Tóm tắt Luận án The study on seagrass communities and carbon storage capacity of them in some typical coastal lagoons in the central of Vietnam

as used to interpret and analyze the spatial distribution of the ecosystems in GIS software. 2.4.6. Data analysis Microsoft Excel and packages of SPSS 20 softwares were used to statistical all of data.ANOVAstatistical was performed to determine the variation of different environments conditions. CHAPTER 3. RESULTS AND DISCUSSIONS 3.1. Seagrass composition and morphological characteristics 3.1.1. Species composition Results of this study showed that a total of 09 species of 6 genera, 4 families in 3 study areas were identified, out of 15 of Vietnam. In particular, Tam Giang - Cau Hai lagoon has 6 species, Thi Nai lagoon has 6 7 species, and Nai lagoon has 6 species, different species composition in different lagoons (Table 3.1). Supplementing the Halodule uninervis for seagrass species composition in Cau Hai lagoon (Tam Giang - Cau Hai lagoon), the Halophila major for seagrass species composition in Nai lagoon. The sorresson homology coefficient among the communities in Tam Giang - Cau Hai lagoon and Thi Nai lagoon is the highest, reaching 0.92. Table 3.1.Status of species composition in 3 study areas STT Taxon Distribution of species composition TG-CH Thi Nai Nai SW NE SW NE SW NE Hydrocharitaceae Juss. Enhalus L.C. Rich. 1 Enhalus acoroides (L.f) Royle +++ +++ Thalassia Banks ex Koenig 2 Thalassia hemprichii (Ehrenb. ex Solms) Asch. + + + + Halophila Du petit Thouars 3 Halophila beccarii Ascherson ++ ++ + + 4 Halophila ovalis (R. Br.) Hooker f. + ++ + + ++ ++ 5 Halophila major (Zoll.) Miquel + Ruppiaceae Horaninov Ruppia Linnaeus 6 Ruppia maritima Linnaeus ++ ++ ++ ++ + + Zosteraceae Domortier Zostera Linnaeus 7 Zostera japonica Ascherson & Graebner +++ +++ ++ ++ 7 Cymodoceaceae N. Taylor Halodule Endlicher 8 Halodule pinifolia (Miki) den Hartog + + ++ ++ + + 9 Halodule uninervis (Forssk.) Ascherson + + + Seasonal species 6 5 7 7 6 6 Total of species 6 7 6 Note: (+): Less; (++): Many; (+++): Very much; TG-CH: Tam Giang-Cau Hai lagoons; SW: southwest wind season (rainy season), NE: Northeast monsoon season (dry season). 3.1.2. The identification keys KEY TO THE FAMILIES BELONG HYDROCHARITALES 1a. Leaves differentiated into a sheath and a blade, without a ligule...........2 1b. Leaves differentiated into a sheath and a blade, with a ligule................3 2a. Flowers dioecious, sometimes monoecious, with a trimerous perianth........................................................................Hydrocharitaceae 2b. Flowers monoecious, without a perianth............................Ruppiaceae 3a. Leaves without tannin cells; each longitudinal vein with several fibrous strands; one xylem lumen........................................Zosteraceae 3b. Leaves with tannin cells; each longitudinal vein with several fibrous strands, but with several xylem lumen..........................Cymodoceaceae HỌ HYDROCHARITACEAE Juss. 1789, Gen. Pl. 67; nom. cons. Typus: Hydrocharis L. KEY TO THE GENENA BELONG HYDROCHARITACEAE 1a. Very coarse plants with a thick rhizome and strap-shaped leaves; leaf margins with very coarse nerves, after decay remaining as persistent strands.............................................................................................. Enhalus 1b. Moderately coarse or even very delicate plants with more slender rhizomes...................................................................................................... 