Mekong Research Articles

Distribution of some commercially valuable fishes (Pangasiidae, Polynemidae, Sciaenidae) in the Mekong Delta (Southern Vietnam) and role of the estuaries in their life cycle

The composition of the fish population of the Mekong River is one of the richest on the planet, however, despite the high diversity (~1200 species) and productivity, the basis of fishing in the Mekong Delta is made up of several dozen species, the most valuable of which are members of the families Pangasiidae, Polynemidae and Sciaenidae. Information on the distribution of these taxa is fragmentary, and the role of various parts of the delta in their reproduction has not been studied. Distribution of families Pangasiidae, Polynemidae and Sciaenidae in the Mekong Delta was assessed in the dry season by midwater trawl catches in 2019 (April, December), 2021 (January, April) and 2022 (March–May). A total of 237 trawls were carried out, in the catch of which 45.9 thousand fish belonging to 36 families were found. Representatives of each of the families Pangasiidae, Polynemidae, and Sciaenidae were noted in 42–57% of all catches within the studied part of the Mekong Delta, however, the frequency of their occurrence in its various parts varies significantly. According to the Mekong Delta zonation scheme, based on the analysis of the taxonomic composition of the fish population, representatives of Pangasiidae are confined to areas of the upper and middle reaches, Polynemidae—to areas of the middle and lower reaches, while significant differences in the horizontal distribution of Sciaenidae in different parts of the delta have not been identified. Significant differences in the vertical distribution of all studied taxa were also not found. The maximum concentrations of juveniles of Pangasiidae and Polynemidae were found in the middle reaches of the delta, and Sciaenidae in the lower reaches, which indicates the important role of the respective areas in the reproduction of representatives of the studied taxa.

On the cause of large daily river flow fluctuations in the Mekong River

Abstract. Natural fluctuations in river flow are central to the ecosystem productivity of basins, yet significant alterations in daily flows pose threats to the integrity of the hydrological, ecological, and agricultural systems. In the dammed Lancang–Mekong River (hereafter LMR), the attribution of these large daily flow changes to upstream regions remains mechanistically unexamined, a factor blamed on challenges in estimating the time required for large daily shifts in upstream river flow to impact the downstream stations. Here, we address this with a newly developed subbasin modeling framework that integrates 3D hydrodynamic and response time models, together with a hydrological model with an embedded reservoir module. This integration allows us to estimate the time required between two hydrological stations and to distinguish between the contributions of subbasins and upstream regions to large daily river flow alterations. The findings revealed a power correlation between upstream river discharge and the time required to reach downstream stations. Significant fluctuations (greater than 1 m) in the river's daily flow were evident before the advent of the era of human activities, i.e., before 1992, with around 92 % of these fluctuations occurring during the wet season, particularly in June, July, and August. This pattern persisted throughout subsequent periods, including the growth period (1992–2009) and the mega-dam period (2010 to 2020), with minimal variation in the frequency of events. The Lancang basin contributed approximately 33 %–42 % of these large river fluctuations at the Chiang Saen station. We found that daily-scale water level and runoff might not fully capture dynamic river flow changes, as significant differences were observed between daily and subdaily river flow profiles. Subbasins significantly contributed to mainstream discharge, leading to substantial shifts in mainstream daily river flows. The outcomes and model derived from the subbasin approach have significant potential for managing river fluctuations and broader applicability beyond the specific basin studied.

A study protocol for a multi-country cluster randomized controlled trial of the impact of a multi-component One Health strategy to eliminate Opisthorchis viverrini and soil transmitted helminths in the Lower Mekong Basin

