Growth and morphological responses of Halophila beccarii to low salinity

. 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.

The loss of seagrass meadows causes negative impacts on coastal ecosystems (e.g., estuaries, coastal lagoons, coral reefs, and mangroves) and human communities living on coastal resources.
Therefore, practical activities to prevent this loss on a global scale are extremely necessary for conserving and managing seagrass meadows [7].
In Vietnam, H. beccarii is commonly found in the lagoons and estuaries in Central Vietnam with a 0.2-20 ppt salinity range [20,21,40] and is documented as one of the dominant seagrasses in the Tam Giang-Cau Hai Lagoon system [20,41].
However, the species distribution in the field is not stable [20], which could be due to the salinity changes. Therefore, in this study, we carried out a mesocosm experiment to examine the growth capacity and morphological responses of H.
beccarii against low salinity conditions in Cau Hai Lagoon (Thua Thien Hue).

Collecting plants and sediments
The materials used for the experiment were weight and then weighed to determine biomass [47].

Statistical analysis
All variables were examined for the normal distribution prior to the analysis by using the  (Fig. 2). The growth rate increased rapidly during the first 2-5 weeks and decreased significantly in the following weeks at all salinity treatments (except 0 ppt). The rate differs significantly among salinity treatments [FA,  2 (4, n = 128) = 134.1, p < 0.0001]. It peaked at 10 ppt (4.4 ± 0.2 mm·day⁻¹) at week five and decreased to a minimum at 0 ppt salinity (0.9 ± 0.1 mm·day⁻¹) (Fig. 3).
The results indicate that H. beccarii could survive and maintain its growth in the salinity range from 0 to 20 ppt. The highest survival rate was observed at 10 ppt. The species could survive but grew poorly at 0 ppt during eight weeks. The tolerance to low salinity was also reported for several other seagrass species, such as Zostera noltii, Amphibolis antarctica, and Posidonia sinuosa [13,48]. In another study, Fakhrulddin et al. revealed that H. beccarii collected from a Malaysian estuary could tolerate the salinity range of 0-45 ppt with no mortality during 303 days of experiment [35]. This difference might be due to the gradual change in treated salinity, from 25 ppt to 0 ppt and from 25 ppt to 45 ppt at twoweek intervals so that the plants would gradually adapt to salinity; as a result, there was no shoot mortality. On the other hand, in our study, the species suffered from sudden salinity changes from 11 ppt (in the field) to tested salinities (0, 5, 10, 15, and 20 ppt), reducing the survival rate of the species.

Response of morphological characteristics
There were significant differences of leaf length (F = 60.9, p < 0.0001), petiole length (F = 35, p < 0.0001), leaf width (F = 23.5, p < 0.0001), and shoot length (F = 92.7, p < 0.0001) among salinity treatments. Leaf length, petiole length, and leaf width peaked in both salinity 10 and 15 ppt, but it was the lowest at 0 ppt; shoot length was highest at 5 and 10 ppt and lowest at 0 ppt ( Fig. 4A-D).    7B, Fig. 6).  The salinity strongly affected the biomass of H. beccarii. The one-way ANOVA revealed a significant difference in biomass among salinity treatments (F = 19.2, p < 0.0001). The biomass of H. beccarii had a maximum value at 5 ppt (4.7 ± 0.5 g) and the lowest at 0 ppt and 20 ppt with 0.2 ± 0.1 and 0.6 ± 0.3 g (Fig. 8