2 8 2a. Leaf-bearing branches arising fromthe rhizome at distances of several internodes; each internode covered by a scale. Leaves distichous, linear; nerves parallel...............................................................................Thalassia 2b. Leaf-bearing branches arising from the thin rhizome at each node. Leaves petiolate, in pairs, in pseudo-whorls or distichously arranged; with a pinnate nervation.......................................................................Halophila ENHALUS L. C. Richard. 1812. Mem. Inst. Paris 12(2): 64, 71, 74. Type species: Enhalus koenigi Rich. (=E. acoroides (L. f.) Royle). Enhalus acoroides (L.f.) Royle, 1839; Phamh., 1993; N.V.Tien, 2002; N.T. Do, 2005; Wang, Q. et al., 2010. _Stratoides acoroides L.f., 1781;_Enhalus koenigi Rich., 1812; _Valisneria sphaerocarpa Blanco., 1937; _Enhalus marinus Griff., 1951. Descriptions: robust dioecious species. Rhizome 1.5 – 1.8 cm in diameter, covered with black long persistent fibrous strands of decayed- leaves and numerous, cord-like, fleshy, thick roots 1.5 – 5 mm in diameter, 8 - 20 cm long. Leaf blades 30 - 150 cm long, 12 – 1.8 cm wide, apex rounded, with longitudinal veins nerves parallel, two sides of the leaf border have 2 long veins, etc. (figure 3.1). Loc.class.: Habitat inter Insulas Zeylonicas, König. Lectotypus: [illustration in] Rumphius. 1750. Herb. Amboin. 6: 179, t. 75, fig. 2. 3 Figure 3.1. Enhalus acoroides – 1. vegetative morphology; 2.a form; 3. habitat form THALASSIA Banks ex Konig., 1805 Leccotype species: Thalassia testudium Banks & Sol. ex Koenig (designated by Rydberg, 1909. Fl. N. Amer. 17: 73). Thalassia hemprichii (Ehrenb.) Aschers, 1871; Phamh., 1961; Ernani G. Menez, R.C. Phillips, Hil. P. Calumpong, 1983; Phamh., 1993; N.V.Tien, 2002; N. T. Do, 2005; Wang, Q. et al., 2010. 9 _Schizotheca Ehrenb., 1834. Descriptions: Rhizome 3 – 5 mm in diameter. Internodes 4 – 7 mm long. Each node with a root, 1,5 mm in diameter. Leaf blades 10 – 40 cm long, 4 – 11 mm wide, with 7 - 17 longitudinal veins,... (figure 3.2) Loc.class.: Eritrea: Massouar. Ehrenberg, C.G., Typus: #s.n. (LT: BM; IT: LE). 1 2 3 Figure 3.2. Thalassia hemprichii – 1,2. a form and leaf apex; 3. habitat form HALOPHILA Thouars. 1806. Gen. Nov. Madag. 2: 2. Type species: Halophila madagascariensis Steudel (=H. ovalis (R. Br.) Hook. f.), validated by Doty and Stone. 1967. KEY TO THE SPECIES BELONG HALOPHILA 1a. Leaf blades are needle-shaped, without cross veins, but with 3 longitudinal veins........................................................... Halophila beccarii 1b. The leaves are oval or ovoid, with cross veins ..............................2 2a. Leaf blades 10 - 12 mm long, 7 – 9 mm wide, with 12 – 16 cross veins angle ranged 45 – 550 with midrib...Halophila ovalis 2b. Leaf blades 15 – 18 mm long, 9 – 12 mm wide, with 16 – 18 cross veins angle ranged 60 – 750 with midrib ..............Halophila major Halophila beccarii Ascherson, 1871; Phamh., 1993; N.V.Tien, 2002; N. T. Do, 2005; Wang, Q. et al., 2010. Descriptions: Dioecious. Thin rhizomes 1 – 2 cm long, with 2 scales covering the base of the erect stem bearing a group of 6 - 10 leaves at the top. Blades lanceolate, up to 3 cm long, 1 - 2 mm wide, with no cross veins, but with 3 paralleled veins in paralleled, apex pointed, etc. (figure 3.3). 