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.

Sediment core samples were collected from 17 stations in the middle and eastern Chukchi Sea during the sixth Chinese National Arctic Research Expedition (CHINARE-Arctic) in summer 2014. The samples were analyzed for composition, abundance, biomass, vertical distribution, size spectra, and ecological indexes of meiofauna. A total of 14 meiofauna taxa were detected, and the free-living marine nematodes comprised the most dominant taxon, accounting for 97.21% of the average abundance. The abundance and biomass of meiofauna were within ranges of (218.12±85.83)–(7 239.38±1 557.15) ind./(10 cm²) and (130.28±52.17)–(3 309.56±1 751.80) μg/(10 cm²), with average values of (2 391.90±1 966.19) ind./(10 cm²) and (1 549.73±2 042.85) μg/(10 cm²) (according to dry weight) respectively. Furthermore, 91.26% of the individuals were distributed in the top layer of 0–5 cm of surface sediment, and 90.84% had sizes of 32–250 μm. Group diversity index of meiofauna in the survey area was low, and the variation of abundance was the main difference in meiofauna communities among all stations. Abundance and biomass of meiofauna were not significantly correlated with environmental factors except concentration of nutrient Si in bottom seawater. Abundance of meiofauna in shallow water of marginal seas in the Pacific sector of the Arctic Ocean is likely at a same level and higher than that in most of China sea areas, suggesting that the shallow water of the summer Chukchi Sea is a continental shelf area with rich resources of meiofauna. The Chukchi Sea is important for studying the ecosystem of the Arctic Ocean and environmental responses. However, studies on meiofauna in the Chukchi Sea are still not enough, and in the future, natural and human disturbances may increase due to global warming, the Arctic channel opening, and other factors. Thus, more studies on meiofauna should be required, in order to know more about how the Arctic benthic community would alter.

Metal-enriched minerals have been widely observed near hydrothermal vent fields. However, the dynamics of particulate metals influenced by hydrothermal activities is poorly constrained. Here, radioactive ²³⁴Th in both dissolved and particulate phases were used to examine the kinetics of particle-reactive metal adsorption, removal, and residence in a newly found hydrothermal plume over the Southwest Indian Ridge. The results showed a relatively low value on ²³⁴Th/²³⁸U ratios (i.e., 0.73–0.88) compared to the deep oceans, indicating an enhanced adsorption of particle-reactive metals onto particulate matter in the plume. Based on the ²³⁴Th-²³⁸U disequilibria, the adsorption and sinking rate constants of ²³⁴Th averaged (0.009±0.001) d^–1 and (0.113±0.024) d^–1 in the hydrothermal plume, corresponding to the residence times of (115±19) d and (16±5) d for dissolved and particulate ²³⁴Th, respectively. This timescale allows vent-discharged particle-reactive metals to disperse hundreds to thousands of miles away. Thus, hydrothermal activities might influence the metal distribution in deep ocean over a very large scope. Also, a high sinking flux of (36.2±5.4) Bq/(m²·d) for ²³⁴Th was observed for the plume, suggesting an enrichment of metal in particles deposited close to the vent. The enhancement of particle sinking could also benefit the transport of organic carbon and nitrogen and fuel the benthic ecosystems under the plume regimes. Thus, hydrothermal plumes may have an impact on both the elemental geochemistry and/or ecosystem to the deep oceans interior than previous expectation.

Living coccolithophores (LCs) are regarded as a group of calcifiers and play important roles in global carbon cycle. This study used microscopic observations of LCs in the western Pacific Ocean to investigate their community structure and biodiversity, especially to test whether local physical traits (mesoscale eddies) could explain their biogeographic distributions during autumn of 2017. The coccolithophore calcite inventory based on carbon-volume transformation was estimated in this study. A total of 28 taxa of coccospheres and 19 types of coccoliths were identified from 161 samples. Gephyrocapsa oceanica was the most predominant species in all the coccolithophore community, followed by Florisphaera profunda, Emiliania huxleyi, Umbilicosphaera sibogae, Gladiolithus flabellatus and Umbellosphaera tenuis. The abundance of coccospheres and coccoliths ranged from 0 to 26.8×10³ cells/L and from 0 to 138.5×10³ coccoliths/L, averaged at 4.2×10³ cells/L and 10.9×10³ coccoliths/L, respectively. This study indicated that coccolithophore community in the survey area can be clustered into four groups. Three ecological niches of coccolithophores were characterized by their vertical profiles and multivariate statistical analysis. Coccolithophore abundance and species composition were remarkably different among warm-eddy region, G. oceanica dominated warm-eddy region, while F. profunda dominated warm-eddy and none-eddy region. The average values of estimated particulate inorganic carbon, particulate organic carbon were 0.197 μg/L and 0.140 μg/L, respectively. The current field study widened the dataset of coccolithophores in western Pacific Ocean.

