MINISTRY OF EDUCATION AND TRAINING

VIETNAM ACADEMY OF SCIENCE AND TECHNOLOGY

GRADUATE UNIVERSITY SCIENCE AND TECHNOLOGY

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CAO VAN LUONG

THE STUDY ON SEAGRASS COMMUNITIES

AND CARBON STORAGE CAPACITY OF THEM

IN SOME TYPICAL COASTAL LAGOONS

IN THE CENTRAL OF VIETNAM

Major: Botany

Code: 9420111

SUMMARY OF BIOLOGY DOCTORAL THESIS

Hanoi, 2019

The thesis is completed at: Graduate University Science and Technology - Vietnam Academy of Science and Technology

Suppervisors : Prof. Dr. Dam Duc Tien

Dr. Tran Thi Phuong Anh

Reviewer 1: …

Reviewer 2: …

Reviewer 3: ….

The thesis shall be defended in front of the Thesis Committee at Academy Level at Graduate University Science and Technology - Vietnam

Academy of Science and Technology at…………………………… 2019

The thesis can be found at:

- the Library of Graduate University Science and Technology

- the National Library.

LIST OF PUBLISHED OF THE AUTHOR

RELATING TO THE THESIS

1. Cao Văn Lương, Đàm Đức Tiến, Nguyễn Thị Nga, 2018. Quần xã thực vật thủy sinh đầm phá Tam Giang – Cầu Hai. Tạp chí Khoa học và Công nghệ Biển (đã phản biện, đang chờ bản in).

2. Cao Văn Lương, Nguyễn Thị Nga, 2017. Bước đầu đánh giá khả năng lưu trữ cacbon của cỏ biển qua sinh khối tại đầm Thị Nại, tỉnh Bình Định. Tạp chí Khoa học và Công nghệ Biển; Tập 17, Số 1; 2017: 63-71.

3. Nguyễn Thị Thu, Cao Văn Lương, Đỗ Văn Mười, 2014. Phục hồi hệ sinh thái thảm cỏ biển bằng mô hình sắp xếp lại hệ thống nò sáo tại đầm phá Tam Giang – Cầu Hai. Tuyển tập Hội nghị Khoa học toàn quốc về Sinh học biển và Phát triển bền vững lần thứ 2; tiểu ban Đa dạng sinh học và Bảo tồn biển. Nhà xuất bản Khoa học tự nhiên và Công nghệ, tr. 227 – 231.

4. Cao Văn Lương, Đàm Đức Tiến, Nguyễn Đức Thế, Nguyễn Văn Quân, 2014. Thành phần loài và phân bố cỏ biển tại đầm Nại – Ninh Thuận. Tuyển tập Hội nghị Khoa học toàn quốc về Sinh học biển và Phát triển bền vững lần thứ 2; tiểu ban Đa dạng sinh học và Bảo tồn biển. Nhà xuất bản Khoa học tự nhiên và Công nghệ, tr. 131 – 137.

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INTRODUCTION

1. The necessery of the thesis Seagrasses are flowering plants and living under marine water.

Although, species diversity of seagrass is not so high, but the ecosystem functions of seagrass are equally important when comparing to coral reefs and mangroves (Nguyen Van Tien et al., 2004).

Recently, seagrass has been recognized for their carbon storage capacity and estimated about 19.9 Pg (1 petagram = 1015 grams) of organic carbon over the world, this capacity is higher 3 times than tropical forest (Fourqurean et al., 2012).

However, there are few specific studies on seagrass community characteristics, the relationship between them with other inorganic and organic factors, and in particular, there has been no research on the carbon storage capacity of seagrass in Vietnam. From the above reasons, I take the thesis with title: “The study on carbon storage capacity and communitiescharacteristicsof seagrass beds in some typical coastal lagoons in the central of Vietnam”. 2. Objectives

This study aims to provide basic knowledge of carbon storage capacity and communities characteristics of seagrass beds in some typical coastal lagoons in the central of Vietnam. These important knowledges will be applied to activities of conservation, caculation of the carbon market, and reducing CO2 emissions in the future in Vietnam.