10 Loc.class.: Indonesia: Borneo: Sarawak, near mouth of Bintula River. Typus: Beccari 3666 (IT: S). 1 2 3 Figure 3.3. Halophila beccarii – 1.a form; 2. leaf, 3.habitat form Halophila ovalis (R.Br.) Hook. f., 1858; Phamh., 1993; N.V.Tien, 2002; N. T. Do, 2005; Wang, Q. et al., 2010 _Caulinia ovalis R. Brown, 1810;_Kernera ovalis Schult., 1829; _Halophila madagascariensis Steud., 1840;_Diplanthera indica Steud., 1840;_Diplanthera sp. Griff., 1851;_Lemnopsis major Zoll., 1851; _Halophila major (Zoll.) Miq., 1855;_Halophila euphlebia Makino, 1912; _Halophila linearis den Hartog, 1957_Halophila hawaiina Doty and Stone., 1966;_Halophila australis Doty and Stone., 1966. Descriptions: Dioecious. Thin rhizome, 1.0 – 1.5 mm in diameter, internodes up to 10 cm long; erect shoot at each node, bearing a pair of petiolated leaves; leaf blades lanceolate to obovate or elliptic, 10 – 12 mm long, 7 – 9 mm wide, margin entire, apex obtuse, base rounded, petiole 2.2 – 3.0 cm long, midrib prominent with 12 - 16 cross veins angle ranged 45 - 550 with midrib, etc. (figure 3.4). Loc.class.: Australia: Tasmania. Typus: R. Brown 5816 (BM). 3 Figure 3.4. Halophila ovalis; 1.a form; 2. leaf, 3.habitat form 11 Halophila major (Zoll.) Miq., 1855; X.V.Nguyen, et al., 2013. _Lemnopsis major Zoll., 1854; _Halophila ovalis var. major (Zoll.) Ascher., 1868;_Halophila euphlebia Mak., 1912. Descriptions: Dioecious. Thin rhizome, 1 – 1.5 mm in diameter, internodes 1 - 5 cm long; erect shoot at each node, bearing a pair of petiolated leaves; leaf blades lanceolate to obovate or elliptic, 15 – 18 mm long, 9 – 12 mm wide, margin entire, apex obtuse, base rounded, petiole 2.2 – 3.0 cm long, midrib prominent with 16 – 18 (25) cross veins angle ranged 60 – 750 with midrib, and distance from intramarginal vein to lamina margin 0.20–0.25 mm, etc. (figure 3.5). Loc.class. Indonesia: Sumbawa: Kambing. Lectotype: H. Zollinger 3430. 1 2 3 Figure 3.5. Halophila major; 1.a form; 2. leaf, 3.habitat form RUPPIACEAE Horaninov., 1834. Typus: Ruppia L. RUPPIA L., 1753. Type species: Ruppia maritima L. Ruppia maritima Linnaeus, 1753; Phamh., 1993; N.V.Tien, et al., 2002; N. T. Do, 2005; Wang, Q. et al., 2010. _Ruppia maritima subsp. rostellata Aschers. & Graeb.; _Ruppia maritima var. rostrata J. Agardh;_Ruppia rostellata W. D. J. Koch ex Reichenbach., _Buccaferrea cirrhosa Petagna, 1787; _Ruppia cirrhosa Grande, 1918. Descriptions: Thin-long rhizome, up to 150 cm, sheath 2 – 10 mm long, Leaves linear 6 – 10 cm long and 0.5 – 0.8 mm wide with acute tips, and a single nerve, leaf sheath not transparent 1.2 cm long. Inflorescence with two hermaphrodite flowers, peduncle supporting the inflorescence not coiled; each inflorescence had from two to eight mature fruitlets, etc.(figure 3.6). 12 Loc.class.: Habitat in Europae maritimis. Lectotypus: Micheli. 1729. Nov. Pl. Gen. t. 35. (designated by Jacobs & Brock. 1982. Aquatic Bot. 14: 329). 1 2 3 Figure 3.6. Ruppia maritima - 1.a form; 2. mature fruitlets, 3.habitat form ZOSTERACEAE Domortier. 1829. Anal. Fam. Pl. 65, 66; nom. cons. Typus: Zostera L. 1753 ZOSTERA L. 1753. Sp. Ed. 1: 986. Type species: Zostera marina L. Zostera japonica Aschers., Graebn., 1907 ; N.V.Tien, 2002; N. T. Do, 2005; Wang, Q. et al., 2010. _Zostera nana Mertens ex Roth., 1868; _Nanozostera japonica (Ascherson & Graebner) Tomlinson & Posluszny, 2010. Descriptions: Perennial. Rhizomes with internodes 5 - 30 mm long, 0.5 – 1.5 mm in diameter, each node with roots. Leaves 5 - 35 cm long, 1 - 2 mm wide, with 2 - 4 secondary nerves on each side, apex rounded to acute, asymmetrical, with a narrow central slit caused by the degeneration of the apical cells; axillary scales 2, linear-lanceolate. Open sheath 2 - 10 cm long, etc. (figure 3.7). Loc.class.: Japan: Honshu: Miyadzu, fr., October 1901s. Typus: U. Faurie 4889. (HT: P; IT: UC). 