The Bohai Sea (BS) is the unique semi-closed inland sea of China, characterized by degraded water quality due to significant terrestrial pollution input. In order to improve its water quality, a dedicated action named “Uphill Battles for Integrated Bohai Sea Management” (UBIBSM, 2018–2020) was implemented by the Chinese government. To evaluate the action effectiveness toward water quality improvement, variability of the satellite-observed water transparency (Secchi disk depth, Z_SD) was explored, with special emphasis on the nearshore waters (within 20 km from the coastline) prone to terrestrial influence. (1) Compared to the status before the action began (2011–2017), majority (87.3%) of the nearshore waters turned clear during the action implementation period (2018–2020), characterized by the elevated Z_SD by 11.6% ± 12.1%. (2) Nevertheless, the improvement was not spatially uniform, with higher Z_SD improvement in provinces of Hebei, Liaoning, and Shandong (13.2% ± 16.5%, 13.2% ± 11.6%, 10.8% ± 10.2%, respectively) followed by Tianjin (6.2% ± 4.7%). (3) Bayesian trend analysis found the abrupt Z_SD improvement in April 2018, which coincided with the initiation of UBIBSM, implying the water quality response to pollution control. More importantly, the independent statistics of land-based pollutant discharge also indicated that the significant reduction of terrestrial pollutant input during the UBIBSM action was the main driver of observed Z_SD improvement. (4) Compared with previous pollution control actions in the BS, UBIBSM was found to be the most successful one during the past 20 years, in terms of transparency improvement over nearshore waters. The presented results proved the UBIBSM-achieved remarkable water quality improvement, taking the advantage of long-term consistent and objective data record from satellite ocean color observation.

The modern fishery stock assessment could be conducted by various models, such as Stock Synthesis model with high data requirement and complicated model structure, and the basic surplus production model, which fails to incorporate individual growth, maturity, and fishery selectivity, etc. In this study, the Just Another Bayesian Biomass Assessment (JABBA) Select which is relatively balanced between complex and simple models, was used to conduct stock assessment for yellowfin tuna (Thunnus albacares) in the Atlantic Ocean. Its population dynamics was evaluated, considering the influence of selectivity patterns and different catch per unit effort (CPUE) indices on the stock assessment results. The model with three joint longline standardized CPUE indices and logistic selectivity pattern performed well, without significant retrospective pattern. The results indicated that the stock is not overfished and not subject to overfishing in 2018. Sensitivity analyses indicated that stock assessment results are robust to natural mortality but sensitive to steepness of the stock-recruitment relationship and fishing selectivity. High steepness was revealed to be more appropriate for this stock, while the fishing selectivity has greater influence to the assessment results than life history parameters. Overall, JABBA-Select is suitable for the stock assessment of Atlantic yellowfin tuna with different selectivity patterns, and the assumptions of natural mortality and selectivity pattern should be improved to reduce uncertainties.

To confirm the seabed fluid flow at the Haima cold seeps, an integrated study of multi-beam and seismic data reveals the morphology and fate of four bubble plumes and investigates the detailed subsurface structure of the active seepage area. The shapes of bubble plumes are not constant and influenced by the northeastward bottom currents, but the water depth where these bubble plumes disappear (630–650 m below the sea level) (mbsl) is very close to the upper limit of the gas hydrate stability zone in the water column (620 m below the sea level), as calculated from the CTD data within the study area, supporting the “hydrate skin” hypothesis. Gas chimneys directly below the bottom simulating reflectors, found at most sites, are speculated as essential pathways for both thermogenic gas and biogenic gas migrating from deep formations to the gas hydrate stability zone. The fracture network on the top of the basement uplift may be heavily gas-charged, which accounts for the chimney with several kilometers in diameter (beneath Plumes B and C). The much smaller gas chimney (beneath Plume D) may stem from gas saturated localized strong permeability zone. High-resolution seismic profiles reveal pipe-like structures, characterized by stacked localized amplitude anomalies, just beneath all the plumes, which act as the fluid conduits conveying gas from the gas hydrate-bearing sediments to the seafloor, feeding the gas plumes. The differences between these pipe-like structures indicate the dynamic process of gas seepage, which may be controlled by the build-up and dissipation of pore pressure. The 3D seismic data show high saturated gas hydrates with high RMS amplitude tend to cluster on the periphery of the gas chimney. Understanding the fluid migration and hydrate accumulation pattern of the Haima cold seeps can aid in the further exploration and study on the dynamic gas hydrate system in the South China Sea.

