Since 2015, green tides with Ulva prolifera as the dominant species in the Qinhuangdao coastal waters have continued to occur. In this study, the relationship between green tides in Qinhuangdao and the Yellow Sea (setting sites in Rudong and Qingdao) was evaluated by genetic analyses of U. prolifera. Single nucleotide polymorphism (SNP) markers were used to analyze genetic diversity and genetic relationships among groups. Genetic differentiation was lower among floating U. prolifera populations in Rudong and Qingdao than in Qinhuangdao. The floating U. prolifera population had higher genetic diversity and polymorphism levels in Qingdao and Rudong than in Qinhuangdao. Physiological experiments showed that the growth rate and net buoyancy of floating U. prolifera were highest in Qinhuangdao and Qingdao, respectively, under the same environmental conditions (temperature and light). Overall, these findings showed that U. prolifera populations in the Qinhuangdao and Yellow Sea green tides (Rudong and Qingdao) differ significantly at the molecular and physiological levels. Therefore, the Qinhuangdao green tide is not correlated with the Yellow Sea green tide and has a different origin and development mode. This study provides insight into the mechanism underlying green tide blooms in coastal waters of China.

The English Channel (the Channel) represents a major sink and transport pathway of anthropogenic radioactive ¹²⁹I. Despite this important role, data concerning the distribution of ¹²⁹I in seawater of the Channel are scarce, and most of existing data are restricted to the eastern part of the Channel. The advection and dispersion of ¹²⁹I from the French coast toward the central and further the English coast, especially in the Channel west of Cap de La Hague, are not fully investigated. We present results of iodine isotopes (¹²⁷I and ¹²⁹I) analyses of surface water samples collected along the central English Channel in October, 2010. The data show high ¹²⁹I concentrations between Dover Strait and La Hague, followed by a dramatic drop towards the Celtic Sea and reveal the dispersal of ¹²⁹I towards central and northern part of the Channel. Our observation also implies that the entire British coast is contaminated by ¹²⁹I. ¹²⁹I levels in the westernmost English Channel, close to the English coast, may reflect combined influences from La Hague and Sellafield. Evolution of ¹²⁹I between 2005 and 2010 suggests a strong link to temporal marine discharges from La Hague plant. The discharges from the nuclear reprocessing facility have continued since 2010 and thus an ecological evaluation of ¹²⁹I radioactive hazards in the environment of the Channel may be needed.

The productivity and health of seagrass depend on the combined inputs of nutrients from the water and sediments in which they grow and the microbiota with which they live intimately. However, little is known about the composition and diversity pattern of single-celled benthic eukaryotes in seagrass meadows. Here, we investigated how the structure and diversity of the benthic microeukaryotic community vary with respect to season, location, and seagrass colonization, by applying 18S rRNA gene amplicon sequencing for 96 surface sediment samples that were collected from three different seagrass habitats through four seasons. We found that benthic microeukaryotic communities associated with seagrass Zostera japonica exhibited remarkable spatial and seasonal variations, as well as differences between vegetated and unvegetated sediments. Diatoms and dinoflagellates predominated in the benthic microeukaryotic communities, but they were inversely correlated and displaced each other as the dominant microbial group in different seasons or habitats. Mucoromycota was more prevalent in vegetated sediments, whereas Lobulomycetales and Chytridiales had higher proportions in unvegetated sites. Total organic carbon and total organic nitrogen were the most important environmental factors in driving the microeukaryotic assemblages and diversity. Our study expands the available knowledge on the biogeographic distribution patterns and niche preferences for benthic microeukaryotes in seagrass systems.

The impacts of the Hong Kong-Zhuhai-Macao Bridge (HKZMB) on suspended sediment content (SSC) were analysed in the Zhujiang River Estuary based on data from HY-1C, which was launched in September 2018 in China, carrying Coastal Zone Imager (CZI) and Chinese Ocean Color and Temperature Scanner on it. A new SSC inversion model was established based on the relationship between in-situ SSC and the remote sensing reflectance in red and near-infrared bands of CZI image. HY-1C satellite data obtained from October to December 2019 were applied to retrieve SSC in the Zhujiang River Estuary. The results show that SSC around the HKZMB is ranging from 20 mg/L to 95 mg/L. SSC change obviously on two sides of the bridge. During flooding and ebbing period, SSC increases obviously downstream of the bridge. SSC difference between upstream and downstream is ranging from 5 mg/L to 20 mg/L. Currents flowing across the HKZMB, the change trend of SSC in most places upstream and downstream is almost the same that SSC downstream of the bridge is higher than SSC upstream. The tidal currents interact with bridge piers, inducing vortexes downstream, leading the sediment to re-suspend downstream of the bridge piers. Other factors, including seafloor topography and wind, can also contribute to the distribution of SSC in the Zhujiang River Estuary.

