With the Regional Ocean Modeling System (ROMS), this paper investigates the sensitive areas in targeted observation for predicting the Kuroshio large meander (LM) path using the conditional nonlinear optimal perturbation approach. To identify the sensitive areas, the optimal initial errors (OIEs) featuring the largest nonlinear evolution in the LM prediction are first calculated; the resulting OIEs are localized mainly in the upper 2 500 m over the LM upstream region, and their spatial structure has certain similarities with that of the optimal triggering perturbation. Based on this spatial structure, the sensitive areas are successfully identified, located southeast of Kyushu in the region (29°–32°N, 131°–134°E). A series of sensitivity experiments indicate that both the positions and the spatial structure of initial errors have important effects on the LM prediction, verifying the validity of the sensitive areas. Then, the effect of targeted observation in the sensitive areas is evaluated through observing system simulation experiments. When targeted observation is implemented in the identified sensitive areas, the prediction errors are effectively reduced, and the prediction skill of the LM event is improved significantly. This provides scientific guidance for ocean observations related to enhancing the prediction skill of the LM event.

This study analyzes the signal quality and the accuracy of BeiDou 3rd generation Satellite Navigation System (BDS3) Precise Point Positioning (PPP) in the Arctic Ocean. Assessment of signal quality of BDS3 includes signal to noise ratio (SNR), multipath (MP), dilution of precision (DOP), and code-minus-carrier combination (CC). The results show that, 5 to 13 satellites are visible at any time in the Arctic Ocean area as of September 2018, which are sufficient for positioning. In the mid-latitude oceanic region and in the Arctic Ocean, the SNR is 25–52 dB Hz and the MP ranges from −2 m to 2 m. As the latitude increases, the DOP values show large variation, which may be related to the distribution of BDS satellites. The CC values of signals B1I and BIC range from −5 m to 5 m in the mid-latitude sea area and the Arctic Ocean, which means the effect of pseudorange noise is small. Moreover, as to obtain the external precise reference value for GNSS positioning in the Arctic Ocean region is difficult, it is hard to evaluate the accuracy of positioning results. An improved isotropy-based protection level method based on Receiver Autonomous Integrity Monitoring is proposed in the paper, which adopts median filter to smooth the gross errors to assess the precision and reliability of PPP in the Arctic Ocean. At first, the improved algorithm is verified with the data from the International GNSS Service Station Tixi. Then the accuracy of BDS3 PPP in the Arctic Ocean is calculated based on the improved algorithm. Which shows that the kinematic accuracy of PPP can reach the decimeter level in both the horizontal and vertical directions, and it meets the precision requirements of maritime navigation.

The Localized Weighted Ensemble Kalman Filter (LWEnKF) is a new nonlinear/non-Gaussian data assimilation (DA) method that can effectively alleviate the filter degradation problem faced by particle filtering, and it has great prospects for applications in geophysical models. In terms of operational applications, along-track sea surface height (AT-SSH), swath sea surface temperature (S-SST) and in-situ temperature and salinity (T/S) profiles are assimilated using the LWEnKF in the northern South China Sea (SCS). To adapt to the vertical S-coordinates of the Regional Ocean Modelling System (ROMS), a vertical localization radius function is designed for T/S profiles assimilation using the LWEnKF. The results show that the LWEnKF outperforms the local particle filter (LPF) due to the introduction of the Ensemble Kalman Filter (EnKF) as a proposal density; the RMSEs of SSH and SST from the LWEnKF are comparable to the EnKF, but the RMSEs of T/S profiles reduce significantly by approximately 55% for the T profile and 35% for the S profile (relative to the EnKF). As a result, the LWEnKF makes more reasonable predictions of the internal ocean temperature field. In addition, the three-dimensional structures of nonlinear mesoscale eddies are better characterized when using the LWEnKF.

In most fish, reproduction is seasonal or periodic under the suitable conditions. In turbot (Scophthalmus maximus) farms, one of the most economically important marine flatfish species, changes in daylength could cause changes in the spawning time. In this study, to characterize the regulation of reproductive physiology following light signals, three melatonin receptors (Mtnr) investigated in turbot were named smMtnr1, smMtnr2, and smMtnr1c. Distinct expression profiles demonstrated that Mtnr mRNAs were concentrated in the brain (as detected in the hypothalamus (Hy) and mesencephalon (Me)), gonad and eye. The most abundant Mtnr1 and Mtnr2 mRNA expression levels were detected in the central nervous system at the beginning of the breeding season, suggesting that Mtnr1 and Mtnr2 may play vital roles in the regulation of turbot gonadal development. In addition, the melatonin profiles gradually increased and reached to the highest level at the spawning stage, indicating that melatonin is a potent hormone in the regulation of fish oocyte growth and maturation. The results of this study suggested that melatonin is the primary factor that transduces the light signal and regulates the physiological functions of turbot seasonal reproduction. Moreover, the results of this study may establish a foundation for further research seeking to identify fish melatonin receptors involved in the gonadal development and gamete maturation.