A laboratory-based microcosm experiment was carried out to examine both the behavioral and antioxidant response of the clam Gomphina veneriformis under the conditions of 3 types of burial material (sand, silt, silt-sand mixture) with 3 burial depths (5 cm, 15 cm, 30 cm). The concentration of dissolved oxygen decreased significantly after 3 d of burial in all experimental groups. In silt and sand-silt mixture groups, the interstitial water quality became worsened with lower pH, and higher $ {\rm {NH}}_4^+$-N concentration, where clam mortality occurred simultaneously. However, clam samples in all sand groups and 5 cm, 15 cm sand-silt mixture groups survived well for 8 d. Obviously fewer individuals left in the bottom sand in the 15 cm, 30 cm silt groups and 30 cm sand-silt mixture groups than in the 5 cm groups. Therefore, it suggests that adding silt and increasing burial depth could stimulate the vertical movement of organisms and cause lethal effects. It was found that the burial depth was the key factor that influenced the activities of antioxidant enzymes, such as superoxide dismutase (SOD) and catalase (CAT). The SOD and CAT activities in the gills and hepatopancreases of organisms both showed significant up-regulation in 30 cm burial depth after buried for 8 d. Higher enzyme activities were found in gills than in hepatopancreases, which indicated that the gills of the bivalve G. veneriformis were more susceptible to burial effects than hepatopancreases. Overall, this study shows that sediment burial could cause effects on the biological behavior and antioxidant enzyme activities.

Phytoplankton communities can response immediately and directly to environmental changes, and thus have been applied as reliable biotic indicators in aquatic systems. This study provided insights into the relationships concerning ecological thresholds of phytoplankton communities and individual taxon in response to environmental changes in coastal waters of northern Zhejiang Province, East China Sea. Results demonstrated that there existed seasonal variations of phytoplankton community ecological thresholds of which spring being higher than those in summer. As for individual species, Prorocentrum donghaiense and Noctiluca scintillans were identified as the most tolerant and sensitive indicator species in spring and summer, respectively. They exhibited strong indications in response to environmental changes. These findings highlighted that phytoplankton community structure in this region was stable when environmental gradients were below the thresholds of sensitive species, whereas potential harmful algal blooms may occur when environmental gradients exceeded the thresholds of tolerant species.

Quantifying the gross and net production is an essential component of carbon cycling and marine ecosystem studies. Triple oxygen isotope measurements and the O₂/Ar ratio are powerful indices in quantifying the gross primary production and net community production of the mixed layer zone, respectively. Although there is a substantial advantage in refining the gas exchange term and water column vertical mixing calibration, application of mixed layer depth history to the gas exchange term and its contribution to reducing indices error are unclear. Therefore, two cruises were conducted in the slope regions of the northern South China Sea in October 2014 (autumn) and June 2015 (spring). Discrete water samples at Station L07 in the upper 150 m depth were collected for the determination of δ¹⁷O, δ¹⁸O, and the O₂/Ar ratio of dissolved gases. Gross oxygen production (GOP) was estimated using the triple oxygen isotopes of the dissolved O₂, and net oxygen production (NOP) was calculated using O₂/Ar ratio and O₂ concentration. The vertical mixing effect in NOP was calibrated via a N₂O based approach. GOP for autumn and spring was (169±23) mmol/(m²·d) (by O₂) and (189±26) mmol/(m²·d) (by O₂), respectively. While NOP was 1.5 mmol/(m²·d) (by O₂) in autumn and 8.2 mmol/(m²·d) (by O₂) in spring. Application of mixed layer depth history in the gas flux parametrization reduced up to 9.5% error in the GOP and NOP estimations. A comparison with an independent O₂ budget calculation in the diel observation indicated a 26% overestimation in the current GOP, likely due to the vertical mixing effect. Both GOP and NOP in June were higher than those in October. Potential explanations for this include the occurrence of an eddy process in June, which may have exerted a submesoscale upwelling at the sampling station, and also the markedly higher terrestrial impact in June.

