Vietnam Journal of Marine Science and Technology

A long-term dataset of in-situ temperature data reveals wind-induced up- and downwelling in the coastal waters of Nha Trang, South Central Vietnam

Andreas Kunzmann — 2024-12-10

In order to implement successful coastal management and protect corals, it is imperative to understand the Nha Trang Bay’s coastal processes and take adequate measures to protect corals and reef structure. This paper aimed to analyze whether sudden variations in physical parameters, such as temperature, could be potentially harmful to coastal coral reefs, in addition to anthropogenic factors such as pollution and intensive fishing. In this paper, the first long-term observation (2008–2019) of temperatures, not only from SST data, but also in situ in coral reefs (10 and 18 m depth) at Nha Trang Bay, South Central Vietnam, was investigated. The data showed that wind-induced upwelling during summer mainly govern the coastal region. In contrast, wind-induced downwelling was found during winter, visible in all three investigated water layers (SST, 10 and 18 m). In winter, the vertical mixing is strong and there is virtually no time-lag between the layers. In summer a scattering layer was formed, the phenomenon where a layer of water with different properties (such as temperature or salinity) is formed, blocking the sinking of water. In summer, correlations with air temperature were not significant, nor were correlations with night cooling, thus having implications for the distribution of nutrients and the health of the coral reefs. However, this was only the situation near the coast. Wavelet analysis shows that the short-term variability is significantly more substantial, caused by the shallow depth of the thermocline, which is much stronger affected by tidal and weather events than in winter. As a result of the combination of large yearly temperature variations (21^oC to 31^oC) plus increased sediment deposition in the rainy seasons, reefs close to the shore are generally not well-developed. This paper strongly advocates for science-based monitoring of coral reef conditions and underscores the need for law enforcement within the Marine Protected Area of Nha Trang Bay.

Distribution characteristics of tropical cyclones affecting the Vietnam region during 1992–2022

Thi Thanh Hoa Pham — 2024-12-10

This research investigates the distribution characteristics of tropical cyclones affecting the Vietnam region, including the mainland and the East Vietnam Sea, from 1992 to 2022. The tropical cyclone activity showed an increasing trend, mainly in the numbers of tropical depressions and typhoons with the sustained maximum wind speed ranging from level 12 to 15 on the Beaufort Wind Scale; however, this trend did not meet the statistical 99% confidence level. The number of tropical cyclones directly affecting the mainland of Vietnam accounted for approximately 32%. Over the East Vietnam Sea, tropical cyclones presented a peak in September and a slump in February. Occurrence frequencies and probabilities were significantly high over the in-shore and off-shore regions from Quang Ninh to Phu Yen, over the North and Center of East Vietnam Sea, where the maximum frequencies could be 0.9 tropical cyclone/year over the Gulf of Tonkin higher than 1.1 tropical cyclone/year passing a grid of 1^o longitude-latitude over the north East Vietnam Sea. Meanwhile, the southern offshore areas from Phu Yen - Ca Mau, Ca Mau - Kien Giang, and Thailand Gulf were less influenced by tropical cyclones. The frequency and probability of typhoons were below 0.1 tropical cyclone/year and 0.2%, respectively.

Application the Mike 3 model to simulate the turtibity dispersion due to sediment dumping activities in Nghi Son Port, Thanh Hoa Province

Hong Lan Nguyen — 2024-12-10

The study applied the Mike 3 hydrodynamic model to simulate the turbidity propagation from sediment mud deposition activities at sea in the Nghi Son port area, Thanh Hoa province. The research results show that during the northeast monsoon period, the sediment mud concentration field is distributed southward from the deposition area, with sediment mud concentrations ranging from 0.01 kg/m³ to 0.4 kg/m³ within an influence radius of approximately 2.5 km. During the southwest monsoon period, the turbidity concentration distribution expands northward from the deposition area due to the influence of wind and currents during this season. The northern area of Nghi Son port is affected by the dispersion of turbidity concentrations below 0.04 kg/m³. The area surrounding Hon Me island has sediment mud concentrations ranging from 0.015 kg/m³ to 0.025 kg/m³. The turbidity concentration distribution with depth ranges from 0.01 kg/m³ to 0.4 kg/m³ and is primarily located at the seafloor depths ranging from -14 m to -18 m. The turbidity concentration gradually decreases towards the water surface. The turbidity concentration in the upper water layer is relatively low, ranging from 0.01 kg/m³ to 0.08 kg/m³, with depths from 0 to -5 m.