Submarine groundwater discharge (SGD) has received increasing attention by studies on coastal areas; however, its effects on biogeochemical zonation have not been investigated to date. The Huanghe River Estuary (HRE) is a world class river estuary with high turbidity, and heavy human regulation. This study investigated how SGD is related to the benthic biogeochemistry of the HRE. Based on the distribution of several parameters (e.g., salinity, temperature, dissolved oxygen (DO) levels, pH, radium isotopes, and nutrients), the HRE was subdivided into six different zones, and the SGD fluxes within each zone were quantified and compared. The highest SGD flux was found in the northwest nearshore zone, where it was more than one order of magnitude higher than in the offshore zone. High SGD resulted in low DO and pH, but high nutrient levels in the benthic boundary layer. The southeast nearshore zone was also characterized by high SGD flux, but benthic waters were more oxic because of the dominating inputs by the Huanghe River. These data suggest that such a zonation would help to understand benthic biogeochemical processes. High SGD may not only contribute to the estuarine nutrient budget, but may also contribute to the formation of hypoxia and acidification.

Compound-specific stable isotope analysis of individual amino acids (CSIA-AA) has been widely used in ecological and biogeochemical studies. It has been proven to be powerful in tracing the diet sources and trophic interactions. However, assessing the N sources of mesopelagic fishes has been inconclusive because the mesopelagic fishes’ unique domain (water depth ranged from 0 to 1000 m) and unresolved nitrogen isotopes of various forms. This study proposes a new method for coupling instruments (ion chromatography and Precon-IRMS) and chemical method of oxidation-reduction of amino acids, and also combined δ¹⁵N of AAs with δ¹³C of fatty acids (FAs) to analyze the trophic interactions of mesopelagic fishes in the South China Sea (SCS). AAs were isolated by ion chromatography with high peak resolution and collected by an automated fraction collector. The chemical method then converted the AAs into N₂O with a robust oxidation yields and suitable molar ratio of NH₂OH to ${\rm {NO}}_2^- $. Finally, the δ¹⁵N of AAs at 20 nmol were measured with a reasonable precision (<0.6‰). With this method, this study report the first batch high precision δ¹⁵N of AAs and δ¹³C of FAs of mesopelagic fishes collected from SCS. Diaphus luetkeni, Chauliodus minimus and Bathygadus antrodes showed similar δ¹³C values of 20:4n-6 (~ −28‰), while Argyropelecus affinis and Stomias had similar values (~ −32‰). These results reflect that mesopelagic fishes had complex diet sources. An increase of 4‰ in δ¹⁵N of glutamic acid (Glu) was found between piscivorous and planktivorous fishes, which might suggest a trophic discrimination factor of mesopelagic fishes in the SCS. This study used δ¹³C of 20:4n-6 to reveal the diet sources of mesopelagic fishes and δ¹⁵N of Glu to clarify trophic level between piscivorous and planktivorous fishes. Thus, this combinative method could therefore ultimately be applied in a variety of deep-sea ecosystem.

The quality of environmental data and its possible impact on the marine species habitat modelling are often overlooked while the sources for these data are increasing. This study selected sea surface temperature (SST) from two commonly used sources, the NOAA OceanWatch and IRI/LDEO Climate Data Library, and then constructed habitat suitability index model to evaluate the influences of SST from the two sources on the outcomes of Ommastrephes bartramii habitat models for the months of July–October in the Northwest Pacific Ocean during 1996–2012. This study examined the differences in the amount of estimated unfavourable/favourable habitat area when the SST used for model building and inference were the same or different. Dynamics in suitable habitat area calculated from SST was insensitive to the two different SST products. In the fishing season of O.bartramii, the changes of magnitude and trend of monthly suitable habitat area in August and September were similar over time, whereas there were large differences for July and October. Importantly, there is a substantial lack of consistency in the O.bartramii habitat distribution based on SST of two sources. This study considered the sources of environmental data for habitat modelling and then inferred species habitat distribution whether by the same or different data source.

