As one of the main areas of tropical storm action in the northwestern Pacific Ocean, South China experiences several typhoons each year, and coastal erosion is a problem, making the area a natural testing ground for studying the dynamic geomorphological processes and storm response of promontory-straight coasts. This study is based on three years of topographic data and remote sensing imagery of Gulei Beach and uses topographic profile morphology, single width erosion-accretion and mean change, combined with the Coastsat model to quantify the seasonal and interannual variability and storm response of the beach and to explain the evolution of shoreline change and beach dynamics geomorphology in the last decade. Gulei Beach has been in a state of overall erosion and local accretion for a long time, with relatively obvious cyclical changes; seasonal changes are also obvious, which are mainly characterized by summer accretion and winter erosion, with accretion at the top of the bay and accretion and erosion on the north and south sides of the bay corner, respectively; the seasonal erosion-accretion volume of the beach profile ranges from −80 m³/m to 95.52 m³/m, and the interannual erosion-accretion volume ranges from −69.09 m³/m to 87.31 m³/m. The response of beaches to typhoons with different paths varies greatly depending on the length, slope, orientation and scale of beach development. The large and gently developing Futou beach is less responsive to storms, while the less developed headlands in the southern Gulei Peninsula are more susceptible to disturbance by external factors and respond more strongly to typhoons. Storm distance is more influential than storm intensity. Under the influence of human activities, obvious erosion hotspots develop during normal weather, but storm processes produce redistribution of beach material patterns, and erosion hotspots disappear after storms. The results of this study enrich the theory of beach dynamics geomorphology and provide technical support for disaster prevention and mitigation, as well as ecological restoration of coastal zones.

Extreme storm events in coastal zones play significant roles in shaping the morphology of boulder beaches. However, boulder displacement and the geomorphological evolution of boulder beaches driven by different extreme storm events, especially typhoon events, remain poorly understood. Thus, boulder displacement and the geomorphic response on a boulder beach in Fujian, southeastern China, were explored before, during and after a cold wave event (Dec. 1–7, 2020) and before and after Typhoon In-Fa (Jul. 19–27, 2021), a large tropical storm. This was achieved by tracking 42 tagged boulders distributed in the intertidal and supratidal zones using Radio Frequency Identification (RFID) and topographic surveys using real-time kinematic techniques, respectively. The results showed obvious disparities in boulder displacement in different geomorphic zones due to cold wave and typhoon events that were mainly characterized by migration magnitude, range, direction, and mode of transport. The typhoon event led to rapid and substantial changes in the overall morphology of the boulder beach, while the cold wave event impacted the intertidal morphology of the boulder beach to only a small extent. The surrounding structure of boulders, beach slope and beach elevation had a combined dominant effect on boulder displacement under the same extreme event. Hydrodynamic factors (effective wave energy fluxes, incident wave direction, storm surge and water level) had dominant effects on boulder displacement during different extreme events. In terms of a single event, the magnitude of the boulder displacement driven by the typhoon was much greater than that driven by the cold wave. However, considering the frequency and duration of cold waves in winter, the impact of multiple consecutive cold waves on the geomorphology of the boulder beach cannot be ignored in this study area. Alternating and repeated interactions between these two processes constitute the complete geomorphic evolution of the boulder beach. This study contributes to improved predictions of the morphodynamic response of boulder beaches to future storms, especially large tropical storms, and facilitates better coastal management.

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 persistence and habitability of coral reef islands in future extreme oceanographic conditions has received increasing attention in the recent decade, concerning that the sea level rise (SLR) and more frequent and intense storms in the context of global climate change are expected to destabilize those islands. Here, we conduct a set of wave-flume laboratory experiments focusing on the morphodynamic change of reef islands to varying ocean forcing conditions (wave height and SLR). Subsequently, a phase-resolving XBeach numerical model is adopted to simulate the monochromatic wave process and its associated sediment dynamics. The adopted model is also firstly validated by laboratory experimental results as reported in this study. It is then used to examine the impacts of island morphological factors (island width, island height, island location and island side slope) on the island migration. The combined laboratory/physical and numerical experiment outputs suggest that reef islands can accrete vertically in response to the sea level rise and the increased storminess.

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} $.

The horizontal structure of mangrove forests is an important characteristic that reflects a significant signal for coupling between mangroves and external drivers. While the loss and gain of mangroves has received much attention, little information about how the horizontal structure of mangrove forests develops from the seedling stage to maturity has been presented. Here, remote sensing images taken over approximately 15 years, UVA images, nutrient elements, sediments, and Aegiceras corniculatum vegetation parameters of the ecological quadrats along the Nanliu Delta, the largest delta of the northern Beibu Gulf in China, are analyzed to reveal changes in the horizontal structure of mangroves and their associated driving factors. The results show that both discrete structures and agglomerated structures can often be found in A. corniculatum seedlings and saplings. However, the combination of seedlings growing into maturity and new seedlings filling in available gaps causes the discrete structure of A. corniculatum to gradually vanish and the agglomerate structure to become stable. The aggregated structure of seedlings, compared to the discrete structure, can enhance the elevation beneath mangroves by trapping significantly more sediments, providing available spaces and conditions for seedlings to continue growing. Furthermore, by catching fine sediments with enriched nutrients, the survival rate of A. corniculatum seedlings in the agglomerated structure can be much higher than that in the discrete structure. Our results highlight the significance of the agglomeration of A. corniculatum, which can be beneficial to coastal mangrove restoration and management.