BackgroundOpisthorchis viverrini (OV) and soil-transmitted helminths (STH) are two of the most common helminths contributing to the Neglected Tropical Disease (NTDs) burden in the Lower Mekong Basin. Although mass drug administration is the cornerstone of control programs to reduce morbidity caused by these infections, this approach has limitations in preventing re-infections. Elimination requires additional measures such as reservoir host treatment, improved hygiene and health education to reinforce MDA's impact. This study aims to examine the impact of a scalable multi-component One Health Helminth Elimination program in the Lower Mekong Basin (HELM) that combines human praziquantel (PZQ) and albendazole (ALB) treatment with a program that includes the “Magic Glasses” and the “Lawa Model” interventions with health promotion at their core.MethodsThis study will employ a cluster randomized controlled trial (cRCT) in 18 rural communities (with sub-district or villages as cluster units) across Cambodia, Laos and Thailand. The control arm will receive one round of PZQ/ALB treatment, while in the intervention arm, multi-component HELM program will be implemented, which includes PZQ/ALB treatment together with the Magic Glasses and Lawa Model interventions. OV and STH infections levels will be evaluated in individuals aged 5–75 years at baseline and will be repeated at follow-up (12 months after the HELM intervention), using modified formalin ethyl-acetate concentration technique and quantitative PCR. The primary outcome of the study will be cumulative incidence of human OV and STH infections. Outcomes between the study arms will be compared using generalized linear mixed models, accounting for clustering.DiscussionEvidence from this trial will quantify the impact of a multi-component One Health control strategy in interrupting Ov and STH infections in the Lower Mekong Basin.Trial registrationAustralian New Zealand Clinical Trials Registry (ANZCTR): ACTRN12622000353796. Prospectively registered 28 February 2022.

The comparison of the prevalence, awareness, treatment and control of hypertension among adults along the three provinces of the Lancang–Mekong River countries—China, Laos and Cambodia

BackgroundUnder the background of similar geography and culture in Lancang-Mekong countries and rapid changes in the regional economy and lifestyle, this study aimed to describe and compare the prevalence, awareness, treatment, and control of hypertension and assess the hypertension care cascade in three provinces of China, Laos, and Cambodia.MethodsA cross-sectional study was conducted between 2021 and 2023 in the three provinces of Lancang–Mekong River countries using consistent investigative procedures. We included 11,005 participants aged ≥ 18 years from three provinces, and data were collected through questionnaires, physical examinations, and biochemical tests. We analyzed the cascade of hypertension care and compared the prevalence, awareness, treatment, and control of hypertension.ResultsThe hypertension care cascade indicated that 46.3%, 51.6%, and 63.1% of patients in Yunnan Province (China), Oudomxay Province (Laos), and Ratanakiri Province (Cambodia), respectively, were not diagnosed, and 10.7%, 12.8% and 21.1% of patients, respectively, did not receive treatment. After sex-age standardization, the prevalence rates of hypertension in the three provinces were 33.4%, 34.5%, and 23.6%, respectively. Higher awareness rate in Yunnan Province (53.4%) and Oudomxay Province (46.5%) than in Ratanakiri Province of Cambodia (39.7%). The treatment rate of hypertension in Yunnan Province (42.4%) was higher than that in Oudomxay Province (34.5%), and Ratanakiri Province (16.9%). In addition, less than 20% of the patients in the three provinces had their blood pressure under control. The factors associated with hypertension differed across the three provinces.ConclusionsIn the three provinces of the Lancang-Mekong River Basin countries, there is a high burden of hypertension and a significant unmet need for hypertension care. Targeted and precise intervention strategies are urgently needed to improve the awareness, treatment, and control of hypertension in low- and middle-income regions.Graphic

Estimation of carbon stock and economic value of Sanjiangyuan National Park, China

Sanjiangyuan National Park (SNP) is one of China’s national parks with high ecological significance and ecological vulnerability. In order to promote the healthy development of national park ecosystems, it is extremely important to investigate the carbon sequestration potential and the economic value of carbon sequestration in the SNP. Here, we comprehensively analyzed the characteristics of land use change from 1985 to 2022 based on the land use transfer matrix and land use dynamic degree, calculated the ecosystem carbon storage based on the InVEST model, and estimated the economic value of carbon storage in each period combined with compound interest present value method. The results show that: (1) From 1985 to 2022, carbon stocks generally went through three phases: increasing, stabilizing, and decreasing. (2) Low carbon density areas were concentrated in the Kekexili Natural Reserve in the Yangtze River Source Park and a small part of the Yellow River Source Park and Lancang River Source Park; The high carbon density area was concentrated in the water area and the forest land in the southeast of the Lancang River Source Park. (3) Land use change in SNP had a decisive impact on carbon storage change, and the economic value of carbon storage in SNP continued to increase from 1985 to 2022. In order to provide a scientific basis for the sustainable management of carbon sinks and the reduction of carbon emissions in the SNP area, it is necessary to further study the carbon cycle process, carbon storage and carbon emissions in the region in the future.