As an interoceanic arc, the Kyushu-Palau Ridge (KPR) is an exceptional place to study the subduction process and related magmatism through its interior velocity structure. However, the crustal structure and its nature of the KPR, especially the southern part with limited seismic data, are still in mystery. In order to unveil the crustal structure of the southern part of the KPR, this study uses deep reflection/refraction seismic data recorded by 24 ocean bottom seismometers to reconstruct a detailed P-wave velocity model along the ridge. Results show strong along-ridge variations either on the crustal velocity or the thickness of the KPR. P-wave velocity model is featured with (1) a crustal thickness between 6–12 km, with velocity increases from 4.0 km/s to 7.0 km/s from top to bottom; (2) high gradient (~1 s⁻¹) in the upper crust but low one (<0.2 s⁻¹) in the lower crust; (3) a slow mantle velocity between 7.2 km/s and 7.6 km/s in the uppermost mantle; and (4) inhomogenous velocity anomalies in the lower crust beneath seamounts. By comparing with the mature arc in the Izu-Bonin-Mariana arc in the east, this study suggests the southern part of KPR is a thicken oceanic crust rather than a typical arc crust. The origin of low velocities in the lower crust and upper mantle may be related with crustal differentiation, which implies advanced crustal evolution from normal oceanic crust to partly thicken oceanic crust. High velocities in the lower crust are related to the difference in magmatism.

Subducted sediments play an important role in the magmatism at subduction zones and the formation of mantle heterogeneity, making them an important tracer for shallow crustal processes and deep mantle processes. Therefore, ascertaining the chemical compositions of different subduction end-members is a prerequisite for using subducted sediments to trace key geological processes. We reports here the comprehensive major and trace element analyses of 52 samples from two holes (U1414A and U1381C) drilled on the subducting Cocos Ridge segment at the Southern Central American (SCA) subduction zone during Integrated Ocean Drilling Program (IODP) Expedition 344. The results show that the SCA subducting sediments contain 51% (wt%) CaCO₃, 27% (wt%) terrigenous material, 16% (wt%) opal, and 6% (wt%) mineral-bound H₂O⁺. Compared to the global trenches subducting sediment, the SCA subducting sediments are enriched in biogenic elements (Ba, Sr, and Ca), and depleted in high field strength elements (Nb, Ta, Zr, Hf, and Ti) and alkali elements (K, Rb, and Cs). Meanwhile, the sediments in this area were affected by the carbonate crash event, which could have been caused by a ~800 m rise in the carbonate compensation depth at 11 Ma in the Guatemala Basin. The reason for the sedimentary hiatus at Hole U1381C may be the closure of the Panama Isthmus and the collision between the Cocos Ridge and the Middle America Trench. In addition, the sediments from the subducting Cocos Ridge segment have influenced the petrogenesis of volcanic lavas erupted in the SCA.

Coastal zones are active reactors of continental material including that transported by rivers via a series of microbiota-mediated reactions. Nevertheless, current knowledge of the ecology and functioning of the microbiota in coastal areas affected by large riverine inputs remains insufficient on a global scale. Here, an investigation on sediment microbial composition, including taxonomy and metabolic network, as well as their relationship with major benthic reaction substrates, namely carbon, nitrogen, sulphur and phosphorus, was conducted in the continental shelf affected by the spread of the Changjiang River plume. Surface sediment samples (48 samples) were collected during March 2018, obtaining a mean Operational Taxonomic Units (OTUs) number of 3 341. Proteobacteria, Acidobacteria and Actinobacteria were abundant phyla in the studied sediments. Bray-Curtis distance analysis classified the 48 samples into 4 clusters (MG1 to MG4) at the phylum-level. MG1 and MG2 are found near the river mouth, receiving substantial land-derived particles from the Changjiang River runoff. Particle-attached microbes may be settled in these regions and influenced the observed sediment microbial diversity and biomass, e.g., increased Crenarchaeota relative abundance. The relative enrichment of these two groups in heterotrophic microbes further suggests a reliance of benthic microbiota on substrates with terrestrial origin, particularly specialized on processing sulphur-rich substrates. Regions MG3 and MG4 are located in the outer margin of the area affected by the Changjiang River plume, mainly fed by settling pelagic particles from phytoplankton. Compared to MG1 and MG2, a significant increase in the abundance of Thaumarcheota (phylum-level) and Nitrosopumilus (genus-level) was found in MG3, suggesting nitrogen-related transformations as the key reactions to sustain microbial metabolism in this region. Coupled with the identified variations in the taxonomic composition, significant differences in the keystone taxa between MG1/MG2 and MG3/MG4 were identified via OTU co-occurrence analyses. A higher abundance of Actinobacteria, Thaumarchaeota and Acidobacteria in MG3 and MG4 reinforced the identified spatial variability in benthic metabolism and highlighted the significance of substrate inputs on the sediment microbial structure and biogeography.