Research Report Seagrass bed as a Carbon Sink in Ranong Biosphere Reserve and Trang-Haad Chao Mai Marine National Park; an important role of seagrass. By Anchana Prathep 2012 MAB Young Scientists Award Thailand Submitted to Man Bioshere (MAB) Program, UNESCO Prince of Songkla Unviersity, Thailand
Summary This is a detailed study of the location and the amount of carbon stored in both the healthy (80% coverage) and unhealthy habitats (20% coverage) in the seagrass beds in the Ranong Biosphere Reserve (RBR) and Trang Haad Chao Mai Marine National Park (HCM). We examined the % carbon content of a rich diversity of seagrass species, compared the above and below ground plant parts and the carbon content in the sediments according to the grain sizes > or < 63μm. The results showed that there were differences between the above and below ground tissues, between study sites, among species and between coverages (P<0.05). The healthy seagrass bed had a greater carbon content. The sediment fractions were also different at different study sites and also those associated with different species (P<0.05). Most sediments were composed of particles greater than 63μm. The % carbon was greater in the sediment with particle size less than 63μm. The total amount of carbon stored in live seagrasses at RBR is estimated to be 6.08 Mg and at HCM, 335.16 Mg. A larger scale estimate of carbon stored in the seagrass ecosystems in Thailand is needed and could help guide the conservation and management of this important but understudied ecosystem. Introduction The increase of CO 2 from anthropomorphic carbon gas emissions is known to cause climate change, which greatly influences all aspects of life. Increase in temperature, acidification in the oceans and unpredictable weather are common phenomenon associated with climate change. Thus, understanding the carbon cycle and how much carbon is stored in various ecosystems is very important to help slow down climate change or global warming. Recent studies have pointed out that coastal ecosystems such as seagrasses, mangroves and salt marshes store carbon in a reserve now referred to as Blue carbon (Nellemann et al.2009; Laoffoley and Grimsditch, 2009; Mcleod et al., 2011; Fourqurean et al, 2012; Pendleton et al., 2012). These ecosystems have potentials for carbon storage greater than tropical forests. For example, seagrasses store 40 times greater carbon than the tropical forests, up to 83 gcm -2 y -1 (Duarte et al., 2005; Kennedy and BjÖrk, 2009). The ecosystem services value of seagrass is also high, at approximately 19,000 USD/ha/year, which is almost 10 times greater than that of the
tropical forest. Estimates are that seagrasses store as much as 3 times more than the highly productive coral reef ecosystem (Cotanza et al., l977). Seagrass ecosystems are closely tied to the livelihoods of local fishermen in developing countries in SE Asia and Africa, where they are common and abundant. The hotspot of biodiversity of seagrass is in SE Asia, where a high of 15 species has been recorded (Short et al., 2007). Thailand has 12 species with now another new record, Halophila major, under review (Tuntriprapas et al., in submission). Seagrass beds are a threatened habitat especially in developing countries where extensive coastal developments of land reclamation for industry or tourism and aquaculture. Compared to coral reefs or mangroves the understanding and public awareness of seagrasses is limited (Orth et al., 2006). The decline of seagrass worldwide is very dramatic with an annual average loss of about 3,370 km 2 at a rate of 27 km 2 /year (Waycott et al., 2009). These data represent the situation in the USA, Europe and Australia, where law enforcement, conservation and public awareness are maintained better than in developing countries where we can assume there is greater loss in the other regions of the world. The Blue Carbon Initiative (www.thebluecarboninitiative.org) is an international group funded by various conservation agencies, dedicated to a better understanding of the roles of seagrass, mangroves and salt marshes as carbon sinks. The studies are ongoing and there is a close association between the scientists and policy makers. We adopted the idea of blue carbon and attempt to estimate the carbon content in the seagrass beds in Ranong Biosphere Reserve and Trang Haad Chao Mai Natianal Park in Southern Thailand. We hope that our studies promote further research and prove the importance of their conservation in Thailand and surrounding regions. Materials and Methods Study sites The seagrass bed surveys were carried out from February to July 2013 in 1) the Ranong Biosphere Reserve (RBR) and 2) Trang Haad Chao Mai National Park (HCM) on the Andaman sea, Southern Thailand (Fig. 1). The purpose of the project
was to assess seagrass species diversity and the area of the seagrass beds in relations to carbon storage. RBR is in a sheltered bay close to a mangrove forest (Fig. 2), while HCM is in front of a more exposed beach (Fig. 3). The areas of the seagrass beds were estimated using GPS and the maps indicate the specific sites for the field surveys. Figure 1 The Google Earth map of the study area at Ranong Biosphere Reserve (RBR) and Trang Haad Chao Mai National Park (HCM), located on the Andaman sea, Indian Ocean
Figure 2 The seagrass bed associated with the mangrove forest at Ranong Biosphere Reserve (RBR) Figure 3 The seagrass bed at Trang Haad Chao Mai National Park (HCM) next to the sandy beach.