Integrating machine learning and zoning-based techniques for bias correction in gridded precipitation data to improve hydrological estimation in the data-scarce region

Gridded precipitation products are valuable for hydrological assessments due to their wide spatial coverage and improved spatiotemporal resolution compared to the use of rain gauges. They have been extensively used in hydrology studies over the past decade. However, it’s important to recognize that these products come with inherent uncertainties, which can impact hydrological simulations.To address this limitation, our study introduces a novel bias correction technique that combines machine learning and zoning-based approaches. We applied this technique in a case study to correct bias in the gauge-based gridded precipitation product in a data-scarce region, such as the Mekong Basin. To evaluate the effectiveness of this proposed method, we compared it against conventional bias correction methods, including mean-based and distribution-based approaches, using various statistical indices and hydrological simulations.Our findings consistently reveal a bias pattern within the gridded precipitation product across the study area, which affects streamflow estimations. All correction methods show promise in reducing precipitation biases and improving streamflow predictions. Among these methods, one particularly promising approach involves integrating machine learning techniques and taking into account climate zones. This integrated approach has emerged as a superior strategy, offering significant advantages in terms of both correcting precipitation data and improving broader hydrological assessments.This investigation underscores the crucial role of this integrated method in optimizing hydrological simulations across diverse geographical areas, especially in regions where data availability is limited.

Impact of tropical cyclones and socioeconomic exposure on flood risk distribution in the Mekong Basin

Tropical cyclones have a big impact on flood risk, and understanding how their activity interacts with population exposure under climate change is critical. Here we investigate spatiotemporal changes in flood risk using numerical models together with historical observations and future projections of tropical cyclone tracks. We find that tropical cyclone-related flood risk shifts from the Mekong Delta to the eastern lower Mekong Basin, driven by the interaction between tropical cyclones and population exposure. Historically, extreme precipitation from tropical cyclones increased flood risk in about 14% and decreased in 7% of the basin. Future tropical cyclones may increase flood risk in about 7% and reduce in nearly 18% of the basin. Moreover, population exposure growth has historically increased flood risk in 3% of the basin and is projected to result in a 1% increase. These findings highlight the complex interactions of tropical cyclone hazards and socioeconomic factors influencing flood risk.

The dominant mechanisms of nutrient cycling in high-dam reservoirs: retention, transport or transformation?

Abstract High-dam reservoirs can significantly affect nutrient cycling processes across the globe. However, the research community now has two contradictive views (i.e. retention versus transformation) about the impact of high-dam reservoirs on nutrient cycling due to incomplete information obtained from limited field samplings. To resolve this issue, we develop a physically-based, three-dimensional water quality model to examine the spatiotemporal distributions of biogenic elements (nitrogen and phosphorus) in high-dam reservoirs with high spatial and temporal details. We apply the model to the Xiaowan Reservoir, a representative high-dam reservoir in the Lancang River Basin. By scrutinizing the spatiotemporal distributions of biogenic elements across space and over time, we find a unique ‘retention-transformation-transportation’ process of nitrogen and phosphorus in the high-dam reservoir, with dominant transformation occurring in the water zone before the dam during non-flood period while dominant retention occurring in the middle part of a reservoir during flood period. We further find that transformations of biogenic elements are enhanced only in low-temperature and low-oxygen environments. Our findings show solid scientific basis to resolve the contradictive views about nutrient cycling mechanisms in high-dam reservoirs, and provide important policy implications for the operation of high-dam reservoirs to achieve improved water quality while maintaining clean energy supply.

