Variations in land surface temperatures in small-scale urban areas in Vietnam during Covid-19 restrictions: Case studies from Da Nang, Hue and Vinh City.

Land surface temperature (LST) is an important variable in urban microclimate research. At the end of 2019, the emergence of Covid-19 pandemic has changed the world in a manner that forced many countries to impose restrictions in human activities. As a measure to prevent the expansion of Covid-19 infections, most of the major cities have entered a prolonged lockdown period and reduction in human activities between the early 2020 and the late 2021. These restrictions were strict in most of the cities in Southeast Asia, particularly in Vietnam. The present study investigated the variations in LST and NDVI observed in three rapidly growing urban areas, namely Da Nang, Hue and Vinh, in Vietnam using Landsat-8 imagery acquired between 2017 and 2022. There has been a slight reduction in LST in the study sites, particularly in Da Nang City, during the lockdown period but not as high as observed in recently conducted studies from big metropolitan cities, including in Vietnam. It is also observed that LST estimated from built-up areas and other impervious surfaces remained relatively stable during the study period which is similar to the results from recent studies.

Land cover and crop types mapping using different spatial resolution imagery in a Mediterranean irrigated area.

Crop type identification is critical for agricultural sustainability policy development and environmental assessments. Therefore, it is important to obtain their spatial distribution via different approaches. Medium-, high- and very high-resolution optical satellite sensors are efficient tools for acquiring this information, particularly for challenging studies such as those conducted in heterogeneous agricultural fields. This research examined the ability of four multitemporal datasets (Sentinel-1-SAR (S1), Sentinel-2-MSI (S2), RapidEye (RE), and PlanetScope (PS)) to identify land cover and crop types (LCCT) in a Mediterranean irrigated area. To map LCCT distribution, a supervised pixel-based classification is adopted using Support Vector Machine with a radial basis function kernel (SVMRB) and Random Forest (RF). Thus, LCCT maps were generated into three levels, including six (Level I), ten (Level II), and fourteen (Level III) classes. Overall, the findings revealed high overall accuracies of >92%, >83%, and > 81% for Level I, Level II, and Level III, respectively, except for Sentinel-1. It was found that accuracy improves considerably when the number of classes decreases, especially when cropland or non-cropland classes are grouped into one. Furthermore, there was a similarity in performance between S2 alone and S1S2. PlanetScope LCCT classifications outperform other sensors. In addition, the present study demonstrated that SVM achieved better performances against RF and can thereby effectively extract LCCT information from high-resolution imagery as PlanetScope.

Environmental management in Ramsar designated wetland areas in Vietnam: studies from U Minh Thuong and Tram Chim national parks (Mekong Delta).

This study investigated the possibility of using remotely sensed data and field surveys for understanding the environmental management practices in two Ramsar sites - U Minh Thong and Tram Chim national parks - in the Vietnamese Mekong Delta. Enhanced agriculture, infrastructure development, changes in hydrological regime, forest fires, and natural resources exploitation are the key variables that caused the depletion of these two wetland areas. Land cover, particularly vegetation coverage, has been changed considerably during the post-war period and agriculture has been intensified in the surrounding areas of U Minh Thuong and Tram Chim wetlands. The current water management strategies in U Minh Thuong and Tram Chim were designated to ensure proper water circulation during the dry and wet seasons in a way helpful to agriculture in the buffer zones and to prevent forest fires during the dry season. It is found that the water management strategies to prevent forest fires in both the parks resulted in the accumulation of toxic agrochemicals within the park during the wet season. Both U Minh Thuong and Tram Chim wetlands are invaded by alien plant species which is threatening the natural biodiversity of the area. Proper monitoring and control of invasive species is necessary for protecting the natural biodiversity of these wetland ecosystems. Proper law enforcement and an interactive and inclusive wetland management should be practiced in order to conserve these valuable wetland ecosystems.

Fish species composition, diversity, and migration in the Mekong Delta: a study in the Cua Tieu River, Vietnam.