13 1 2 3 Figure 3.7. Zostera japonica – 1.a form; 2. leaf tip, 3.habitat form CYMODOCEACEAE N. Taylor. 1909. in A. Amer. Fl. 17: 31. Typus: Cymodocea Konig, 1805. HALODULE Endl. 1841. Gen. Pl. Suppl. 1: 1368. Type species: Diplanthera tridentata Steinheil (=H. uninervis (Forssk.) Archerson). KEY TO THE SPECIES BELONG HALODULE 1a. leaf tip tridentate, with 3 well-developed lateral teeth. Blades leaf 0.8 – 1.4 mm wideHalodule uninervis 1b. leaf tip rounded, more or less serrulate, lateral teeth faintly developed or absent. Blades leaf 0.5 – 0.8 mm wide...Halodule pinifolia Halodule uninervis (Forssk.) Aschers., 1882; Ernani G. Menez, R.C. Phillips, Hil. P. Calumpong, 1983; Phamh., 1993; N.V.Tien, 2002; N. T. Do, 2005; Wang, Q. et al., 2010. _Zostera uninervis Forssk., 1775;_Diplanthera tridentate Steinheil., 1883; _Diplanthera madagascariensis Steud., 1840;_Ruppia sp. Zoll., 1854;_Halodule austrais Miq., 1855;_Halodule tridentate F. v. M., 1882;_Diplanthera uninervis Aschers., 1897; F. N. Williams., 1904; Phamh., 1961. Descriptions: Thin rhizomes 0.5 – 0.8 mm in diameter, internodes 2.5 – 3 cm long. Leaf blades 4 – 11 cm long and 0.8 mm - 1.4 mm wide; apex tridentate with a short central tooth and well-developed lateral teeth. Leaf sheath 2 – 3 cm long, etc. (figure 3.8). 14 Loc.class.: Type: Yemen: near Al Mukha: Mocha. Typus: Forsskål (no material found). 1 2 3 Figure 3.8. Halodule uninervis - 1.a form; 2. leaf tip, 3.habitat form Halodule pinifolia den Hartog, 1964; Ernani G. Menez, R.C. Phillips, Hil. P. Calumpong, 1983; Phamh., 1993; N.V.Tien, 2002; N. T. Do, 2005; Wang, Q. et al., 2010. _Diplanthera pinifolia Miki. 1932. Bot. Mag. Tokyo 46: 787. Descriptions: Thin rhizomes up to 1 mm in diameter, internodes 1 – 3 cm long. Leaf blades 2 - 8 cm long and 0.5 mm - 0.8 mm wide; apex rounded with minute serrations and two poorly developed to non-existing lateral teeth. Leaf sheath 1 – 1.5 cm long, etc. (figure 3.9). Loc.class.: China: Taiwan: Takao, 16 Dec 1925. Typus: S. Miki s.n. 1 2 3 Figure 3.9. Halodule pinifolia; 1.a form; 2. leaf tip, 3.habitat form 3.1.3. Variation of seagrass composition 3.1.3.1. Tam Giang - Cau Hai lagoons There are 6 species belonging to 4 genera, 4 families. Zostera japonica is the dominant species. The supplement ofHalodule uninervis increasesthe number of seagrass species here from 6 to 7, excluding Halophila minor. 3.1.3.2. Thi Nai lagoon 15 There are 7 species belonging to 5 genera, 4 families. In the period of 2008- 2009, Thalassia hemprichiiwas not recorded, Halodule pinifolia is the dominant species. 3.1.3.3. Nai lagoon There are 6 species belonging to 5 genera, 3 families. Enhalus acoroides is the dominant species. The supplement the Halophila major increases seagrass species composition here from 5 to 6. * Halophila beccarii, which is is in the "Red list - Red list" (IUCN, 2010) appearstotally in three lagoons, most of which are Tam Giang - Cau Hai lagoon. 