The specific objectives: 1. To assess the community structure of seagrass beds in the Tam Giang - Cau Hai lagoon (Thua Thien – Hue province), Thi Nai lagoon (Binh Dinh province) and Nai lagoon (Ninh Thuan province); 2. To determine the biomass reserves of seagrass beds in the study areas; 3. To investigate the ability of seagrass beds on the storage organic carbon (Corg) and the accumulation of CO2.

3. The main research contents

3.1. Study on the seagrass communities: species composition and morphological, distribution, coverage areas. Establishment of the key identifying of seagrasses in study areas.

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3.2. Investigation and identification of quantitative characteristics of seagrass species in the study area.

3.3. Determination of content (%OC) and Corg of seagrass species;

3.4. Evaluating carbon storage capacity of seagrass communities.

4. Structure of the thesis

The thesis consists of 129 pages, 2 pages in introduction, 28 pages in overeview, 15 page in materials and methods, 69 pages of results and discussion, 2 pages of conclusions and recommendations, 1 pages of published , 12 pages in reference. The thesis has 16 tables, 35 figures and charts, 137 references.

CHAPTER I. OVERVIEW

1.1. An overview of seagrasses

Seagrasses belonging to Anthophyta, Monocotyledoneae, Hydrocharitales. A monocotyledon has two forms of reproductive: dioecious and monoecious. Populations are maintained and developed primarily by the development of shoots. Morphological characteristics of seagrasses were described and compared by C. Den Hartog (1970) and C. Phillips and G. Menez (1988).

1.2. Seagrass beds in the world

At present, there are 66 species of seagrass distributed over 600,000 km2 in the world (den Hartog, 2006). The number of species is not much, but the seagrass bed is one of the most important marine ecosystems. Moreover, the seagrass bed is an essential part of the solution to climate change and can store twice as much carbon as temperate and tropical forests (James Fourqurean (2012).

1.3. Previous studies in Vietnam

In Vietnam before 1995, seagrass was studied a little compared to other marine fauna and flora groups. Pham Hoang Ho recorded the findings of seagrass in Qui Nhon (1960) and in Phu Quoc (1985). Since 1996 the study on seagrass has been officially promoted. The most prominent are the studies of Nguyen Huu Dai (1999, 2002), Nguyen Van Tien (1999, 2000, 2003, 2006, 2008, 2013). From these studies, we have a basic

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understanding of species composition and distribution of Vietnamese seagrass.

Nguyen Xuan Vy (2013) has analyzed DNA and published the species Halophila major (Zoll.) Miquel, which was a new species for Vietnam, thereby supplementing the composition of seagrasses in Vietnam from 14 species up to 15 species. Cao Van Luong (2013) through research and application of GIS tools to synthesize and statistic that there are about 18,630 ha of seagrass distributed along the coast of Vietnam.

Studies on the storage of carbon in marine plants (mangroves, seaweeds and seagrasses) have recently been just at the level of application of IPCC (2006) indices or constant for calculations.

1.4. Some concepts on the lagoon

1.4.1. Lagoon

A lagoon is a part of the sea that is separated from the sea by a form of external barriers (such as sand islands, sand bars, coral reefs, etc.), possibly a freshwater lake separated from a larger lake or a river, can also be an estuary, a river branch in a door or a swamp, etc. sea water is flowing in (Tran Duc Thanh et al, 2010).

1.4.2. Coastal lagoon

The coastal lagoon is a type of brackish, saline or super-saline coastal water body, blocked by a sand dike and connected to the outer sea (Tran Duc Thanh et al, 2010). In Vietnam, coastal lagoons are called "đầm" or "phá" depending on the historical and customary local names.

* In summary, in the world as well as in Vietnam seagrass has been studied a lot, although the number of species is limited and the area of distribution is small, but the seagrass has important meaning and functions for the environment. However, research on seagrass community characteristics in coastal lagoons in general, especially in coastal lagoons in central Vietnam was a few. Therefore, my research address these issues.

