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 Land-ocean interface has been a challenge in determining the boundary conditions for earth system modelling. Due to the uncertainty of material fluxes, the localization effect on counter gradient convergence, and the existence of self-adapted structures, land-ocean interface is unable to be monitored ergodically under the framework of classic linear theory. There is a great difficulty in quantifying real parameters for this interface, which is the key connection between linear and nonlinear systems under harmonic conditions, to better improve the simulation and prediction capability of earth system modelling. In a previous work, based on a two-dimensional multi-constituent tidal model and its adjoint model, together with assimilation using satellite altimeter data and tidal gauge array data, the spatial and temporal variation in bottom friction coefficient was estimated in an improved way. Inspired by the abovementioned study, this contribution proposed a suggestion of complex parameterization for land-ocean interface, in combination with remote sensing observation and numerical modelling.

Since a diagram depicting "The Great Ocean Conveyor" was first appeared as a logo by Wally Broecker in 1987, how the warmed upper ocean circulation ran through the Indonesian Seas with their complex coastline geometry and narrow passages, known as the Indonesian through-flow, becomes one of the difficult settings in boundary conditions of climate change model. With strong nonlinear effect, a shallow narrow passage forms vertical mixed hotspots on the one hand, and on other hand, it becomes a self-adapted low-pass filter if the tidal resonance units are introduced. Qiongzhou Strait, a sufficient sediment supply and shallow narrow passage for the northern shelf of the South China Sea, is reported with strong tidal current and westward through-flow. And its tidal channel and tidal deltas are maintained by the tidal resonance units introduced themself. Strong disturbances such as typhoon and cold wave can cause storm jet flow through the strait, triggering different high nutrition, algal bloom and hypoxia events over the adjacent Beibu Gulf. Qiongzhou Strait seems to be a noteworthy case of studying how the self-adapted low-pass filter introduced and whether human activities can affect the through-flow on the shallow narrow passages.

Marine sands are important submarine mineral resources. The marine sand resources of Hainan Island are widely distributed in coastal zones and continental shelf areas, with large scale, good quality and rich in heavy minerals. The basic distribution characteristics of marine sand resources around Hainan Island are described, and the progress and main achievements of marine sand exploration around Hainan Island are reviewed with emphasis on heavy mineral sand and aggregate sand. Through the exploration, 15 abnormal areas of heavy mineral placer have been delineated in the sea area around Hainan Island, and one large zirconiumtitanium placer deposit has been proved. However, due to the low level of investigation and research, the resource potential of heavy mineral placer is not clear. The scale of sediment survey around Hainan Island has basically reached 1∶250 000, which provides a solid foundation for the exploration of aggregate sand resources. In recent years, billions of cubic meters of aggregate sand resources have been discovered in the east entrance of Qiongzhou Strait and tidal sand ridge area of the western Hainan, and the resource potential is predicted to reach tens of billions of cubic meters. According to the comprehensive analysis, 8 potential exploration areas of marine sand resources can be delineated around Hainan Island. Except the east mouth of Qiongzhou Strait and the tidal sand ridge area of the western Hainan, the northeast continental shelf of Hainan, the southwest continental shelf of Hainan and Beibu Gulf area are all favorable enrichment areas of marine sand resources, and may contain heavy mineral resources potential. According to the specific sedimentary environment, the exploration objectives and tasks of each prospective area are discussed respectively. Combined with the current work progress and existing problems, the suggestions for the future marine sand exploration are put forward.

Climate reconstructions for the last millennium are essential for understanding climate change and will provide an important basis for predicting future climate change. However, there are spatial differences in monsoonal precipitation, and the hydrothermal configurations of two important characteristic periods of the past millennium, the Medieval Warm Period and the Little Ice Age, are highly controversial, especially in southern China. Therefore, the Core BBG-02 taken from the northeastern Beibu Gulf were analyzed for chronology, grain size, major elements and trace elements to understand the physical source of the Core BBG-02 and the climate and environmental changes in its source area. The analysis of chondrite-normalized distribution patterns of rare earth element and geochemical indicators reveal that the Red River may be the main source of the Core BBG-02, and the changes in geochemical indicators of the core reflect changes in the intensity of Indian Summer Monsoon. The results indicate that the climate was warm and humid during the Medieval Warm Period and cold and arid during the Little Ice Age, which is consistent with the results of previous studies on the variation of Indian Summer Monsoon and precipitation. The variation of solar radiation intensity is the essential reason of the variation of Indian Summer Monsoon, which is also influenced by ENSO activity and Indian Ocean Dipole.