Resource exploitation in the Clarion-Clipperton Zone (CCZ) is of major research interest worldwide, but its influence on the environment is poorly understood, especially due to the lack of baseline values for metals in the surrounding sediment. This work aimed to establish the baseline values of 17 metals (Ba, Ca, K, Mg, Mn, Na, Ag, As, B, Cd, Co, Cr, Cu, Hg, Ni, Pb, and Zn) using normalization, the cumulative frequency curve method considering a total of 172 samples taken from 8 multitube cores and 1 box sediment core collected in the western CCZ during the COMRA-45 cruise campaign from August to September 2017. The baseline values of the evaluated metals were as follows: 1 932 mg/kg for Ba, 29 512 mg/kg for Ca, 18 150 mg/kg for K, 17 120 mg/kg for Mg, 6 747 mg/kg for Mn, 28 546 mg/kg for Na, 0.571 mg/kg for Ag, 5.00 mg/kg for As, 94.4 mg/kg for B, 0.626 mg/kg for Cd, 104 mg/kg for Co, 76.1 mg/kg for Cr, 370 mg/kg for Cu, 0.028 mg/kg for Hg, 190 mg/kg for Ni, 27.5 mg/kg for Pb and 156 mg/kg for Zn. Our findings would fill the baseline value gap in the study area and further improve accuracy of environmental impact assessments on the impact of resource exploitation.

The Clarion-Clipperton Zone (CCZ) hosts one of the largest known oceanic nodule fields worldwide and is regulated by the International Seabed Authority. A baseline assessment of diversity and distribution patterns is essential for reliable predictions of disturbed ecosystem response scenarios for sustained commercial activities in the future. In the present study, the spatial patterns and diversity of phytoplankton communities were analyzed along with upper ocean biogeochemistry, in the licensed China Ocean Mineral Resources R&D Association (COMRA) contract area and the surrounding western CCZ between August 21 and October 8, 2017. Results indicated this was a typical low-nutrient low-chlorophyll a (Chl a) environment, characterized by low levels of phytoplankton abundance and diversity. In total 112 species belonging to 4 phyla were recorded (>10 μm), with species counts including 82 diatoms, 27 dinoflagellates, 1 cyanobacteria and 2 chrysophyte. Dominant taxa in successive order of descending abundance and occurrence included Nizschia marina, Cyclotella stylorum, Dactyliosolen mediterraneus, Rhizosolenia setigera, Pseudo-nitzschia delicatissima, Thalassiothrix frauenfeldii, Synedra sp., Chaetoceros simplex and Pseudo-nitzschia circumpora. The depth-averaged abundance and Chl a concentrations were (265±233) cells/L and (0.27±0.30) μg/L, respectively. Diatoms accounted for 90.94% of the community with (241±223) cells/L, while dinoflagellates accounted for 5.67% and (15±13) cells/L. The distribution pattern exhibited the same trend as abundance, Chl a and species richness, showing subsurface maximum levels at around 100 m, with stations near 10°N having higher levels than in the north. Cluster analysis was performed in two assemblages, relating to geographic locations to the south and north of 12°N. The subsurface maximum of abundance, Chl a, species richness, dissolved oxygen and nitrite were generally corresponding to the presence of high salinity North Pacific Central Water at depths of 50−120 m. Higher availability of nitrate, phosphate and silicic acid in the subsurface may account for the shift in phytoplankton distribution, as shown by redundancy correspondence and spearman correlation analysis. Diel variation in an anchor station demonstrated prominent species succession without significant differences in oceanographic variables, among which diatoms succession resulted from the light limitation, while dinoflagellate diel variation mainly related to lateral transport of water masses. The observed patchiness in spatial phytoplankton distributional patterns was attributed to upper ocean environmental gradients in the CCZ. The baseline generated in this study could be analyzed using current conservation strategy programs associated with deep-sea mining.

In the Xiangshan Bay at the east coast of China, coastal marine pollution is conspicuous and severe in recent years. As transport of the pollutants is closely related to the coastal circulation, there is a great practical significance to investigate the circulation in this area. In this work, the surface pattern and vertical profiles of Lagrangian residual velocity (LRV) were studied based on field observation data from the inner Xiangshan Bay. By tracking GPS-GPRS drifters’ trajectories, the surface LRV pattern is going out in the central deep trough and flowing inwards near the shoreside. Combined with data from two mooring stations, vertical profiles of LRV is flowing out at surface and flowing in at the bottom, consistent with the gravitational circulation induced by baroclinic effects at the estuary. However, according to the diagnostic analysis, the main mechanism driving the residual current is barotropic rather than baroclinic. The LRV equation is controlled by the tidally-averaged barotropic pressure gradient force, tidal body force and tidally-averaged turbulent stress, while the tidally-averaged baroclinic pressure gradient force is one order of magnitude less than other forces. Additionally, the tidally mean eddy viscosity coefficient which is used in the expression of tidally-averaged turbulent stress might be not adequate and requires further studies.

