (3) Based on the analysis above, this study proposes the conception of building fishing pastures downstream of the DAA, and proposes a general migration plan in different seasons to scientifically and rationally utilize and protect the surrounding waters of DAA.