CHAPTER 2. MATERIAL AND METHODS

2.1. Research subject and scope Study areas were seagrasses beds in the Tam Giang - Cau Hai lagoon

(Thua Thien Hue Province), the Thi Nai lagoon (Binh Dinh Province), and the Nai lagoon (NinhThuan Province).

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Research period: 4 years, from 2014 to 2018 2.2. Research materials

To carry out this study, we used data and materials inherited from some projects where the author was the main participant directly collecting and analyzing the samples. A total of 504 samples were collected, including 378 quantitative samples and 126 qualitative samples. Samples were preserved at the Department of Ecology and Marine Plant Resources, Institute of Marine Environment and Resources - Vietnam Academy of Science and Technology. 2.3. Research methods 2.3.1. The methodology of field survey

Investigating and collecting specimens according to the " Phương pháp nghiên cứu cỏ biển" by Nguyen Van Tien and Tu T.L. Huong in 2008; “Survey manual for tropical marine resources” by English. et. al., in 1997; “SeagrassNet - Manual for Scientific Monitoring of seagrass habitat” by Short. et. al., in 2002; “Methods of studying plant communities” by Hoang Chung in 2008, and “Aquatic biology basic” by Dang Ngoc Thanh and Ho Thanh Hai in 2007. 2.3.2. Qualitative analysis

The comparative morphological method was used follow the method of Nguyen Van Tien et. al. in 2002: “Cỏ biển Việt Nam”;“Seagrass taxonomy and identification key” by Kuo, J., and den Hartog, C., in 2001; “Taxonomy and Biogeography of Seagrasses” by Den Hartog, C., và Kuo, J., in 2006; “The seagrass of the world” by Den Hartog C., in 1970; and “Seagrasses” by Phillips R.C. and Menez E.G., in 1988;

Analysis, classification and data processing was performed in the laboratory of the Institute of Marine Environment and Resources - Vietnam Academy of Science and Technology.

Establishment of the identification key for detected seagrass species in the study areas. Order and names of taxons are arranged according the “International Code of Botanical Nomenclature (Vienna Code)” in 2006. Some was according the “Cây cỏ Việt Nam” by Pham Hoang Ho in 2000.

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2.3.3. Quantitative analysis Quantitative analysis method according ”Global Seagrass Research

Methods” by F. T. Short, & R. G. Coles in 2001; ”Phương pháp nghiên cứu Cỏ biển” by Nguyen Van Tien and Tu T.L. Huong in 2008. 2.3.4. Analysis of carbon contents

+ Analysis of organic carbon content in seagrass according to two methods:

- Analysis of carbon content according to TCVN 8726: 2012. - Analysis by Michael Ensminger (2011). The principle of the method

used the SHIMADZU TOC-CVS auto-analyzer module + Calculation of the organic carbon stock according to the formula:

C = m x %OC x S (2) - From the amount of CO2, the carbon dioxide content could be

calculated and the carbon dioxide content can be determined. The CO2 content is calculated by the following formula:

MCO2 = C * 3.67 (tCO2/ha) (3) - Determine the commercial value of C based on the formula:

T (USD) = CO2 (tonnes / ha) x price (USD / tonne carbon credits) (4) In this case, we are using the forecasted carbon credit price by 2030 of

Societe Generale of 60 Euro 2.4.5. Design of distribution map of seagrass

The available satellite imagery data was 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

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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 +++ +++ ++ ++

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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

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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.

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_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).

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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).

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Loc.class.: Indonesia: Borneo: Sarawak, near mouth of Bintula River. Typus: Beccari 3666 (IT: S).

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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).

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Figure 3.4. Halophila ovalis; 1.a form; 2. leaf, 3.habitat form

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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.

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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).

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Loc.class.: Habitat in Europae maritimis. Lectotypus: Micheli. 1729. Nov. Pl. Gen. t. 35. (designated by Jacobs & Brock. 1982. Aquatic Bot. 14: 329).

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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).

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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 wide…………………………………………Halodule 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).

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Loc.class.: Type: Yemen: near Al Mukha: Mocha. Typus: Forsskål (no material found).

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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.

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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

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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

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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).

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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

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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.

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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).