3.2. Distribution characteristics of seagrass 3.2.1. Tam Giang - Cau Hai lagoons The area of seagrass distribution in the period of 1996 - 2010 tended to decrease sharply, was 2,200 ha in 1996 (Nguyen Van Tien, 2004), 1,200 ha in 2003 (IMOLA, 2007) and 1,000 ha in 2010 (Cao Van Luong, 2010). In present, this area has increased significantly, up to 2,037 ha (figure 3.11). Figure 3.11. Distribution characteristics of seagrass in Tam Giang – Cau Hai lagoons 3.2.2. Thi Nai lagoon Through statistics and satellite analysis of this study, the total area of seagrass in Thi Nai lagoon is 180 ha. Meanwhile, according to N.H. Dai (1999), N.V.Tien (2008), N.X.Hoa (2011), the area of seagrass here in 2010 was in range of 200 - 215 ha (figure 3.12). 3.2.3. Nai lagoon 16 Through statistics and satellite analysis of this study, the total area of seagrass in Nai lagoon was estimated at 90 ha. There are 02 areas which have hight density of seagrass, approximately tens of hectares located in the southwest area of Tri Thuy bridge and in the ponds at Dong Khanh bridge. According to Trong Nho (1994) and Dang Ngoc Thanh (2003), from 2000s onward, there was about 60 ha of area of seagrass (figure 3.13). Figure 3.12. Maps of seagrass distribution in Thi Nai lagoon Figure 3.13. Maps of seagrass distribution in Nai lagoon 3.3. Coverage and density of shoots 3.3.1. Tam Giang - Cau Hai lagoons The Zostera japonica is the dominant species and has the highest density of shoots and coverage at 9,905 ± 550 shoots/m2, 75%; Halodule pinifolia with 6,010 ± 722 shoots/m2, 50% and lowest is Ruppia maritima with 1,112 ± 309 shoots/m2. In comparing of density of shoots from 2009 throught 2017, there was in different species. In 2009, the density of shoots Zostera japonica averaged 8,550 shoots/m2, but it was 9,905 ± 550 shoots/m2 by 2016, increased by 1.15 times (Nguyen Van Tien, 2013). 17 3.3.2. Thi Nai lagoon Seagrass in Thi Nai is mainly distributed on sandy and muddy substrate along shallow coastal waters in aquaculture ponds and on floating dunes such as in the southwest of Thi Nai bridge, Ha Thanh estuary with 25 - 90% coverage. The Zostera japonica is the dominant species and the coverage is inthe range of 31 - 75%, reaching 3,051 ± 907 shoots/m2, the Halodule pinifoliais 20 - 60% coverage, reaching 350 – 1,500 shoots/m2. Halophila ovalis and Halodule uninervisare sparsely distributed, the Halophila beccarii is recorded in the rainy season only. 3.3.3. Nai lagoon The coverage of seagrass from 50% to 80%, some transects up to 100%. The lowest coverage was in the Enhalus acoroides in the Tri Thuy (25%). The average coverage at the sections is about 65%, with 150 ± 5 shoots/m2. 3.4. The quantitative characteristics of seagrasses 3.4.1. Tam Giang - Cau Hai lagoons Quantitative indicators in Zostera japonica varies strongly according to spatio-temporal distribution. The results showed that the dry season is suitable for seagrass growth and development. The average density, length and biomass reached 9,905 ± 550 shoots/m2, 20,71 ± 2,15 cm and 1,779.1 ± 305,5 g.dry/m2 respectively. The dry season is suitable for Halodule pinifolia growth and development. The average density, length and biomass reached 6,010 ± 722 shoots/m2, 12.02 ± 1.5 cm, 831.3 ± 155.3 g.dry/m2 respectively. The temporaldistribution does not affect the growth and development of Halophila ovalis.The average density, length and biomass reached 3,407 ± 843 shoots/m2, 3.48 ± 0.2 cm, 256.6 ± 34.7 g.dry/m2 respectively. The average density, length and biomass of Halophila beccarri reached 5.,25 ± 434 