Riverine sediment geochemistry and its dispersal pattern on the western Sunda Shelf

Understanding sediment provenance in continental shelf basins is essential for reconstructing paleoenvironmental changes, enhancing insights into sedimentary dynamics, and elucidating their contributions to the global carbon cycle. To decipher sediment provenances and enhance comprehension of the sediment dispersal patterns and the factors governing geochemical compositions on the Sunda Shelf, we conducted an exhaustive analysis of trace elemental concentrations and the isotopic ratios of strontium (87Sr/86Sr) and neodymium (εNd) in the silicate fractions of 35 surface sediment samples. These samples were collected from the western Sunda Shelf and its proximate major river end-members, namely, the Mekong, Rajang, Pahang, and Kelantan Rivers. Through the application of various statistical methodologies, including classical cluster analysis, Principal Component Analysis (PCA), the La-Sc-Th discrimination diagram, and an array of elemental ratios, we identified three distinct geochemical provinces on the Sunda Shelf. Each province is defined by unique geochemical signatures indicative of varied sediment sources or provenances. This distinction was primarily attributed to pronounced sediment heterogeneity, reflecting lithologic variances from the diverse river end-members. In pursuit of a holistic understanding of sediment provenance in the region, SrNd isotopic data was also integrated from prior studies encompassing the eastern Sunda Shelf and the southern South China Sea. By utilizing the SrNd mixing model, complemented with Monte-Carlo simulations, we estimated the sediment contributions from surrounding river end-members to the southern South China Sea basin. According to the model, the Mekong River emerges as the principal sedimentary source of the Sunda continental shelf and the southern South China Sea, attributed to its substantial sediment outputs. Additionally, the model has identified significant contributions from the Rajang, Pahang, and Kelantan rivers, particularly in offshore regions near their estuaries. Further, this study revealed the previously underappreciated influence of South China's rivers, namely the Red and Pearl Rivers, on the eastern Sunda Shelf and deeper southern South China Sea region beyond the continental shelf. This study not only delineates the dominant sediment sources influencing the Sunda Shelf and the South China Sea but also underscores the importance of considering a broad spectrum of river end-members to understand sedimentary dynamics in an active marine environment.

Matrix representation of the graph model for conflict resolution based on intuitionistic preferences with applications to trans-regional water resource conflicts in the Lancang–Mekong River Basin

With the continual growth in global demand for water resources, the governance of trans-regional water resources has become an important field of cooperation between countries, and conflicts caused by competition for water resources have become a focal issue in international relations. The root causes of trans-regional water resource conflicts include unclear ownership of water resources and the limited rationality of the parties involved. The water rights trading model has proven to play a stabilizing role in resolving trans-regional water resource conflicts versus the traditional negotiation model. To demonstrate the rationality and theoretical strength of the water rights trading model in allocating trans-regional water resources in uncertain environments, this paper proposes a novel matrix representation of intuitionistic stability definitions through a graph model for conflict resolution (GMCR). First, decision-makers’ (DMs’) intuitionistic preferences, unilateral movements, and intuitionistic unilateral improvements in a GMCR with two DMs are represented in a matrix. The joint unilateral movements and joint intuitionistic unilateral improvements for a coalition in a GMCR with multiple DMs are also represented. Next, according to the logical forms of intuitionistic stability definitions, four stability definitions in a GMCR with intuitionistic preferences were redefined to enable matrix representations of graph models with either two DMs or multiple DMs. Finally, we analyzed the state transitions and strategies of all stakeholders in trans-regional water resource conflicts in the Lancang-Mekong River Basin within the intuitionistic graph model. The contributions of this paper are twofold. First, in terms of theoretical research, this article enriches and develops the GMCR and proposes a set of definitions of matrix intuitionistic stability. Second, in terms of practical application, the intuitionistic stability analysis results can provide a reference for water rights trading to solve trans-regional water resource conflicts in uncertain environments and transform trans-regional water resource governance systems.