Southern Vietnam, particularly the Mekong Delta, is popular in inland fish species diversity. In this study, fish species diversity across six stations from the estuary to the upstream of Cua Tieu River, which is situated in the Tien Giang province in Southern Vietnam, has been studied from January 2018 to June 2020. Altogether, 2088 specimens were collected and classified, and these belonged to 115 species, 98 genera, and 54 families of 15 orders that were recognized and identified. The names of species, genera, families, and orders are cited. In the total of 115 species, the Perch-like order (Perciformes) is the most diverse group, with 50 species (consist of 43.48% of total species). The catfish order (Siluriformes) is the second-most diverse group, consisting of 19 species (16.52% of total species). The carp order (Cypriniformes) consists of 8 species (6.96%), and the herring order (Clupeiformes) consists of 7 species (6.09%). Three orders of fishes, needle fish order (Beloniformes), Spiny eel order (Synbranchiformes), and flounder order (Pleuronectiformes), consist of 6 species (5.22%), and other orders consist of 1-3 species (0.87-2.61%). Among the 115 species recorded, 4 species were classified as vulnerable (VU) in the Red Data Book of Viet Nam (2007) (which is 3.48% of the total species collected). These are Elops saurus, Anodontostoma chacunda, Datnioides polota, and Toxotes chatareus. According to the IUCN Red list (2020), 1 species is endangered (EN) (0.87%), 1 species is vulnerable (VU) (0.87%), 3 species of fish are near threatened (NT) (2.61%), 6 species are data deficient (DD) (5.22%), and 58 species are least concern (LC) (50.43%). The result also recorded 41 fish as migratory species (26 freshwater species and 15 marine species), which is 35.65% of total species collected. Some migratory species are important and endemic species of the Mekong River, such as Bagarius yarrelli, Boesemania microlepis, Yasuhikotakia modesta, Cyclocheilos enoplos, Pangasianodon hypophthalmus, Pangasius conchophilus, Pangasius krempfi, Pangasius pleurotaenia, Phalacronotus bleekeri.

Anthropogenic influences on coastal environmental changes in the Mekong Delta: a study from Ben Tre Province, Southern Vietnam.

Low-lying coastal environments are highly dynamic and sensitive to natural as well as anthropogenic perturbations. Climate change, sea level rise, storms and tsunamis are the natural phenomena that affect the deltaic coasts in Southeast Asia in general and Vietnam in particular. The effects of these phenomena can be exacerbated by human activities such as mangrove deforestation, aquaculture and infrastructure development. Conversely, the low-lying coastal areas are important in the economic development of Southeast Asian countries. In the Vietnamese Mekong Delta, coastal areas have been affected by a number of factors, such as climate change, sea level rise, aquaculture, pollution and tourism-related activities in recent decades. The present study investigated shoreline changes, expansion of aquaculture ponds, soil salinity changes and salinity intrusion in the river systems along the coastal areas of Ben Tre Province in the Vietnamese Mekong Delta between 1998 and 2020 using satellite imagery and field data. Variations in erosion and accretion were found to be not unique along the coast of Ben Tre. There was a rapid expansion of aquaculture ponds between 1998 and 2015 and a slight decline since then. Soil salinity has been increased between 1998 and 2020; it is seen from recent satellite data that soil is becoming more saline in the inland areas of Ben Tre. Saltwater intrusion into the rivers of Ben Tre is considered associated with El Niño-La Niña conditions. It is suggested that reforestation of abandoned shrimp ponds in Ben Tre by mangrove vegetation can be effective as a bioshield against coastal hazards, such as sea level rise and shoreline erosion.

Impact of a dam construction on the intertidal environment and free-living nematodes in the Ba Lai, Mekong Estuaries, Vietnam.

The impact of high siltation and accumulation of organic and waste material in the intertidal of the dammed Ba Lai River in Vietnam as part of the Mekong estuarine system was investigated by means of marine free-living nematodes. Nutrients content (nitrate, ammonium, total phosphorus, total nitrogen), total suspended solids, total organic carbon, coliform, bacteria E. coli, pH, dissolved oxygen, total dissolved solids, methane and hydrogen sulfide concentration, and the nematode communities were characterized in sediment at selected stations along the river above and below the dam. Our results found elevated methane concentrations at the upstream side of the dam while hydrogen sulfide concentrations found to be highest in the downstream side of the dam. Furthermore, methane and hydrogen sulfide concentrations were correlated to nematode community characteristics such as trophic composition densities and genera composition. There was a clear difference between the communities above and below the dam. The discontinuous nematode community distribution indicated that the Ba Lai River is impacted by dam construction. Potentially the high deposition and eutrophication could turn the area into a methane-rich area related to predicted impact on nematodes.

Did the Covid-19 restrictions influence land surface temperatures in Southeast Asia? A study from Ho Chi Minh City, Vietnam.