Based on remote sensing images and geographic information system technology, the coastline and reclamation information of Fangcheng Bay in China and Haiphong Port in Vietnam from 1980s to 2010s were compared. The results showed that in the past 30 years or so, the coastline of the two places mainly migrated to the sea, of which the average end point rate of Fangcheng Bay was 6.49 m/a, and that of Haiphong Port was 6.21 m/a, showing a trend of rising first then falling and falling first then rising, respectively. In the 1980s and 1990s, the average end point rate of Fangcheng Bay was higher than that of Haiphong Port, while in the 2000s and 2010s, it was the opposite. In the past 30 years or so, the average new reclamation area of the two bays was 122.48 hm²/a and 164.24 hm²/a, respectively, and the reclamation intensity index of the two places showed a rising trend. In the 1980s and 1990s, the reclamation intensity index of Fangcheng Bay was smaller than that of Haiphong Port, while in the 2000s and 2010s, it was the opposite. From 1980s to 2010s, the reclamation area and the average spatial expansion rate per year increased first in Fangcheng Bay and then decreased, while Haiphong Port continued to fluctuate. Reclamation was the main factor for the change of the coastline of the two places, and port construction was the main driving force.

 Hydrological observations were carried out in the northeastern of Beibu Gulf in July and October 2018, February 2019 to reveal its seasonal variations. Sectional measurements of temperature, salinity and current velocity were deployed to the north of 18°N and east of 108°E. The results show that there is obvious seasonal disparities on the character of water masses in this region: the sea water temperature in summer decreases from northern shallow water to southern deep region and the onshore brackish water dominates the area shallower than 30 m, and a occlusive cold water mass appears in deep layer to the west of Hainan Island; while in winter the water temperature increases from north to south, the brackish water shrinks to the northern coast shallower than 10 m, the area is occupied mainly by the mixed water, and a warm tongue appears in upper layer on the west of Hainan Island. The circulation in the research region is mainly counterclockwise in all three seasons, though the strength of current varies in different seasons, and is significantly influenced by wind. The water exchange between the northeastern gulf and outer ocean is not active, since the observed salinity is entirely less than 34. The water mass is generally shear instable, and static instability appears in winter. This suggests that the diapycnal mixing in this region might be strong. 

Based on the runoff, the chemical oxygen demand (COD), total nitrogen (TN) and total phosphorus (TP) data in the Maolingjiang River Estuary section from 2014 to 2020, the inflow fluxes of COD, TN and TP were calculated by the LOADEST model. Results showed that: (1) The regression equation of COD, TN and TP fluxes into the sea from the Maolingjiang River was well fitted by the LOADEST model, there were consistencies between the simulated and measured values. (2) The simulated multi-year average COD, TN and TP inflow fluxes were 34 537 t/a, 3 302 t/a and 235 t/a, and the monthly average inflow fluxes were 2 878 t/month, 275 t/month and 19.58 t/month, respectively. COD, TN and TP inflow fluxes in the wet season accounted for 77%, 80% and 71% of the total annual inflow fluxes, respectively. The multi-year average inflow fluxes of COD, TN and TP were 21 059~25 271 t/a, 1 775~2 130 t/a and 70~84 t/a from non-point sources, respectively. COD and TN were mainly controlled by non-point sources. TP was controlled not only by non-point sources but also by point sources. (3) Comparatively, the multi-year average inflow fluxes of COD, TN and TP of the Qinjiang River were about 1.16, 2.15 and 2.37 times those of the Maolingjiang River, respectively. The difference of inflow fluxes of COD, TN and TP from the Qinjiang River and Maolingjiang River were mainly affected by the number of people living along the river.

Taking Beibu Gulf, South China Sea as a case, the occurrence, source, and ecological risks of 12 organophosphate esters (OPEs) were investigated in 34 surface sediment samples. The Σ12OPEs in the sediments ranged from <LOQ to 35.1 ng·g^-1. Triphenyl phosphate (TPHP), tri-n-butyl phosphate (TNBP), tris (2-chloro-propyl) phosphate (TCIPP) and tris (2-chloroethyl) phosphate (TCEP) were the dominant OPEs in the surface sediments. Generally, the total mass fraction of OPEs detected in sediments from the offshore was higher than that in the near shore, and the mass fraction for major OPEs were also different between the above two areas. On the one hand, the main reason might be related to their physiochemical properties. TPHP and TNBP compounds with higher lipophilicity might be more likely to accumulate in the sediments. On the other hand, it might be related to the source. For example, the OPEs detected in the offshore might be related to the oil exploitation and transportation, submarine optical cable and oil pipeline laying, shipping activities and the transportation of circulation in Beibu Gulf, while the discharge of life and industrial sewage as well as marine aquaculture might be the major sources for OPEs detected in the inshore area. Risk assessment revealed that most individual OPE could pose low ecological risks, but medium ecological risks of TPHP and the mixture of OPEs on aquatic organisms were found, which requires more attention. Therefore, the OPEs pollution situation in Beibu Gulf needs to be paid attention to, especially the pollution emission and control TCIPP and TCEP from inshore area, as well as TPHP and TNBP from offshore area, so as to reduce OPEs pollution from the source.