The clay mineralogy of 28 sandy-muddy transitional beach (SMT-Beach) sediments and surrounding mountain river sediments along the coasts of southeastern China was systematically investigated to reveal the sediment source-to-sink process variations of such beaches and their morphological indications. The results show that the clay mineral assemblages of these SMT-Beaches mainly comprise of almost equal illite (~30%), kaolinite (~28%), chlorite (~22%), and smectite (~20%) contents. From the surrounding mountain rivers to the SMT-Beaches, clay mineral assemblages show distinct spatial changes characterized by a large decrease (~40%) in kaolinite, whereas the other three clay minerals present relative increases, especially clear for smectite. The muddy sediment sources of SMT-Beaches inferred from the clay mineralogy are mainly derived from nearby mountain rivers coupled with long-distance transport and penetration of the Changjiang River. The sandy sediments of these beaches are predominantly sourced from nearby mountain rivers, the weathering products of surrounding rocks in both mainland and island environments, and erosion of the “Old Red Sand” and “Red Soil Platform”. However, the sandy sediment sources of the SMT-Beaches are largely reduced because of the remarkable decrease in the river fluvial supply associated with intensive human activities such as dam construction and coastal reclamation. Subsequently, the sandy sections of SMT-Beaches present clear erosion and have revealed by both time series remote sensing images and a compilation of published literature. In contrast, the muddy sediment supply of SMT-Beaches is temporarily stable and relatively constant, resulting in the landward migration of the mudflats with relative transgression or accumulation. These findings highlight that the natural evolution processes of SMT-Beaches have been greatly reshaped by intensive human activities.

The change of coastal wetland vulnerability affects the ecological environment and the economic development of the estuary area. In the past, most of the assessment studies on the vulnerability of coastal ecosystems stayed in static qualitative research, lacking predictability, and the qualitative and quantitative relationship was not objective enough. In this study, the “Source-Pathway-Receptor-Consequence” model and the Intergovernmental Panel on Climate Change vulnerability definition were used to analyze the main impact of sea level rise caused by climate change on coastal wetland ecosystem in Minjiang River Estuary. The results show that: (1) With the increase of time and carbon emission, the area of high vulnerability and the higher vulnerability increased continuously, and the area of low vulnerability and the lower vulnerability decreased. (2) The eastern and northeastern part of the Culu Island in the Minjiang River Estuary of Fujian Province and the eastern coastal wetland of Meihua Town in Changle District are areas with high vulnerability risk. The area of high vulnerability area of coastal wetland under high emission scenario is wider than that under low emission scenario. (3) Under different sea level rise scenarios, elevation has the greatest impact on the vulnerability of coastal wetlands, and slope has less impact. The impact of sea level rise caused by climate change on the coastal wetland ecosystem in the Minjiang River Estuary is mainly manifested in the sea level rise, which changes the habitat elevation and daily flooding time of coastal wetlands, and then affects the survival and distribution of coastal wetland ecosystems.

The northern Andaman Sea off Myanmar is one of the relatively high productive regions in the Indian Ocean. The abundance, biomass and species composition of mesozooplankton and their relationships with environmental variables in the epipelagic zone (~200 m) were studied for the first time during the Sino-Myanmar joint cruise (February 2020). The mean abundance and biomass of mesozooplankton were (1916.7±1192.9) ind./m³ and (17.8±7.9) mg/m³, respectively. A total of 213 species (taxa) were identified from all samples. The omnivorous Cyclopoida Oncaea venusta and Oithona spp. were the top two dominant taxa. Three mesozooplankton communities were determined via cluster analysis: the open ocean in the Andaman Sea and the Bay of Bengal (Group A), the transition zone across the Preparis Channel (Group B), and nearshore water off the Ayeyarwady Delta and along the Tanintharyi Coast (Group C). Variation partitioning analysis revealed that the interaction of physical and biological factors explained 98.8% of mesozooplankton community spatial variation, and redundancy analysis revealed that column mean chlorophyll a concentration (CMCHLA) was the most important explanatory variable (43.1%). The abundance and biomass were significantly higher in Group C, the same as CMCHLA and column mean temperature (CMT) and in contrast to salinity, and CMT was the dominant factor. Significant taxon spatial variations were controlled by CMCHLA, salinity and temperature. This study suggested that mesozooplankton spatial variation was mainly regulated by physical processes through their effects on CMCHLA. The physical processes were simultaneously affected by heat loss differences, freshwater influx, eddies and depth.

Macroalgae have long been used as biological indicators of marine ecosystem health worldwide due to their ecological importance and sensitivity to environmental stress. A number of previous studies have utilized macroalgal communities in monitoring surveys of environmental conditions. This study examined the characteristics and patterns of marine macroalgal communities in the Yellow Sea off the western coast of Korea. Macroalgae were analyzed for the number of species, biomass, and coverage ratio by macroalgal type. During the study period, 82 macroalgal species (10 green algae, 17 brown algae, and 55 red algae) were identified at the five study sites, with the highest number of species found at Gwanrido and Uido (both containing 41 species) and the lowest at Daeijakdo (27 species). The average biomass (via dry weight) was 98.63 g/m², consisting of green algae (8.39 g/m²), brown algae (35.08 g/m²), and red algae (55.16 g/m²). The dominant macroalgae species in terms of biomass were Corallina pilulifera, Sargassum thunbergii, and Ulva australis in the intertidal zones, and Botryocladia wrightii and Gelidium elegans in the subtidal zones. Richness, evenness, and diversity indices based on the biomass of abundant species were 5.08, 0.65, and 2.30, respectively, over the entire study area. Based on the evaluation of the environmental states by the community indices, overall, the Ecological Evaluation Index of macroalgae communities in the study area was marked as “Good-Moderate”, but was determined as “Moderate-Low” at several sites during summer. The results can be a direct approach in the assessment of coastal habitats in which anthropogenic as well as climate change influences persist.