Mangroves are crucial for protecting coastal areas against extreme disasters such as tsunamis and storm surges. An experimental study was conducted to determine how mangroves can mitigate the tsunami wave propagation. The test was performed in a flume, where mangrove models were installed on a slope, and dam-burst waves were used to simulate tsunami waves. To study how mangrove forests reduce the impact of tsunamis, this paper measured the heights of the incoming waves under different initial conditions (tsunami wave intensity and initial water depth) and plant factors (arrangement and distribution density) and described the reduction process. The results show that, after passing through the mangrove, the tsunami bore height will decrease within a certain range as the initial water depth increases. However, there is no correlation between the increase of inundation level and the drop of water level. The bore height attenuation is more significant at higher density of mangroves, but after tsunami passing through the mangroves, the relative bore height will decrease. When the distribution density of mangroves is constant, the wave attenuation at different locations (before, on and after the slope) shows different relationships with the initial water depth and wave height for different models. The transmission coefficient ($ {K }_{i} $) shows a parabolic correlation with its density. The proportion of the energy loss caused by the mangrove resistance to the total energy ($ {E }_{b} $) is defined as $ {C}_{m2} $. The variation trend of $ {C}_{m2} $ corresponds to the tsunami wave energy attenuation rate ($ {C}_{a} $) and $ {K }_{i} $.

Reliable wave information is critical for marine engineering. Numerical wave models are useful tools to obtain wave information with continuous spatiotemporal distributions. However, the accuracy of model results highly depends on the quality of wind forcing. In this study, we utilize observations from five buoys deployed in the northern South China Sea from August to September 2017. Notably, these buoys successfully recorded wind field and wave information during the passage of five tropical cyclones of different intensities without sustaining any damage. Based on these unique observations, we evaluated the quality of four widely used wind products, namely CFSv2, ERA5, CCMP, and ERAI. Our analysis showed that in the northern South China Sea, ERA5 performed best compared to buoy observations, especially in terms of maximum wind speed values at 10 m height (U10), extreme U10 occurrence time, and overall statistical indicators. CFSv2 tended to overestimate non-extreme U10 values. CCMP showed favorable statistical performance at only three of the five buoys, but underestimated extreme U10 values at all buoys. ERAI had the worst performance under both normal and tropical cyclone conditions. In terms of wave hindcast accuracy, ERA5 outperformed the other reanalysis products, with CFSv2 and CCMP following closely. ERAI showed poor performance especially in the upper significant wave heights. Furthermore, we found that the wave hindcasts did not improve with increasing spatiotemporal resolution, with spatial resolution up to 0.5°. These findings would help in improving wave hindcasts under extreme conditions.

Understanding the connectivity of fish among different typical habitats is important for conducting ecosystem-based management, particularly when designing marine protected areas (MPA) or setting MPA networks. To clarify of connectivity among mangrove, seagrass beds, and coral reef habitats in Wenchang, Hainan Province, China, the fish community structure was studied in wet and dry seasons of 2018. Gill nets were placed across the three habitat types, and the number of species, individuals, and body size of individual fish were recorded. In total, 3 815 individuals belonging to 154 species of 57 families were collected. The highest number of individuals and species was documented in mangroves (117 species, 2 623 individuals), followed by coral reefs (61 species, 438 individuals) and seagrass beds (46 species, 754 individuals). The similarity tests revealed highly significant differences among the three habitats. Approximately 23.4% species used two habitats and 11.0% species used three habitats. A significant difference (p<0.05) in habitat use among eight species (Mugil cephalus, Gerres oblongus, Siganus fuscescens, Terapon jarbua, Sillago maculata, Upeneus tragula, Lutjanus russellii, and Monacanthus chinensis) was detected, with a clear ontogenetic shift in habitat use from mangrove or seagrass beds to coral reefs. The similarity indices suggested that fish assemblages can be divided into three large groups namely coral, seagrass, and mangrove habitat types. This study demonstrated that connectivity exists between mangrove–seagrass–coral reef continuum in Wenchang area; therefore, we recommend that fish connectivity should be considered when designing MPAs or MPA network where possible.