shoots/m2, 3.34 ± 0.1 cm, 206.6 ± 17.6 g.dry/m2 respectively. The biomass is higher than 57.7 g.dry/m2 in Nguyen Van Tien (2006). For the first time, samples of the Halodule uninervis were collected and initial quantitative analysis was conducted. The average density, length and biomass reached 1,200 ± 125 shoots/m2, 12.4 ± 1.5 cm and 294.05 ± 27.8 g.dry/m2 respectively 18 Figure 3.15. The quantitative correlation of Zostera japonica Tam Giang – Cau Hai lagoons Figure 3.19. The quantitative correlation of Zostera japonica Thi Nai lagoon Evaluation of the correlation of the indicators showed that the shoot density was a factor that strongly affected biomass rather than the length with R2 = 0.87 (r = 0.92) and R2 = 0.55 (r = 0.74) (figure 3.15). At the same time, the ratio of SKT/SKD also shows that, in the dry season (1.92), seagrass grows better than the rainy season (1.07). 3.4.2. Thi Nai Lagoon Quantitative indicators of seagrass in Thi Nai lagoon tend to decrease when compared to previous studies, such as N.V.Tien (2004, 2002, 2006, 2008). There is a significant change in biomass and density cause of season, the biomass of Zostera japonica is higher in the dry season. The average density, length and biomass reached 3,051 ± 907 shoots/m2, 22.87 ± 1.5 cm and 228.03 ± 32.69 g/m2 respectively. The average density, length and biomass of Halodule pinifolia reached 907 ± 322 shoots/m2, 9.10 ± 1.15 cm and 81.42 ± 18.56 g.dry/m2 respectively. The average density, length and biomass of Halophila ovalis reached 505 ± 32 shoots/m2, 3.12 ± 0.07 cm and 141.21 ± 7.80 g.dry/m2 respectively. The average density, length and biomass of Halophila beccarri reached 156 ± 11 shoots/m2, 3.40 ± 0.25 cm và 23.57 ± 1.52 g.dry/m2 respectively. The average density, length and biomass of Ruppia maritima reached 964 ± 67 shoots/m2, 42.37 ± 12.1 cm and 812.33 ± 21.95 g.dry/m2 respectively. 19 The average density, length and biomass of Halophila uninervis reached 925 ± 33 shoots/m2, 9.7 ± 0.8 cm and 393.0 ± 26.7 g.dry/m2 respectively. Thalassia hemprichii is distributed in a narrow range in the lagoon, where salinity is high and stable. It has lowest shoots density but the biomass is quite hight (86 ± 11 shoots/m2 with 156.06 ± 48.17 g.dry /m2). The value of coefficient R2 = 0.69 (r = 0.83) showed the strong correlation between density and biomass in Zostera japonica in Thi Nai lagoon (figure 3.19). 3.4.3. Nai lagoon The average density, length and biomass of Enhalus acoroides reached 150 ± 5 shoots/m2, 84.23 ± 9.85 cm and 2,791.4 ± 145.1 g.dry/m2 respectively (higher than most other areas such as Cu Mong - Phu Yen, Ba Thin - Cam Ranh, Kien Giang), more 2.4 times then the biomass of Enhalus acoroides on the world (464.4 g.dry/m2), more 1.5 times than the biomass of E. acoroides in Papua New Guinea with 773.6 g.dry/m2 (figure 3.20) (Nguyen Van Tien, 2006; Nguyen Huu Dai, 2002; Duarte C. M, 1999; Brouns JE M, 1997). There is a close correlation (r = 0.75) between the density of shoot and biomass in the Enhalus acoroides (figure 3.21). Figure 3.20. Comparative graph of biomass of Enhalus acoroides Hình 3.21. The quantitative correlation of Enhalus acoroides in Nai lagoon The average density, length and biomass of Thalassia hemprichii reached 112 ± 17 shoots/m2, 18.22 ± 2.2 cm and 353.7 ± 48.7 g.dry/m2 respectively. The average density, length and biomass of Halophila ovalis reached 1,116 ± 336 shoots/m2, 2.52 ± 0.18 cm, 116.1 ± 34.4 g.dry/m2 respectively. This result is higher than some studies in other areas in Vietnam (Nguyen Van Tien, 2006). 