Based on specimens collected in Yinggehai, Hainan, China from 2013 to 2016, a stable epiphytic taxon is found on the surface of the individual of marine green alga Cladophora aokii Yamada. According to the morphological characteristics, the taxonomy of Cl. aokii and its epiphytes is carried out. There are some epiphytes attached on Cl. aokii Yamada including Cl. fascicularis (Mertens ex C. Agardh) Kützing, Chaetomorpha pachynema (Montagne) Kützing, Ceramium camouii Dawson, Licmophora abbreviata Agardh, Lyngbya sp. and Chattonella sp.. The formation of the individual of Cl. aokii is dissected and explained, which can help to analyze the adaption in details among this species, its epiphytes and native marine environment. The results reveal the marine macro-epiphytic taxonomy in Hainan, China, and preliminarily explain the adaptive relationship between macroalgae and environment.

The feedback between morphological evolution and tidal hydrodynamics in a wave-dominated tidal inlet, Xiaohai, China is investigated through data analysis and numerical model experiments. Historically, Xiaohai Inlet had two openings, located at the north and south of Neizhi Island (a rocky outcrop), respectively. The evolution of Xiaohai Inlet was dominated by the natural process before 1972. In addition to the natural process, human interventions, including the closure of the north opening, 50% of freshwater reduction, and increase of land reclamation, have altered tidal hydrodynamics and morphological evolution since 1972. A series of numerical model simulations were conducted to investigate the influence of morphological changes on the hydrodynamics and the influence of human activities on the inlet evolution. The natural process has caused narrowing and shoaling of the inlet throat, development of the flood-tidal delta, and shoaling of the tidal channel inside the lagoon. Human interventions have accelerated these changes. Consequently, the tidal propagation from the offshore into the lagoon has been impeded and the tidal energy has been dissipated substantially. Tidal current has changed from ebb-dominant to flood-dominant in most parts of the inlet system whereas the inlet throat has remained as ebb-dominant, the tidal prism has decreased consistently, and sediment has continued to deposit inside the inlet. As a result, the changes of morphology, hydrodynamics, and sediment transport show a positive feedback. The human interventions have had both advantageous and adverse influences on the stability of the inlet. The closure of the North Opening has decreased the longshore sediment input to the inlet, and increased the tidal prism, ebb velocity, and sediment transport in the south opening, thus enhancing the inlet's stability. However, reducing the river discharge and landfill of the tidal flats has resulted in a decrease of the tidal prism, the ebb velocity, and the ability to export sediment, thus having the tendency to deteriorate the inlet's stability. A stability analysis based on a closure curve methodology has shown that Xiaohai Inlet is in a state of dynamic equilibrium at present.

Knowledge of coastline changes and vulnerability is of great importance to local government departments that are responsible for the management and development of coastal zones. To study the nature of change and vulnerability along the coasts of the Hainan Island, we collected a large number of sediment samples through the last few years, and reconstructed the changes of the coastline by combining the data of sediment grain-size analysis and the nautical charts/TM RS imaginary. Contrary to being almost free from erosion (as expected from the findings that the coastlines are in a relatively stable state), four major cities in Hainan (i.e., Haikou, Wenchang, Sanya and Changjiang) turned out to be suffered from a moderate coastal vulnerability primarily because of the large populations that impose considerable pressure on the coastlines. Thus, the assessment methodology utilized in this study, including both anthropogenic and natural factors, serves as a useful tool to obtain a comprehensive understanding of coastline vulnerability for local government, in terms of coastal management and adaptation.

Coastal erosion on islands is increasing due to sea level rise, frequent extreme events, and anthropogenic activities. However, studies on the multifactorial coastal erosion risk and the vulnerability of islands are limited. In this study, the Coastal Erosion Risk Assessment (CERA) method was applied for the first time to the study area in China to assess the erosion risk on the coast of Hainan Island; to explore the effects of coastal ocean dynamics, sediment movement characteristics, and anthropogenic construction; and to discuss the suitability of the method and countermeasures for coastal protection. The results show that the coast of Hainan Island shows high sensitivity, high value, low exposure, and moderate erosion. The whole island showed high vulnerability but low erosion risk, with the eastern region being more affected by erosion, particularly the eastern side of Wulong Port and Yalin Bay in Wenchang, and the shore section of Yalong Bay in Sanya, having a very high risk of coastal erosion. In addition, Monte Carlo simulation was used to check the applicability of the CERA method, and it was found that the rate of shoreline change, population density, and number of storms significantly contributed to coastal erosion, but only the short-term effects of sea level rise were considered. The effects of sea level rise and sediment grain size were primarily analyzed as influencing factors. The effects of sea level rise continue to strengthen, with coastal retreat expected to be greater than 2 m by the mid-21st century. Moreover, Hainan Island is primarily composed of the fine and medium sand types, which have little resistance to coastal erosion. Currently, the impact of sediment grain size is rarely considered in coastal erosion risk assessment studies. However, it can be incorporated into the indicator system in the future, and the spatial variation of indicators can be fully considered to strengthen the refinement study.

The flow field over Hainan Island and the Leizhou Peninsula in summer and winter is discussed with three-dimensional mesoscale model developed in the University of Virginia and using the representative meteorological data of January and July.
Simulation results indicate that the local weather characteristics over the Hainan Island are distinctly influenced by the Wuzhi Mountain terrain. The cloudy or rainfall weather over the northeast of the Wuzhi Mountain occurs easily, under proper large-scale conditions of flow, temperature and humidity. while west wind prevails. The overcast or rainfall weather is often induced by strong convection in the afternoon over west of the Hainan Island under easterly prevailing wind.