Zooplankton plays an important role in aquatic food webs by fluxing of energy from primary producer to subsequent trophic levels in the food chain. The annual pattern of zooplankton communities and potential environmental drivers were studied in the Kohelia channel, Bangladesh from summer 2014 to spring 2015. Samples were collected using net at a depth of 1 m. A total of 32 species belonged to 18 orders, 27 families and 15 taxonomic groups were identified. Of these species, 22 distributed in all four seasons of which 8 were dominant and highly contributing to the total communities. Species number peaked in summer next to winter and fall in spring while maximum abundance was in summer and minimum in spring. Multivariate analyses showed that there was a clear annual pattern in the zooplankton communities. Species diversity and evenness peaked in spring but fall in autumn while the high value of species richness was found in winter. Biological-environmental best matching (BIO-ENV) analyses conformed that community pattern of zooplankton was mainly driven by transparency salinity, and temperature individually or combined with water nutrients. These results demonstrate that annual pattern of the zooplankton community shaped by channel environmental factors in subtropical channel ecosystems, thus might be used for community-based subtropical coastal water bioassessment.

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.

To explore the geochemical characteristics and genesis of the elements in ferromanganese nodules from the Northwest Pacific, this study analyses the mineral composition, elemental content, occurrence phase and genetic mechanisms of samples by X-ray diffraction (XRD), inductively coupled plasma-optical emission spectrometry (ICP-OES), inductively coupled plasma-mass spectrometry (ICP-MS) and phase analysis methods. The results show that ferromanganese nodules are mainly hydrogenetic, and Mn/Fe content ratio ranges from 0.95 to 2.05. The major minerals are vernadite (δ-MnO₂) and amorphous ferric oxyhydroxide (FeOOH), and the secondary minerals include todorokite, birnessite, quartz and plagioclase. Ferromanganese nodules contain high contents of Co (0.24%–0.42%), Cu (0.23%–0.73%), Ni (0.33%–0.86%) and rare earth elements (REEs, 1192–1990 µg/g), which have positive Ce and negative Y anomalies but no Eu anomaly. A cluster analysis suggests that the elements in ferromanganese nodules can be divided into three groups: hydrogenetic components, including Fe, Ti, Zr, P, Pb, Co, Ba, Sr, V and REEs; diagenetic components, including Mn, Ni, Mg, Zn and Cu; and detrital components, including Al, Na, K and Ca. According to chemical leaching, ferromanganese nodules can be divided into four phases: Na, Ca, Mg and Sr are mainly enriched in the carbonate phase; Mn, Co, Ni and Ba are mainly enriched in the Mn-oxide phase; Fe, P, Ti, Cu, Pb, V, Zn, Zr and REEs are mainly enriched in the Fe-oxide phase; and Al and K are mainly enriched in the residual phase. A combination of the two different methods reveal selective enrichment of metal elements from seawater by ferromanganese nodules, featuring multisource mineralization. Moreover, through ion exchange and adsorption, approximately 71.2% of REEs are enriched in the Fe-oxide phase, 15.4% in the Mn-oxide phase and 12.4% in the residual phase, while REE contents in the carbonate phase are relatively low. In addition, under the oxic conditions of seawater, the oxidation of soluble Ce³⁺ to insoluble CeO₂ together with Fe-Mn minerals results in Ce enrichment in ferromanganese nodules. This study provides a reference for the metallogenesis of ferromanganese nodules from the Northwest Pacific.

Beach erosion has occurred globally in recent decades due to frequent and severe storms. Dongsha beach, located in Zhujiajian Island, Zhejiang Province, China, is a typical embayed sandy beach. This study focused on the morphodynamic response of Dongsha beach to typhoon events, based on beach topographies and surficial sediment characteristics acquired before and after four typhoon events with varying intensities. The four typhoons had different effects on the topography and sediment characteristics of Dongsha beach. Typhoons Ampil and Danas caused the largest (−51.72 m³/m) and the smallest erosion (−8.01 m³/m), respectively. Remarkable alongshore patterns of beach profile volumetric changes were found after the four typhoon events, with more erosion in the southern and central parts of the beach and few changes in the northern part. Grain size coarsening and poor sorting were the main sediment patterns on the beach influenced by different typhoons. Typhoons that occurred in the same year after another typhoon enhanced the effect of the previous typhoon on sediment coarsening and sorting variability, but this cumulative effect was not found between typhoons that occurred during different years. A comparison of the collected data revealed that the topographic state of the beach before the typhoon, typhoon characteristics, and tidal conditions were possible reasons for the difference in the responses of Dongsha beach to typhoon events. More severe beach erosion was caused by typhoons with higher intensity levels and longer durations, and high tide levels during typhoons can determine the upper limit of the beach profile erosion site. Taken together, these results can be used to improve beach management for storm prevention.