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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

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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

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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 ± 2.6 52.92 194.2 13,011

TN17 48 Z.j 264.5 ± 34.1 38.3 ± 1.1 48.6 178.4 11,953

TN18 5 H.o 97.4 ± 5.7 29.9 ± 0.9 1.46 5.4 362

TN19 1 Th.h 206.6 ± 48.2 40.6± 0.4 0.84 3.1 208

total 2 136,7 501,7 33,621

TT1 15 E.a 2,791.4 ± 145.1 44.7 ± 2.7 187.1 686.7 46,009

TT2 2 H.o 116.1 ± 34.4 26.7 ± 1.9 0.62 2.3 154

DN 38 H.p 125.7 ± 2.4 33.1 ± 2.0 15.8 58.0 3,886

DK 5 Th.h 353.7 ± 848.7 40.8 ± 5.2 7.2 26.4 1,769

Total 3 210.7 773.3 51,818

Total 1 + 2 + 3 10,416 38,228 2,561,263

Note: E.a – Enhalus acoroides, Th.h – Thalassia hemprichii, H.p – Halodule pinifolia, H.u – Halodule uninervis, H.o – Halophila ovalis, H.m – Halophila beccarii, H.mj – Halophila major, R.m – Ruppia maritima, Z.j – Zostera japonica, OL – O Lau, CT – Con Te, DS – Dam Sam, TG – Tam Giang, CH – Cau Hai, TN – Thi Nai, TT – Tri Thuy, DN – Dong Nha, DK – Du Khanh, (*) data has made decrease decimal.

- In Tam Giang - Cau Hai lagoon: %OC in seagrasses reaches from 21.7 ± 0.5% to 39.4 ± 0.9%, the lowest in Halophila beccarii and highest in Zostera japonica. The average %OC in seagrasses is 26.55 ± 2.3% (table 3.16).

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- In Thi Nai lagoon: %OC in seagrasses reaches from 26.63 ± 2.32% to 40.64 ± 0.45%, the lowest in Halophila beccarii and highest in Thalassia hemprchii. The average %OC is 34.30 ± 1.82% (table 3.16, figure 3.26).

- In Nai lagoon: %OC in seagrasses reaches from 26.7 ± 1.9% to 44.7 ± 2.7%, the lowest in Halophila ovalis and the highest in Enhalus acoroides. The average %OC is 36.32 ± 4.1% (table 3.16)

- The average % OC in seagrasses in the three lagoons is 32.8 ± 1.3%, showing that the default conversion factor of 0.47 (47%) of IPCC (2006) is used to estimate reserves organic carbon (Corg) in seagrass is not yet suitable.

There is correlation (R2 = 0.51, r = 0.71) between biomass and %OC (figure 3.27), however, there is almost no correlation between shoots density and %OC (R2 = 0,06) (figure 3.28).

On the basis of the determination of organic carbon content (%OC) of each species, biomass (g.dry/ m2) and distribution areas (ha). The results showed:

- In Tam Giang - Cau Hai lagoon: a total of Corg in seagrasses is 10,068.8 tons, equivalent to 36,952.5 tons.CO2. The Zostera japonica has the highest in Corg (5.9 tons.Corg/ha, equivalent to 21.6 tons.CO2/ ha). On average of seagrasses is 4.9 tons.Corg/ ha (table 3.16).

28,08

33,6

26,63

39,21

29,98

40,64

0

5

10

15

20

25

30

35

40

45

H. uninervis H. pinifolia H. beccarii Z. japonica H. ovalis T hemprichii

y = 0,0533x + 27,358

R2 = 0,512

0

20

40

60

0 50 100 150 200 250 300

Sinh khối (g.khô/m2)

%C

Figure 3.26. %OC in seagrasses

in Thi Nai Figure 3.27. Correlation between

biomass and %OC of seagrass in Thi Nai

y = 0,0011x + 32,469

R2 = 0,0688

0

20

40

60

0 500 1000 1500 2000 2500 3000 3500 4000 4500 5000

Mật độ (chồi/m2)

C%

Figure 3.28. Correlation between shoots

and %OC of seagrass in Thi Nai Figure 3.29. Charts of carbon vulue

(tons.Corg/ha)

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- In Thi Nai lagoon: a total of Corg in seagrasses is 136.7 tons, equivalent to 501 tons.CO2. In the Zostera japonica is the highest, with 0.89 tons.Corg/ha (equivalent to 3.3 tons.CO2/ha). On average of seagrasses is 0.76 tons/ha (table 3.16).