The stock enhancement programs for black sea bream Acanthopagrus schlegelii have been conducted in China for a few years. However, little information has been reported concerning the effectiveness and genetic effect of black sea bream stock enhancement. In order to detect the contribution of released individuals in Zhujiang River Estuary (ZRE) and Daya Bay (DB), six microsatellite markers were used to identify the hatchery-released individuals. In addition, this pedigree of hatchery populations (broodfish and hatchery-released offspring) was traced to detect the number of effective parents (N_e), the inbreeding coefficient and the decrease of genetic variability in the reproduction. The pedigree reconstruction showed that at least 69 (out of 93) broodfish had offspring. The estimated N_e was 54.8, consequently the inbreeding coefficient was 0.91%. The genetic diversity of hatchery-released offspring was lower than that in that of broodfish (heterozygosity alleles, 0.727–0.774), some alleles (number of alleles, 61–69) and genetic variance were lost during reproduction. It was observed that wild samples had higher levels of genetic diversity compared with hatchery populations as well as recaptured samples in releasing area. A total of 128 hatchery-released black sea bream were identified among 487 recaptured samples in ZRE, while a total of 15 samples were identified among 96 samples in DB. In summary, there was a high survival of released fish. Nevertheless, the results provided evidence to consider a loss of genetic variation in hatchery-released stock and a negative genetic effect of the stock enhancement.

To characterize environmental factors controlling decadal-scale variations in the buried flux of marine organic carbon (${\rm{BF}}_{\rm{C_m}} $) in the eastern shelf sea areas of China (ECSS), four well preserved sediment cores collected from the central Yellow Sea mud (CYSM) area, the Yellow Sea Coastal Current (YSCC) area and the Changjiang River Estuary (CRE) were investigated in this study. In the CYSM, variations in ${\rm{BF}}_{\rm{C_m}} $ were found to be dependent on variations in primary productivity and to exhibit a cyclical trend possibly related to fluctuations in the Pacific Decadal Oscillation (PDO) and the East Asian winter monsoon index (EAWM). In the YSCC, ${\rm{BF}}_{\rm{C_m}} $ likewise depends on primary productivity. Prior to the 1950s, variations in ${\rm{BF}}_{\rm{C_m}} $ were similar to that of the EAWM. After the 1950s, ${\rm{BF}}_{\rm{C_m}} $ increased rapidly and exhibited maximum values in the surface layer, consistent with an increase in primary productivity caused by the input of terrestrial nutrients associated with China’s economic development. In the CRE, variations in ${\rm{BF}}_{\rm{C_m}} $ were affected by several competing factors making it difficult to identify clear relationships between variations in ${\rm{BF}}_{\rm{C_m}} $ and primary productivity. In contrast, long-term variability in ${\rm{BF}}_{\rm{C_m}} $ is more similar to changes in the Changjiang River sediment load. Thus, it is speculated that the construction of dams along the Changjiang River may be the main cause of variations in ${\rm{BF}}_{\rm{C_m}} $ in this area. Given the disproportionate effects of human activities on marine environments and decadal variations in ${\rm{BF}}_{\rm{C_m}} $ in the ECSS, careful attention should be paid to regional differences in organic carbon preservation and environmental changes lest estimates of these values be made imprecise or inaccurate.

The waters near the Antarctic Peninsula have always been a study hot spot because of their variable and unique oceanographic conditions. To determine the distribution and possible influencing factors on phytoplankton size and abundance near the Antarctic Peninsula, a large-scale survey was conducted during the austral summer of 2018. Samples were collected in 27 stations located in the Drake Passage (DP), South Shetland Islands (SSI), and South Orkney Islands (SOI). Phytoplankton communities were described using chlorophyll a (Chl a), flow cytometry and light microscopy to cover a size range from pico- to microphytoplankton. Nanophytoplankton, especially small nanophytoplankton (2−6 μm) with abundance ranging from 0.66 ×10³ cells/mL to 8.46 ×10³ cells/mL, was predominant throughout the study area. Among different regions, there was an obvious size shift. The proportion of picophytoplankton near the Elephant Island (EI) and DP was higher than other regions, and larger cells were found mainly in east of SOI. The distribution of phytoplankton abundance detected by flow cytometry was not completely consistent with Chl a concentrations due to the contribution of larger cells to Chl a. Possible influencing factors on the phytoplankton size distribution were discussed. The properties of water masses such as temperature and salinity can influence the phytoplankton size distribution. Correlation analysis revealed that only picophytoplankton is significantly correlated with salinity. Light and Fe availability might affect phytoplankton abundance and size distribution especially near the waters of SSI and EI in this study. It was also speculated that the abundance of cryptophytes is possibly related to ice melting.