Species diversity and distribution of the macro algae from Bach Long Vi Island, Vietnam

Quynh Nga Nguyen — 2024-12-10

Research results on seaweed at four cross-sections of Bach Long Vi Island from 1994 onwards have shown that 129 species of macroalgae belong to 4 phyla. Among them, Cyanobacteria has ten species, accounting for 7.75% of the total number of species; Red seaweed (Rhodophyta) has 70 species (54.26%); Brown seaweed (Ochrophyta/Phaeophyta) has 27 species (20.93%), and Green seaweed (Chlorophyta) has 22 species (17.06%). They belong to 21 orders, 42 families, and 72 seaweed genera. The number of species in the studied transects ranges from 41 species (transect 4) to 99 species (transect 2) and averages 72.25 species/transect. The similarity coefficient of seaweed species at four surveyed transects ranged from 0.26 (between transects 1 and 4) to 0.74 (between transects 2 and 3) and averaged 0.48. Of the 129 known species, there were 44 species only distributed in the intertidal zone, accounting for 34.10% of the total number of species; 108 species are only distributed in the subtidal zone (83.72%) and 26 species are distributed in both intertidal and subtidal zones (20.15%); most species distributed from the low intertidal zone downward and are concentrated mainly in the low intertidal zone and the upper range of the subtidal zone. In terms of structure, the number of taxa ranges from 2 (Cyanophyta) to 12 (Rhodophyta). On average, 5.25 orders/phylum; of families from 4 (Cyanophyta, Ochrophyta) to 25 (Rhodophyta) and 10.5 families/order; of genera from 6 (Cyanophyta) to 42 (Rhodophyta) and 18; of species from 10 (Cyanophyta) to 70 (Rhodophyta) and 32.25. The average number of species/genera is quite large (32.25). However, some genera have only one species (Hydrocoryne, Asparagopsis, Parvocaulis, etc.), and some other genera have a more significant number of species (Dictyota: 10 species; Hypnea: 8 species; Peyssonnelia: 5 species, etc.). Among the families, some have only one genus (Peyssonneliaceae, Scytosiphonaceae, Codiaceae, etc.), but others have more genera (Dictyotaceae: 5, Boodleaceae: 4, Sargassaceae: 2, etc.). The total number of orders of the four phyla (Cyanophyta, Rhodophyta, Ochrophyta, and Chlorophyta) is 21. Of which, Cyanophyta has 2 orders, accounting for 9.5% of the total number of recorded orders: Rhodophyta (12 and 57.1%), Ochrophyta (4 and 19%), and Chlorophyta (3 and 14.4%).

Species composition and distribution of Sponges in some islands in Vietnam sea

Cong Thung Do — 2024-12-10

Research results from 2003 to present, in same Vietnamese islands, have identified 205 Sponges species belonging to 85 genera and 56 families, 22 orders of 3 classes, including Dermospongia class (203 species), two classes Calcarea and Homoscleromorpha has only 1 species in each class. Among these, there are 3 species new to science: Cladocroce pansinii Bertolino & Calcinai, 2023, Cladocroce lamellata Bertolino and Calcinai, 2023 and Spongilla marconii. The diversity of Sponges is quite high, reaching 2.4 species/genus, 3.66 species/family and 9.32 species/order. Orders with the highest number of species include Haplosclerida (55 species), followed by Poecilosclerida (29 species), Dictyoceratida (21 species), and Halichondrida (20 species). The South Central islands have the highest number of species (121 species), followed by the North Central islands (65 species), the Northeastern islands of Vietnam (57 species), Spratly Islands (37 species) and Phu Quoc (15 species). Species Cinachyrella australiensis, Dysidea fragilis, Dysidea cinerea can live at a water depth of 134 m. The biomass of Sponges varies from 0.14–2.5 kg/m².

Characterizing of ITS2 secondary structures reveals the geographical differentiation of seagrass Halophila ovalis (Hydrocharitaceae) in the world

Xuan-Thuy Nguyen — 2024-12-10

The seagrass Halophila is one of the genera of Hydrocharitaceae that shows the highest number of species, with around 20 species. Among them, H. ovalis, H. major, H. minor, and H. nipponica are closely related species. It is the first time ITS2 secondary structures and their phylogenetic utility in this genus were reported worldwide. Phylogenetic analysis based on 205 bp of ITS2 showed four clades corresponding to above species. ITS2 secondary structures showed insight into Halophila ovalis from the East coast of Africa. Halophila ovalis from the East coast of Africa showed a distinct variant in Helix 1, 2, and 3 compared to the worldwide populations. Therefore, the ITS2 locus should be used as a DNA barcode for identifying Halophila species.