Summary of Biology doctoral thesis: The study on seagrass communities and carbon storage capacity of them in some typical coastal lagoons in the central of Vietnam

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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.

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Nội dung Text: Summary of Biology doctoral thesis: The study on seagrass communities and carbon storage capacity of them in some typical coastal lagoons in the central of Vietnam

MINISTRY OF EDUCATION AND VIETNAM ACADEMY TRAINING OF SCIENCE AND TECHNOLOGY GRADUATE UNIVERSITY SCIENCE AND TECHNOLOGY ----  ---- 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.
1 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.
2 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
3 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).
4 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.
5 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
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 Distribution of species composition STT Taxon 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 Thalassia hemprichii (Ehrenb. ex Solms) 2 + + + + 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 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).
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 Figure 3.13. Maps of seagrass distribution distribution in Thi Nai lagoon 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