- In Nai lagoon: a total of Corg in seagrasses is 210.7 tons, equivalent to 773.3 tons.CO2. The Enhalus acoroides is the highest, with 2.47 tons.Corg/ha (equivalent to 45.7 tons.CO2/ha). On average of seagrasses is 2.34 tons.Corg/ha.

- The above results are very low compared to the world average, from 331.6 tons.Corg/ha (Fourqurean, 2012) to 372.4 ± 74.5 tons.Corg/ha (Kennedy H., 2010) (figure 3.29).

4.3.3. Evaluation of carbon storage capacity

The price of carbon credits depends on the exchange market, such as the EEX market, the BLUENEXT market and the EUAs market. Accordingly, the current price of carbon credits only ranges from 4 to 6 Euro (Figure 3.30 - Figure 3.32). However, the construction of carbon credit price also depends on the views of each country (Figure 3.33).

Figure 3.30. price of carbon – EEX

estimate Figure 3.31. price of carbon – EUAs

estimate

Figure 3.32. price of carbon –

BLUENEXT estimate Figure 3.33. the price of carbon of each

country

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According to a study by Societe Generale, the average price of carbon credits worldwide in 2020 in the EUAs market is 45 Euro/ton, 2030 will be 60 Euro/ton and 2050 will be 250 Euro/ton (figure 3.34). In Australia, the government imposed a carbon tax throughout the territory of 23 AUD/ton from 2012 (figure 3.35).

Figure 3.34. forecast of carbon credit

price in 2050 Figure 3.35. the price of carbon of the

Australian

In this study, we use the forecasted carbon credit price by 2030 of Societe Generale of 60 Euro (equivalent to 67 USD). The results showed that the value depends on each species of seagrass and their carbon storage capacity (table 3.16). In Enhalus acoroides in Nai lagoon is about 3,067 USD/ha, in Halophila ovalis in Thi Nai lagoon is 72 USD/ha (table 3.16). The seagrass beds in Tam Giang - Cau Hai lagoon have the highest estimated value, cause Zostera japonica has a very large area (1,655 ha) and relatively high in organic carbon content (32.7 - 39.4%), then the estimated value when converting based on the carbon credits they bring is about 2.2 million USD.

The total value from the carbon storage capacity of seagrass in the three lagoons in this study achieved 2,561,263 USD, equivalent to about 59 billion VND.

Within the scope of this study, only the amount of organic carbon in seagrass biomass could be assessed, the amount of organic carbon which is captured in the sediment of the seagrass beds has not been estimated. However, it can be seen that the equivalence and sale the carbon credits which are created from each hectar of seagrass will bring income to the community in laloons: 20.8 million VND/ha, 3.2 million VND/ha and 9.8 million VND/ha; the average is 19 million VND/ha. In comparision to direct use values and other indirect use values that seagrass ecosystems, their carbon absorption and storage values are very high. It not only is an appreciable income for coastal community but also

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contributes to improving the role of seagrass compared to other ecosystems (coral ecosystems, mangrove ecosystems).

CONCLUSIONS AND RECOMMENDATIONS

Concluded

1. In the three research lagoons, we have identified and built the identification keys according to the latest nomenclature for 09 seagrass belonging to 04 families, 06 genera. Includes: Enhalus acoroides (L.f) Royle, Halophila beccarii Ascherson, Halophila ovalis (R. Br.) Hooker f., Halophila major (Zoll.) Miquel, Thalassia hemprichii (Ehrenb. ex Solms) Asch., Ruppia maritima Linnaeus, Zostera japonica Ascherson & Graebner, Halodule pinifolia (Miki) den Hartog, Halodule uninervis (Forssk.) Ascherson.