The coupled ocean–atmosphere–wave–sediment transport (COAWST) modeling system is employed to investigate the role of wave-mixing playing in the upwelling off the west coast of Hainan Island (WHU). Waves, tides and sea surface temperature (SST) are reproduced reasonably well by the model when validated by observations. Model results suggest the WHU is tidally driven. Further investigations indicate that inclusion of wave-mixing promotes the intensity of the WHU, making the simulated SST become more consistent with remote-sensed ones. Dynamically, wave-mixing facilitates the “outcrop” of more upwelled cold water, triggering stronger WHU and leading to a three-dimensional dynamical adjustment. From the perspective of time, wave-mixing contributes to establishing an earlier tidal mixing front strong enough to generate WHU and that is, WHU may occur earlier when taking wave-mixing into consideration.

The diversity, community composition and 16S rRNA gene abundance of bacterioplankton along a transect across an upwelling area off the eastern coast of Hainan Island (the Qiongdong upwelling) were investigated in August of 2016 using high throughput sequencing and quantitative PCR assay of 16S rRNA genes. Compared with the offshore stations, the inner-shelf stations had higher bacterial gene abundance (up to 3 fold) and operational taxonomic unit richness, a result of the influence of upwelled and fresher waters. Overall, a majority of the reads were affiliated with Gammaproteobacteria (11%–74%) and Alphaproteobacteria (14%–43%). The structure of the bacterial community was significantly affected by salinity, dissolved oxygen, ${\rm{NO}}_3^-$ and ${\rm{NH}}_4^+ $, which also defined the physicochemical features of the upwelled waters. Horizontally, the relative abundances and gene abundances of Rhodobacteraceae, SAR86, Cyanobacteria and Bacteroidetes in eutrophic zone decreased from the inner to outer shelves and slope, whereas Alteromonas, Vibrio and Oceanospirillaceae exhibited an opposite trend. This study stresses the riverine influence on the oceanographic condition and spatial variability of bacterioplankton diversity and distribution in the Qiongdong upwelling.

Coastal lagoon deposits provide evidence for the magnitude and frequency of past tropical cyclones prior to instrumental records and historical documentation. In the present study, we attempt to analyze the sedimentary records containing typhoon information for the northern South China Sea region. For this purpose, sediment cores were collected from two coastal lagoons in the southeastern Hainan Island, and were analyzed in laboratory to derive the data sets about grain size, organic and inorganic carbon contents, and deposition rates. The grain size and organic-inorganic carbon data were used to formulate the proxies of typhoon events. The deposition rates, as calculated using the CRS ²¹⁰Pb method, are around 0.5 mm/a for both lagoons, on the basis of which an age model is established. Within the cores, sedimentary layers associated with 35 typhoon events have been identified. On such a basis, a 350 year history of local typhoon activities is reconstructed by incorporating the 210Pb dating results, typhoon-induced sedimentation patterns and the historical documents. A comparison of the frequency of typhoon occurrence with the regional climate records indicates that the observed changes in tropical cyclone activity patterns, as revealed by the lagoon sedimentary records, may be related to El Niño, Pacific Decadal Oscillation (PDO), North Atlantic Oscillation (NAO), sunspot, and other potential climate drivers that affect the tropical cyclone variability. This study demonstrates that the sedimentary record of storms can be analyzed in combination with historical documents, to provide meaningful information on past storm activities and their long-term variability.

Some of the islets in the eastern Beibu Gulf are covered by Quaternary volcano strata. The rock samples from these islets mainly consist of quartz tholeiites (at Shenjiandao), olivine tholeiites (at Linshidao and Xieyangdao) and alkali basalts (at Yangpubi and Jianshidao), and basically represent four periods of the Quaternary volcanism of Hainan Island and its adjacent regions. Except for the samples from Shenjiandao, most of the Quaternary volcanics of these islets belong to alkali magma series. The trace element characteristics of all of these samples show they are OIB (oceanic island basalt) -like, which implies that their deep geodynamic setting may be related to a mantle plume. The Sr-Nd-Pb isotopic compositions show that the mantle source beneath the Quaternary strata can be regarded as a result of binary mixing between a depleted, DMM (depleted MORB mantle)-like source and an enriched mantle type 2 (EM2). The EM2 may be originated from the Hainan mantle plume, and has been metasomatized by carbonaceous fluids released from ancient recycled oceanic crust at an asthenospheric mantle level. These features, together with typical trace element ratios, reflect that the parent magma was not subjected to crustal contamination during its ascent to the surface. This study provides further petrological and geochemical evidence for the existence of the Hainan mantle plume.

More than 50% of the typhoons landing in China have landed on the southeast coast, where they have caused great pressure on the coastal environment. Seagrass, one of the most important constituents of coastal ecosystems, is also greatly affected by typhoons. In order to clarify how seagrass distribution variation is affected by typhoons in coastal areas in southeast China, data of Typhoons Dawei and Tianying (category 4 and category 2 respectively, which just ran through the southern part of Hainan province) have been studied. In situ observation and satellite remote sensing data (CBERS-China Brazil Earth Resources Satellite) in 2004 and 2006 were used to retrieve seagrass distribution in Xincun Bay, Hainan province. In situ observations showed that leaf length, stem biomass and above ground biomass on average showed evidence of reduction after Typhoons Tianying and Dawei. However, seagrass density showed no evidence of reduction after typhoon Tianying and Dawei passed by and increased rapidly in January 2006. From results of satellite remote sensing data, seagrass distribution can be detected with high accuracy, and the area of seagrass distribution on the south coast of Xincun Bay in 2006 after the typhoon passed by was smaller than that in 2004 in region A and B. However, in region C, area of seagrass coverage under 20% increased. These results demonstrated that typhoons Tianying and Dawei damaged seagrass bed and helped seagrass to get rid of aged and dead leaves, and this correspondingly facilitated seagrass growth.