Vietnam’s hedging amid U.S.-China Mekong rivalry: risk management under uncertainties

This paper examines how Vietnam has navigated U.S.-China rivalry in the Mekong basin while seeking to ensure its security, prosperity, and autonomy. I use the theoretical lens of hedging, defined as an insurance-maximising behaviour under conditions of high uncertainty. While existing scholarship has extensively examined Vietnam’s hedging on maritime security issues and the broader U.S.-China strategic and economic competition, this paper makes an empirical contribution by focusing on the relatively understudied angle of hedging in the Mekong basin context. I argue that Vietnam has hedged by (1) diversifying its Mekong partnerships with both the United States and China and (2) selectively cooperating with China under the Lancang-Mekong Cooperation. These acts aim to minimise Vietnam’s exposure to the long-term consequences of several non-traditional security risks, namely ecological challenges, water insecurity, economic vulnerabilities, and energy insecurity. Vietnam’s Mekong hedging also insures against the traditional security risks of big-power entrapment and military aggression as U.S.-China competition intensifies.

Mekhong Cruise Servicing along the Mekhong River in Luang Prabang Heritage City, Luang Prabang Province

This research study to providing boat tours along the river. The Luang Prabang Provincial Boat Association has 3 locations: Ban Don Nui Port, Luang Port and Wat Nong Port. The case study of Wat Nong Port which includes to study the satisfaction of tourists to the service of guided tours along the Mekong River. It is a quantitative and qualitative study in collecting data from 180 questionnaires, using descriptive statistics and giving actual meaning from data analysis to be accurate and precise. The results of the study on the satisfaction of tourists to the service of guided boats along the Mekong River from the analysis of the results of the study shows that tourists are satisfied with all 7 aspects: there is satisfaction of tourists to the service of the guided boat, an average of 3.98, the confidence of customers in the service of the guided boat is at a very high level, an average of 3.92, there are physical things offered to tourists who come to use the service at a very high level, an average of 3.77, the service of the staff towards the satisfaction of the tourists is at a high level, an average of 3.66, the satisfaction of the service of the staff inside the boat is at a very high level Average 3.59, satisfaction with other service charges of tourists is at a high level, average 3.67 and things that should be improved in the service of boat tours along the Mekong River at a high level, average 3.47. Also, the customer also offers to make a map to show the location of the boat service for convenience, providing information throughout the travel period and saving time in finding the place.

Population genetics of Macrognathus siamensis (Synbranchiformes: Mastacembelidae): Implications for non-migratory fishery resources in the Mekong River basin

The spotted spiny eel, Macrognathus siamensis is an economically important freshwater fish in the Mekong River basin, which is undergoing dramatic biodiversity changes due to anthropogenic impacts. The species is non-migratory, with a moderate larval duration that facilitates downstream genetic connectivity. Restriction site-associated DNA sequence datasets that includes 3736 and 1244 single nucleotide polymorphisms representing putatively neutral loci, from all geographic sites and without the Tachileik site, indicate strong population structure in this sedentary fish, with a significant isolation-by-distance signature. This structure reflects isolation of tributary populations and downstream dispersal of these distinct populations to mix in the Mekong mainstem and contributes to increased genetic diversity in the lower reaches of the Mekong basin. Genetic data indicates that the dispersal of eggs and larvae downstream is not impeded by Khone Falls and that the falls prevent upstream dispersal of eggs and larvae. Despite this downstream connectivity, there is a strong population structure among above- and below-Khone Falls groups. An analysis of outlier loci putatively under selection provides evidence that the difference between connected metapopulations above and below the falls is due to the fundamental differences in environmental regimes. We hypothesize that instead of a physical barrier, the falls simply represents a demarcation between previously recognized upstream limited-floodplain environments and downstream extensive-floodplain environments. These findings emphasize the need to monitor genetic diversity of key sedentary fishery species to assess whether proposed dams along the river could affect genetic diversity downstream. This genetic diversity is particularly important for the resilience of downstream populations because of the many environmental impacts caused by dams, land use, and climate change. Early detection of reduced downstream genetic diversity could trigger remediation programs to ensure resilience and continued fisheries productivity of important non-migratory fish resources.