2. The number of species in different lagoons is not significantly different: Tam Giang - Cau Hai lagoon has 6 species, Zostera japonica is dominant species; Thi Nai lagoon has 7 species, Halodule pinifilia is dominant species; and Nai lagoon has 6 species, with Enhalus acoroides dominant species.

3. The distribution area and coverage of seagrass tended to be lower than before, except for Tam Giang - Cau Hai lagoon. The total distribution area of seagrass in three lagoons is 2,307 ha, mainly distributed in the intertidal zone of the lagoons where the depth is from 0.5 to 2 m. In Tam Giang - Cau Hai lagoon: distributed in 15 areas with 2,037 ha, coverage is 58.3%; in Thi Nai lagoon: distributed in 11 areas with 180 ha, coverage of 28.1%; in Nai lagoon: distributed in 4 areas with 90 ha, coverage of 43.3%.

4. Biomass of seagrass ranged from 23.6 ± 1.5 g.dry/m2 (in Halophila beccarii) to 2,791.4 ± 145.1 g.dry/m2 (in Enhalus acoroides), average is 604.9 ± 174.7 g.dry/m2. The ratio of above and below biomass from 0.48 to 1.95, an average of 1.1 has indicates a good growth trend in dry and poor seasons in the rainy season.

5. Total biomass of seagrass in lagoons are 10,566.9 tons. In particular, biomass of seagrass in Tam Giang - Cau Hai lagoon account for a large part with 10,153.7 tons.dry, followed by Nai lagoon with 281.1 tons.dry and Thi Nai lagoon is 132.1 tons.dry.

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6. The average organic carbon content of seagrass in Tam Giang - Cau Hai lagoon is 26.55 ± 2.3%; in Thi Nai lagoon is 34.30 ± 1.82%, and in Nai lagoon is 36.32 ± 4.1%. The total amount of organic carbon in the existing seagrass in the three lagoons is 10,416 tons.Corg (equivalent to 38,228 tons.CO2). In particular, Tam Giang - Cau Hai lagoon has 10,056.6 tons.Corg (equivalent to 34,192.4 tons.CO2); in Thi Nai lagoon is 136.7 tons.Corg (equivalent to 501 tons.CO2); and in Nai lagoon is 210.7 tons.Corg (equivalent to 716.5 tons.CO2).

7. Value of the ability to absorb CO2 of seagrass in 03 lagoons when using the credit price in 2030 to convert equivalent to more than 59 billion VNĐ.

Recommendations

1. We have to expanding the scope of investigation to study the characteristics of seagrass communities and their carbon storage capacity in coastal areas, coastal lakes and islands across the entire territory of Vietnam.

2. According to studies, the amount of organic carbon stored by seagrass is 2 - 3 times higher than terrestrial forest, mostly storaged in the underground. Meanwhile, seagrass in Vietnam with different species composition, biomass, distribution and habitat. Have to fully assessed the organic carbon stocks in seagrass, in addition to determining the right roles and functions of seagrass, it also aims to establish a solid and rigorous scientific basis. Thereby, helping regulatory agencies plan appropriate policies for the exploitation and conservation of seagrass, there is a need for specific studies of the amount of retained carbon stored in sediments beneath the carpets seagrass beds.

3. Using the various methods in studying the carbon storage capacity of seagrass, which requires investment in automatic TOC analyzers, with high accuracy and reliability.

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NEW SCIENTIFIC CONTRIBUTIONS OF THE THESIS

- This is the first of scientific study, comprehensive investigation of the status of species composition and distribution characteristics of seagrass communities, their biomass, etc. at 03 study areas in the central of Vietnam. Detecting and supplementing Halodule uninervis (Forssk.) Ascherson into seagrass composition in Tam Giang - Cau Hai lagoons (Thua Thien - Hue province) and Halophila major (Zoll.) Miquel into seagrass composition in Nai lagoon (Ninh Thuan province). Build up the classification key according latest nomenclature from Famalies to Species and describing in detail 09 seagrass species.

- For the first time, the determination of organic carbon content in seagrass species, assessment of organic carbon stocks, conversion of carbon credits and assessment of the value of CO2 absorption capacity of seagrass communities in the region research.