20 The average density, length and biomass of Halodule pinifolia reached 525 ± 35 shoots/m2, 17.75 ± 3.98 cm and 125.7 ± 2.4 g.dry/m2 respectively. The average density, length and biomass of Halophila major reached 975 ± 113 shoots/m2, 3.21 ± 0.13 cm, and 175.3 ± 47.5 g.dry/m2 respectively. The average density, length and biomass of Ruppia maritima reached 557 ± 12 shoots/m2, 58.32 ± 12.55 cm and 765.2 ± 128.1 g.dry/m2 respectively. 3.4.3. The ratio of above and below biomass Research on ratio of above ground and below ground biomass (SKT/SKT) is a factor to assess the health and growth direction of seagrass (Dumbauld, 2003). In Tam Giang - Cau Hai lagoon, the ratio of SKT/SKD in all species is 1.01, the highest in Zostera japonica with 1.5; the lowest in the Halodule uninervis with 0.6 (figure 3.23). In Thi Nai lagoon, the ratio of SKT/SKD of all species is 1.46, Zostera japonica is at an average of 1.48 (figure 3.24). The lowest in Halophila beccarii with 0.77, highest in Ruppia maritima with 1.95 showing a sweetening in the place where they are distributed. Figure 3.23. Ratio of SKT/SKD of seagrasses in Tam Giang – Cau Hai Figure 3.24. Ratio of SKT/SKD of seagrasses in Thi Nai Figure 3.25. Ratio of SKT/SKD of seagrasses in Nai In Nai lagoon, the SKT/SKD ratio of seagrasses is 0.84; Enhalus acoroides is 0.75; Thalassia hemprichii is 1.0 and lowest in the Ruppia maritima with 0.48 (figure 3.25). The results show the development of underground of seagrasses in Nai lagoon. 3.5. Cacbon storage in the seagrass 3.5.1. Biomass of seagrass With the total area of sea grass covered (100%) is 1,276.3 ha, which converted from 2,307 hectares of mixed seagrass. The reserve of seagrass 21 is estimated in 03 lagoons in turn reached: Tam Giang - Cau Hai lagoon is 10,153.7 tons.dry, in Thi Nai lagoon is 132.1 tons.dry và Nai lagoon is 281.1 tons.dry. If using the default conversion factor of 0.47 of IPCC (2006) to convert into Corg from biomass, the calculation result is 4,966.4 tons.Corg, respectively, equivalent to 18,226.7 tons.CO2 3.5.2. Organic carbon content in seagrass The results of organic carbon content (%OC) in seagrass in 3 lagoons are summarized in table 3.16: Table 3.2. Stock of organic carbon in seagrass and value evaluation Site Area (ha) Specices Biomass (g.dry/m2) Carbon content (%OC) Stock of carbon (tons) Amount of CO2 converted (tons) Value (USD)* OL3 5 H.p 631.3 ± 31.5 27.4 ± 0.6 0.86 3.2 214 OL4 30 R.m 1,963.8 ± 18.0 22.3 ± 1.5 13.14 48.2 3,229 OL5 15 Z.j 1,779.1 ± 305.5 32.7 ± 4.4 9,017.3 33,093.5 2,217,265 CT 70 Z.j DS1 5 Z.j DS2 10 Zj TG4 1.450 Z.j TG5 45 H.u 294.1 ± 27.8 26.8 ± 2.1 35.4 129.9 8,703 TG5 17 H.p 831.3 ± 155.3 35.8 ± 1.2 50.6 185.7 12,442 CH1 61 H.u 294.1 ± 27.8 29.1 ± 2.2 52.2 191.6 12,837 CH2 105 Z.j 1,779.1 ± 305.5 39.4 ± 0.9 735.9 2,700.8 180,954 CH3 37 H.b 206.6 ± 17.6 21.7 ± 0.5 16.6 60.9 4.080 CH4 187 H.o 256.6 ± 34.7 30.6 ± 1.7 146.8 538.8 36,100 Total 1 10,068.8 36,952.5 2,475,824 TN3 5 H.p 77.6 ± 5.04 28.5 ± 1.8 1.25 4.6 308 22 TN4 20 Z.j 162.8 ± 46.1 38.1 ± 2.1 12.41 45.6 3,055 TN5 4 H.u 64.2 ± 6.3 28.1 ± 2.5 0.72 2.6 174 TN6 30 H.p 130.9 ± 7.4 37.7 ± 0.4 14.82 54.4 3,645 TN7 13 H.p 59.1 ± 10.1 40.6 ± 5.9 3.12 11.4 764 TN10 2 H.p 48.2 ± 7.2 27.5 ± 1.7 0.26 1.0 67 TN11 2 H.b 55.1 ± 13.1 26.6 ± 2.3 0.29 1.1 74 TN14 50 Z.j 256.7 ± 14.3 41.2 ±