The near-inertial waves (NIWs) are important for energy cascade in the ocean. They are usually significantly reinforced by strong winds, such as typhoon. Due to relatively coarse resolutions in contemporary climate models, NIWs and associated ocean mixing need to be parameterized. In this study, a parameterization for NIWs proposed by Jochum in 2013 (J13 scheme), which has been widely used, is compared with the observations in the South China Sea, and the observations are treated as model outputs. Under normal conditions, the J13 scheme performs well. However, there are noticeable discrepancies between the J13 scheme and observations during typhoon. During Typhoon Kalmaegi in 2014, the inferred value of the boundary layer is deeper in the J13 scheme due to the weak near-inertial velocity shear in the vertical. After typhoon, the spreading of NIWs beneath the upper boundary layer is much faster than the theoretical prediction of inertial gravity waves, and this fast process is not rendered well by the J13 scheme. In addition, below the boundary layer, NIWs and associated diapycnal mixing last longer than the direct impacts of typhoon on the sea surface. Since the energy dissipation and diapycnal mixing below the boundary layer are bounded to the surface winds in the J13 scheme, the prolonged influences of typhoon via NIWs in the ocean interior are missing in this scheme. Based on current examination, modifications to the J13 scheme are proposed, and the modified version can reduce the discrepancies in the temporal and vertical structures of diapycnal mixing.

A wave-current-sediment coupled numerical model is employed to study the responses of suspended sediment transport in the wet season to changes in shoreline and bathymetry in the Zhujiang (Pearl) River Estuary (ZRE) from 1971 to 2012. It is shown that, during the wavy period, the large wave-induced bottom stress enhances sediment resuspension, resulting in an increase in the area of suspended sediment concentration (SSC) greater than 100 mg/L by 183.4%. On one hand, in spring tide, the change in shoreline reduces the area of SSC greater than 100 mg/L by 17.8% in the west shoal (WS) but increases the SSC, owing to the closer sediment source to the offshore and the stronger residual current at the Hengmeng (HEM) and Hongqili (HQL) outlets. The eastward Eulerian transport is enhanced in the WS and west channel (WC), resulting in a higher SSC there. The reclamation of Longxue Island (LXI) increases SSC on its east side and east shoal (ES) but decreases the SSC on its west and south sides. Moreover, in the WC, the estuarine turbidity maximum (ETM) is located near the saltwater wedge and moves southward, which is caused by the southward movement of the maximum longitudinal Eulerian transport. In neap tide, the changes are similar but relatively weaker. On the other hand, in spring tide, the change in bathymetry makes the SSC in the WS increase, and the area of SSC greater than 100 mg/L increases by 11.4% and expands eastward and southward, which is caused by the increases in wave-induced bottom stress and eastward Eulerian transport. On the east side of the WC, the eastward Eulerian transport decreases significantly, resulting in a smaller SSC in the middle shoal (MS). In addition, in the WC, the maximum SSC is reduced, which is caused by the smaller wave-induced bottom stress and a significant increase of 109.88% in southward Eulerian transport. The results in neap tide are similar to those in spring tide but with smaller changes, and the sediment transports northward in the WC owing to the northward Eulerian transport and vertical shear transport. This study may provide some references for marine ecological environment security and coastal management in the ZRE and other estuaries worldwide affected by strong human interventions.

The wave-induced setup and circulation in a two dimensional horizontal (2DH) reef-lagoon-channel system is investigated by a non-hydrostatic model. The simulated results agree well with observations from the laboratory experiments, revealing that the model is valid in simulating wave transformation and currents over reefs. The effects of incident wave height, period, and reef flat water depth on the mean sea level and wave-driven currents are examined. Results show that the distributions of mean sea level and current velocities on the reef flat adjacent to the channel vary significantly from those in the area close to the side walls. From the wave averaged current field, an obvious alongshore flux flowing from the reef flat to the channel is captured. The flux from the reef flat composes the second source of the offshore rip current, while the first source is from the lagoon. A detailed momentum balance analysis shows that the alongshore current is mainly induced by the pressure gradient between the reef flat and the channel. In the lagoon, the momentum balances are between the pressure and radiation stress gradient, which drives flow towards the channel. Along the channel, the offshore current is mainly driven by the pressure gradient.