Impacts of hydroelectric stations on vegetation phenology in the Lancang River Basin

Hydropower projects can be critical in regulating ecological systems and managing water resources. The combined effects of natural conditions and human activities make the impact of hydropower on vegetation phenology much more complex. In this study, we selected four representative hydropower stations (Huangdeng (HD), Xiaowan (XW), Nuozhadu (NZD), and Jinghong (JH)) along the Lancang River to assess the effects of dams on vegetation phenology. The buffer zone analysis and Geodetector methods were used to evaluate the spatiotemporal variations of vegetation phenology and to analyze the possible causes of vegetation phenology. This study found that: (1) Throughout the study area, the relative contributions of factors affecting the start of the growing season (SOS) were ranked as air temperature (50.46 %), elevation (50.16 %), precipitation (49.90 %), and human activities (34.45 %). (2) The SOS of the rugged areas (HD) had been significantly delayed since water storage. The disparities of vegetation phenology in different areas had been identified by introducing the Core Impact Zone (CIZ) and the Peripheral Impact Zone (PIZ). Within the CIZ, the most pronounced delay was observed in grasslands (+6.57 days/yr). The delay days in the PIZ was 2.68 days/yr, which revealed that the effect of water storage diminished with increasing distance from the water source. (3) Human activities significantly influenced the vegetation phenology around the economically developed region (JH). This research provided new insights into the impact of hydropower development on vegetation phenology in river basins.

Power and the River Lords

The Mekong River is experiencing a crisis, with water flows and flood cycles rendered unstable owing to large-scale hydropower development in China and Laos. As communities face the radical decline of fisheries and unexpected floods and ebbs, residents and regional NGOs alike turn toward voices of moral authority and power to seek to mitigate these losses. But the kinds of authority sought out by fishers on the river are not the same as those identified by environmental groups; “spirit lords” and other divine figures enter into the conversation just as much as activists and lobbyists. In addition, these religious figures present multiple perspectives on how to live in a changed world and a model of resistance that challenges how we might see political power and its heroes.

Repatriation via Saigon

The South Korean Vietnam War deployment (1965–73), typically understood as a mercenary expedition, was also an exercise in the management of refugees and other displaced populations. While the South Korean Army managed its own refugee villages in South Vietnam, the overseas dispatch of military labor also promised to ameliorate the problem of mass displacement at home, an ongoing legacy of the Korean War (1950–53). The northern exile writer Sŏnu Hwi (1922–86) was a notable proponent of this vision of the war as a frontier, framing Vietnam as a surrogate for an inaccessible North Korea and an outlet for the social contradictions of Cold War developmentalism in South Korea. Focusing on the embedded novel Wave to the Mekong River (1966–67), this article argues that Sŏnu's expansionist fantasy deviated from official justifications of Vietnam as a war for development. At the same time, it suggests, Sŏnu's ideological equalization of exile and displacement also informed an emergent antiwar literature showing that the deployment aggravated, rather than resolved, the marginalization of mobilized youth.

Prediction of impulse waves generated by the potential failure of large deposits in the Rumei Reservoir, Lancang River, China

The impulse wave generated by the reservoir landslide seriously endangers the surrounding safety. In this paper, the failure mode of a large deposit in Rumei Reservoir, China was first explored by centrifuge test. Thereafter, a physical model based on Froude similarity (scale 1:200) and a numerical model based on fluid‒solid coupling were constructed. The failure motion process and the generated impulse waves of the deposits at high (2895 m) and low (2750 m) water levels are studied in detail. The results of the two models are similar. The velocity and wave height of the deposits at the 2750 m water level are greater than those at the 2895 m water level. The maximum wave heights generated in the river under both conditions exceed 46 m and 22 m, respectively, and the maximum run-ups formed on the opposite bank are 71 m and 30 m, respectively. This also suggests that landslides in the near field area are dangerous. Waves at both water levels will not overtop the dam, but attention should be given to the effect of the first wave at the 2895 m water level on the dynamic water pressure in the key area of the dam.