There have been a prolonged lockdown period and reduction in human activities in most of the major cities in the world during the Covid-19 pandemic period between the early 2020 and the late 2021. Such a reduction in human activities was believed to have influenced pollution levels and land surface temperatures (LST) in urban areas. This paper describes the variations in LSTs before, during and after the Covid-19 lockdown in Ho Chi Minh City in southern Vietnam, which is the economic hub of the country. For this purpose, Landsat-8 OLI and TIRS images acquired between 2015 and 2022 were used. It is observed that there was a significant reduction of 1 to 1.8 °C in LST in open areas, excepting impervious surfaces and built-up areas, during the strict lockdown period in Ho Chi Minh City, and an increase in LST since then. The observed reduction in LST during the lockdown period in Ho Chi Minh City is in agreement with the reduction in greenhouses gases during the same period in recent studies. Human mobility and industrial activities have been restored in November 2021 in the study area which would explain the regain in LST in the post-lockdown period.

Remote sensing for mapping algal blooms in freshwater lakes: a review.

A large number of freshwater lakes around the world show recurring harmful algal blooms, particularly cyanobacterial blooms, that affect public health and ecosystem integrity. Prediction, early detection, and monitoring of algal blooms are inevitable for the mitigation and management of their negative impacts on the environment and human beings. Remote sensing provides an effective tool for detecting and spatiotemporal monitoring of these events. Various remote sensing platforms, such as ground-based, spaceborne, airborne, and UAV-based, have been used for mounting sensors for data acquisition and real-time monitoring of algal blooms in a cost-effective manner. This paper presents an updated review of various remote sensing platforms, data types, and algorithms for detecting and monitoring algal blooms in freshwater lakes. Recent studies on remote sensing using sophisticated sensors mounted on UAV platforms have revolutionized the detection and monitoring of water quality. Image processing algorithms based on Artificial Intelligence (AI) have been improved recently and predicting algal blooms based on such methods will have a key role in mitigating the negative impacts of eutrophication in the future.

Landsat-8 and Sentinel-2 data for mapping plastic-covered greenhouse farming areas: a study from Dalat City (Lam Dong Province), Vietnam.

Plastic-covered greenhouse (PCG) farming has been practiced worldwide, particularly in highland areas, for planting fruits and garden plants. Highland areas in south-central Vietnam have been witnessing an expansion of PCG farming in recent decades. For sustainable management of plastic-covered greenhouse farming areas, a proper and continuous monitoring of their spatial patterns is necessary. In this study, we used spaceborne multispectral imagery (Landsat-8 OLI and Sentinel-2 MSI) for automatic detection and mapping of plastic covered greenhouse farming areas in and surrounding areas of Dalat City in Lam Dong province, south-central Vietnam. Unsupervised (K-Means algorithm) and automatic delineation using spectral indices were applied to the Sentinel-2 and Landsat-8 data for mapping plastic-covered greenhouses in Dalat City and surrounding areas. The results were evaluated using field data; automatic spectral index-based mapping of PCGs using Sentinel-2 and Landsat-8 data was found as robust in terms of the overall accuracy. The overall accuracies of PCG maps derived from Sentinel-2 and Landsat-8 data by applying spectral indices were 90.8% and 88.7%, respectively, and those of unsupervised K-Means classification were estimated as 73.2% and 71.7%, respectively. This study estimated that about 2425 ha of plastic-covered greenhouses existed within in Dalat City in 2020.

Ecological impact assessment of irrigation dam in the Mekong Delta using intertidal nematode communities as bioindicators.

The ecological response of nematode communities to dam construction has limited attention. In this study, the response of intertidal nematode communities in the Ba Lai River (Mekong Delta, Vietnam) to the construction of an irrigation dam was investigated. Nematode communities and environmental parameters were investigated during the rainy season of 2015. The obtained results showed that the Ba Lai dam had caused negative impacts on the local environment by disrupting longitudinal connectivity, the accumulation of nutrients (total organic carbon, total nitrogen, total phosphorus), and heavy metals (copper, iron, arsenic, lead) in the upstream and the lateral sides of the dam, consequently leading to changes in the nematode communities. The response of nematode communities to the dam's presence was clear based on their abundance, diversity, dominant genera, and community composition. Furthermore, changes in the abundance and diversity of nematodes in the Ba Lai River appeared to be controlled primarily by acidity (pH), total suspended solids (TSS), iron (Fe), clay, and salinity most responsible for changes to nematode communities. Because the nematode communities are well adapted to the physicochemical disturbances caused by dam construction, they are a potential tool for ecological monitoring and understanding the influence of dams on aquatic ecosystems.