Halophila beccarii Ascherson is classified as a threatened seagrass species by IUCN because of the reductive tendency of ​​its distribution area. This seagrass is considered a euryhaline species adapted to a wide range of salinities from freshwater and brackish water to marine water. Previous studies showed that the species tends to grow better under low salinity; however, its optimum salinity has not been determined. In Vietnam, H. beccarii grows in habitats with low salinity (0–20 ppt). The results show that salinity affects the growth, survival rate, shoot density, biomass, and morphological characteristics of the grass. The leaf dimension is more prolonged and broader; the petiole and shoot length are longer at 10 ppt salinity. In contrast, both the number of shoots and biomass peak at 5 ppt and decrease at lower and higher salinities. The study reveals that H. beccarii can grow better under mesohaline conditions than freshwater and hypersaline conditions with an optimum salinity at 5–10 ppt. These findings would explain the species’ distribution dynamics in coastal environments and be helpful information for conserving the seagrass populations in habitats with fluctuating salinity as coastal lagoons in Central 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.

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.