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Najas indica (Willd.) Cham. is a freshwater submerged aquatic vegetation. N. indica is an annual plant. Therefore, seeds in the sediment and the survival and growth of seedlings play an important role in re-establishing new populations of the species. The purpose of this study was to determine the seed density in the sediment, examine the effect of salinity on seedling germination from seeds, and assess the impact of the salinity on the growth of early seedlings of N. indica under the experimental condition. The seed density of N. indica was highest from September to November and the lowest in May to July 2018. Seedlings of the species were observed in the range of salinity from 0–15‰. No seedling was recorded at 20‰ salinity during the experimental period. The number of seedlings, growth rate, shoot length, and the number of internodes of seedlings of N. indica, all had maximum values at a salinity of 5‰, while leaf length tended to decrease with increasing salinity. The study showed that the optimum salinity for seedling germination from seeds and growth of seedlings of N. indica species from the Cau Hai lagoon was at 5‰ salinity. The study results supply the necessary information for the protection and development of meadows of the N. indica species in the lagoon environment of Vietnam.

Halophila beccarii Aschers., 1871 was one of the seagrass species that was vulnerable and threatened list by the International Union for Conservation of Nature (IUCN). During the survey period from March 2018 to September 2019, our results showed that H. beccarii was widely distributed in the Cau Hai lagoon and concentrated mainly in the Southeast area of the lagoon. The distribution of H. beccarii was seasonal. The species’ growth started from the early rainy season of the previous year until the end of the dry season of the following year. However, the period when the species had a high cover, biomass, and shoot density was from September 2018 to March 2019, and those values peaked in January 2019. The lowest cover and shoot density were recorded in November 2019, while the lowest biomass was in July 2019. The rainfall had a close positive correlation with the biomass of H. beccarii, while water salinity and water temperature were moderate negatively correlated with the biomass of the species.

Lagoonal environments exhibit high levels of instability depending on hydrological, climatic and ecological factors, thereby influencing the distribution and structure of submerged plant communities. Conditions typically fluctuate widely due to the interaction of freshwater from rivers with saltwater from the sea, as well as from aquaculture activities that together influence submerged hydrophyte community spatial and temporal variability depending on plant survival strategies. Ruppia species feature either underwater pollination mediated by an air bubble or by the release of pollen floating at the water surface, the former promoting self-pollination. Tropical Asian Ruppia brevipedunculata Yu and den Hartog was assumed to pollinate below the water surface and identified as a separate lineage among selfed Ruppia taxa. We used nine nuclear microsatellites to estimate inbreeding levels and connectivity of R. brevipedunculata within a large SE Asian lagoon complex. Ruppia brevipedunculata meadows were strongly inbred as could be derived from the many monomorphic or totally fixed loci for unique alleles in different parts of the lagoon, which appears consistent with selfing behavior. Those from aquaculture ponds were highly inbred (FIS = 0.620), though less than open lagoon sites that showed nearly total inbreeding (FIS = 0.942). Ruppia brevipedunculata from two major lagoon parts were highly differentiated with spatially structured gene pools and a strong barrier between parts of the lagoon over a 30 km distance. Migration-n analysis indicated unidirectional though limited gene flow and following potential hydrological connectivity. Overall, private alleles under homozygote conditions explained a stronger genetic differentiation of populations situated inside aquaculture ponds than of open lagoon populations. Kinship values were only relevant up to 5 km distance in the open lagoon. Within a confined area of aquaculture ponds featuring dense vegetation in stagnant water, there would be opportunity for mixed pollination, thereby explaining the higher diversity of unique multilocus genotypes of aquaculture pond habitats. Low connectivity prevents gene pools to homogenize however promoted sites with private alleles across the lagoon. Complex hydrodynamic systems and human-made habitats enclosed by physical structures impose barriers for propagule dispersal though may create refugia and contribute to conserving regional genetic diversity.

Human-induced land use in coastal areas is one of the main threats for seagrass meadows globally causing eutrophication and sedimentation. These environmental stressors induce sudden ecosystem shifts toward new alternative stable states defined by lower seagrass richness and abundance. Enhalus acoroides, a large-sized tropical seagrass species, appears to be more resistant toward environmental change compared to coexisting seagrass species. We hypothesize that reproductive strategy and the extent of seedling recruitment of E. acoroides are altered under disturbance and contribute to the persistence and resilience of E. acoroides meadows. In this research, we studied eight populations of E. acoroides in four lagoons along the South Central Coast of Vietnam using 11 polymorphic microsatellite loci. We classified land use in 6 classes based on Sentinel-2 L2A images and determined the effect of human-induced land use at different spatial scales on clonal richness and structure, fine-scale genetic structure and genetic diversity. No evidence of population size reductions due to disturbance was found, however, lagoons were strongly differentiated and may act as barriers to gene flow. The proportion and size of clones were significantly higher in populations of surrounding catchments with larger areas of agriculture, urbanization and aquaculture. We postulate that large resistant genets contribute to the resilience of E. acoroides meadows under high levels of disturbance. Although the importance of clonal growth increases with disturbance, sexual reproduction and the subsequent recruitment of seedlings remains an essential strategy for the persistence of populations of E. acoroides and should be prioritized in conservation measures to ensure broad-scale and long-term resilience toward future environmental change.

Halophila beccarii is a threatened tropical seagrass which has a narrow, restricted distribution in the shallow coastal areas of the Indo-Pacific. The monoecious H. beccarii reproduces both sexually or asexually and such mixed reproduction mode is important to stay resilient in a dynamic coastal environment. This study examined genetic diversity and structure of H. beccarii to determine its survival strategy within a tropical coastal lagoon of Southeast Asia (Central Viet Nam). Seven microsatellite loci gave an overall low level of allelic richness ( A r = 1.9), gene diversity ( H E = 0.233) and clonal diversity (R = 0.27). A small neighborhood size and short dispersal distance together with a significant fine-scale genetic structure within each site indicated that its survival strategy is through local seed recruitment over very short distances and a limited local clonal extension ( H. beccarii meadows is through sexual reproduction and within site dispersal. Implications for conservation are that environmental conditions of the lagoon should allow to maintain a local recruitment, establishment and development of H. beccarii beds at site level.

SUMMARY Cellular and body scale structure of a new armored dinoflagellate Heterocapsa huensis, collected from Hue, Vietnam were investigated. Morphology of motile cell was observed by light, fluorescent and scanning electron microscopy, and body scale structure was examined by whole mounts of transmission electron microscopy. Cells of H. huensis were ellipsoid with a spherical nucleus located in the posterior and multiple pyrenoids located above the nucleus; this arrangement was similar to that of Heterocapsa pygmaea. Transmission electron microscopy revealed ultrastructure of the body scales consisted of a rounded triangular basal plate and three-dimensional ornaments. Structure of the basal plate resembles that of Heterocapsa illdefina; however, the number of the peripheral spine is different from that of H. illdefina and this structure has never been reported from Heterocapsa species. A new Heterocapsa species, H. huensis Iwataki et Matsuoka sp. nov., is described based on positions of organelles and body scale ultrastructure.