²¹⁰Po is a typical natural particle active radionuclide, which can be used as a tracer for the study of atmospheric aerosol deposition dynamics and particle dynamics in marine environment. In this study, the water quality parameters and the specific activities distributions and behavior of different phases for ²¹⁰Po in six offshore sections of the north coastal Beibu Gulf were investigated. Results showed that the specific activities of dissolved, particulate and total ²¹⁰Po were 0.16-2.78 mBq/L, 0.14-2.43 mBq/L and 0.51-3.94 mBq/L, respectively. There was no significant correlation between the specific activities of dissolved ²¹⁰Po and the water quality parameters, but the specific activities of particulate ²¹⁰Po showed significant correlation with water temperature, salinity and mass concentrations of TSM, respectively. At the same time, the specific activities of total ²¹⁰Po were significantly correlated with water temperature and mass concentrations of TSM, respectively. The distribution trends of ²¹⁰Po in different sections were not consistent, but in general, the particulate phase was the main existence form of ²¹⁰Po in the nearshore water, and the offshore ²¹⁰Po was confirmed mainly in the form of dissolved phase. Further analysis showed a negative correlation between the distribution coefficients of ²¹⁰Po in the solid-liquid phase and the mass concentrations of total suspended particulate matter. Based on the one-dimensional steady-state irreversible removal model, a longer residence time of dissolved ²¹⁰Po relative to particulate ²¹⁰Po was obtained, indicating that the atmospheric deposition and riverine input of ²¹⁰Po into the coastal Beibu Gulf would partially enter the dissolved phase after entering the seawater, so that its residence time in the water was prolonged.

Understanding the spatial evolution of mangrove patches is important to comprehensively and accurately evaluate the results of protection and restoration efforts in regional mangroves. In this study, the recent 30-year changes in area and number of mangrove patches in Maowei Sea and their driving factors were analyzed. The 1990, 2000, 2010 and 2019 Landsat remote sensing images were used as the main data sources, whereas the landscape dynamic analysis, spatial overlay analysis and main transformation route counting were adopted as the analytical approaches. The results demonstrated that: (1) The area of mangrove in Maowei Sea increased from 479.7 hm² to 1 477.5 hm² between 1990 and 2019 with the centroid shifting northward and westward. (2) Anthropogenic and natural factors were identified as the main driving force leading the changes in mangrove patch quantity in Maowei Sea from 1990 to 2010 and 2010 to 2019, respectively. (3) The number of mangrove patches increased from 204 in 1990 to 405 in 2019. However, the simultaneous occurrence of declining patch density and increasing mean patch size indicated that the patch fragmentation had been alleviated caused by the driving of natural factors. The artificial plantation of mangroves, especially using Sonneratia apetala, had contributed to the overall increase in mangrove area in Maowei Sea. Nevertheless, research on the artificial forestation process, expansion mechanism of mangrove patches and their ecological effects should be emphasized in future.

Chlorophyll a (Chl a), as the main pigment of phytoplankton photosynthesis, is often used to indicate the existing amount of phytoplankton in water column. The concentration of Chl a in Beibu Gulf from August to September 2021 was analyzed and its spatial distribution characteristics and influencing factors were discussed based on the summer cruise of the Shiptime Sharing Program on 2020 Beibu Gulf Scientific Research Expedition. The results showed that the mass concentration of Chl a ranged from 0.03 to 10.59 μg/L with an average value of 0.84 μg/L, among which the surface layer mass concentration ranged from 0.08 to 10.59 μg/L (average 0.97 μg/L) and the bottom layer mass concentration ranged from 0.03 to 4.69 μg/L (average 0.99 μg/L). Combined with the analysis of remote sensing data, the spatial distribution of Chl a mass concentration in Beibu Gulf was generally high in the nearshore and low in the offshore area. The high value area was mostly distributed in the nearshore area of Guangxi, namely the northern part of Beibu Gulf, while the mass concentration in the central and southern part of Beibu Gulf was relatively low. The correlation analysis between water environmental parameters and Chl a indicated that water nitrogen limitation was an important factor affecting the mass concentration of Chl a in Beibu Gulf from August to September.