MoLam Phee Fa: The Process of Learning about the Culture and Rituals of Healing Patients using Mediums

The objectives of this study were to investigate (i) The knowledge and wisdom of treatment with a blue spirit medium and (ii) The learning methods for treating patients with the wisdom of MoLam Phee Fa. The study was conducted by analyzing documents and collecting field data by surveys, interviews, observations, and group discussions. The results of the descriptive analysis of the data collected from a group of 40 participants indicated that: (i) Belief in Phee Fa Phaya Thaen is considered to be an ancient religion that exists in southern China along the Mekong River Basin. People who respect it believe that Phaya Thaen will bring prosperity as well as help in times of illness. This knowledge has continuously been passed on by learning the steps in ceremonies to help cure the sick. (ii) MoLam Phee Fa is a group of people who have inherited the knowledge of black magic healing by inviting the Phee Fa spirit into a medium to predict the cause of the illness of those who come for treatment. Those who come to receive the knowledge transfer will strictly adhere to the rules. Disciples must learn the skills while treating patients and memorize the literature used, including memorizing the contents of the poems and melodies and using the principles of psychiatry to empower patients to have the strength to live their lives. In conclusion, MoLam Phee Fa are in possession of a method for curing patients using magical methods that relies on building morale. This wisdom, therefore, deserves to be learned and passed on as an alternative treatment.

Monthly and Interannual Variations in Upwelling and Chlorophyll off the Vietnam Coast in Summer Based on Deep Learning

AbstractThe deep learning technique is used to automatically identify upwelling regions off the Vietnam coast using Sea Surface Temperature (SST) data from satellite imagery. After delineating these upwelling areas, this study is pioneering in deriving the upwelling probability off the Vietnam coast, pinpointing two significant upwelling hotspots. To deepen the understanding of the individual impacts of wind and sea surface height anomalies on monthly and spatial variations in upwelling probability and chlorophyll distribution, the Empirical Orthogonal Function method was employed to analyze this data set. The upwelling probability demonstrates a close relationship with wind and sea surface height anomalies, while chlorophyll concentration correlates with the strength of the southwesterly wind. The southwesterly wind, Ekman pumping, eddy dipole, and northeastward jet have been identified as key drivers of the formation and spatial variability of upwelling in summer, albeit with varying contributions to different parts of the coastal region. The distribution of upwelling probability correlates with chlorophyll variation off the northern Vietnam coast. High chlorophyll concentration off the southern Vietnam coast is primarily influenced by the southwesterly wind that carries the Mekong River plume eastward. Furthermore, two abnormal scenarios in upwelling and chlorophyll concentration during 2010 and 2018 were analyzed, attributed to the abnormal southwesterly monsoon and coastal circulation influenced by an El Niño event and a weak La Niña event with a positive Indian Ocean Dipole, respectively. This study elucidates the dynamic processes underlying the intricate chlorophyll distribution over the continental shelf, which is influenced by both the river plume and upwelling.

Greenhouse gas concentrations and diffusive fluxes in the middle reach of the Lancang River before and after damming

With increasing concerns of greenhouse gas (GHG) emissions from reservoirs, the debate surrounding whether hydropower qualifies as green energy has become increasingly contentious. However, quantifying the impact of dam construction on river GHG emissions still poses challenging due to the absence of direct observational data on river GHG emissions pre- and post-damming. In the present study, the differences in GHG diffusive fluxes from pre- and post-dammed river channels in the middle reaches of the Lancang River were quantified through three field campaigns in 2016, 2018, and 2023. The results indicate that reservoir construction increases the diffusive fluxes of GHG from the natural river, the total GHG emissions (expressed as carbon dioxide equivalent: CO2eq) from post-dammed river ranged from 234.42 to 527.55 mg CO2eq m−2 d−1 and were 1.4–5.2 times higher than that from pre-dammed river. However, the GHG diffusive fluxes from the reservoirs in the Lancang River remain below the average of global reservoirs. Moreover, river damming enhances the methane (CH4) emissions, the average value of CH4/CO2 ratio (in mg m−2 d−1) was 0.003 post-dam and was 13 times higher than that of pre-dam. Our results revealed that alterations in sediment distribution following reservoir construction emerge as a pivotal factor contributing to the variations in spatial distributions of GHG diffusive fluxes in the studied river channel. Overall, the findings of the study provide a primary and direct evidence of river damming impact on GHG emissions and underscore the necessity of long-term continuous in-situ measurements of GHG fluxes in the rivers undergoing hydropower development.