Plants and sediment collection and analyses To understand the differences between the healthy (80% coverage) and unhealthy (20% coverage) seagrass beds, 3 repetitions of random samples were removed within 20 cm X 20 cm quadrats (Fig. 4). Figure 4 Assessment of coverage using 20 cm X 20 cm quadrat in a bed with 20% coverage of Cymodocea serrulata at RBR. Also, to understand the differences in carbon storage of each species, every seagrass species was collected. The seagrass plants were then separated out into 4 parts: leaf, sheath, rhizome and root (Fig. 5). Each part was then thoroughly cleaned and dried at 60 C until constant weight was obtained, to provide the dry biomass. The biomass was then divided into 2 groups: the above ground biomass and the below ground biomass (Fig. 6). Plants were then prepared for % carbon content analyses using a CHN Analyzer (CHNS-O Analyzer, CE Instruments Flash EA 112 Series, Thermo Quest, Italy) at Prince of Songkla University Central Equipment Laboratory, Prince of Songkla University, Thailand.
Figure 5 Plants were separated into 4 parts: leaf, sheath, rhizome and root; and presented into above and below ground biomass. Th= Thalassia hemprichii, Ea=Enhalus acoriodes, Cr=Cymodecea rotundata and Ho=Halophila ovalis Since it was previously reported by Kennedy et al., (2010) that there was a high percentage of carbon in the sediments, sediment was also collected and sent for % carbon content examination. Three repetitions of plastic cores 7 cm in diameter (Fig. 7) of sediment in the healthy and unhealthy seagrass beds were taken from 30 cm depth, packed in plastic bags and brought back to the laboratory. Sediments were cleaned and dried at 60 C until constant weight was obtained. The sediments were then sieved using sediment shakers (Retsch A5 200 digit, Germany) to estimate the grain sizes. Particles were separated according to a series of >2 mm, 1-2 mm, 1-0.25 mm, 0.25-0.5 mm, 125-250 μm, 63-125 μm and <63 μm. They were weighed and the proportion of each grain size was calculated. Sediments were grouped into 2 size classes: >63 μm and < 63 μm for further % carbon content analysis (Fig. 8). To estimate the carbon storage at each study site, the % carbon found from plants and sediments were used to calculate the seagrass area of each site.
Figure 6. The experimental design of sample collections. All seagrass species were divided into leaf, sheath, rhizome and root except Halophila ovalis (Ho), which is no sheath.
Figure 7. The 7 cm diameter plastic core for sediment sampling. Statistical analyses Three-way ANOVAs were employed to test of the effects of sites, species and coverage, on the above and below grounds biomass (g/m2); % elemental carbon above and below ground; carbon content above and below ground (Mg/ha); sediment grain sizes and % carbon in the sediment. When necessary, data were transformed to meet the assumptions of the parametric test. Statistical results were presented based on the transformed analyses, but for clarity graphical output was based on the untransformed means. Non-parametric analyses (Kruskal-Wallis) were employed if data did not meet the assumptions.
Figure 8. The experimental design of sample collections. Sediment particles were separated using the grain size shakers and grouped later into > 63μm and < 63μm size classes. Results 1. Seagrass diversity and the area of the beds There were differences in species diversity and areas between RBR and HCM. At RBR the only seagrass species were Cymodocea serrulata and Halodule uninervis. The seven species found at HCM were Cymodocea rotundata, C. serrulata, Thalassia hemprichii, Enhalus acoroides, Syringodium isoetifolium, Halodule uninervis, Halophila ovalis. (Figs. 9-15). The estimated area at RBR was 79 ha and at HCM 882 ha (Figs.16 and 17).
Figure 9. Enhalus acoroides (L.f.) Royle, the largest seagrass in the Indo- Pacific region. Figure 10. Thalassia hemprichii (Ehrenb.) Asch & Magnus, the second largest seagrass species in the region, provides habitat and food for sea turtles.
Figure 11. Cymodocea rotundata Asch. & Schweinf., a medium-sized species, common in the region. Figure 12. Cymodocea serrulata (R.Br.) Asch. & Magnus, similar to C. rotundata but with serrated tips on the leaves.
Figure 13. Syringodium isoetifolium (Asch.) Dandy, the only cylindrical leaved species in this region, commonly found at the lower intertidal to subtidal regions. Figure 14. Halodule uninervis (Forssk.) Boiss, mostly found at lower intertidal to subtidal.
Figure 15. Halophila ovalis (R.Br.) Hook.f., a common species in this region. It is well known as a dugong food. Figure 16. The Google earth map of the study area, circled in red, at RBR.
Figure 17. The Google earth map of the study area,circled in red, at HCM. 2. Seagrass biomass, sediment grain size and % carbon There were significant differences between the above and below ground biomass between sites, species, coverage and their interactions (Table 1). HCM had a greater biomass than RBR. Since it is the largest tropical species, Enhalus acroides, (it reaches 1.5 + m in height), had greater above and below ground biomass. The highest above ground biomass was found in the healthy E. acroides stand at 200 g/m 2 (Fig. 18A). The smallest species, Halodule uninervis, provided the smallest biomass (Fig. 18B). A similar pattern was also observed in the below ground plant parts: the larger plants had a larger below ground biomass reflecting their coarse, tough rhizomes. The highest average below ground biomass was 419 g/m 2, while the smallest average below ground biomass was only 1.0 g/m 2 found in the C. serrulata stand with 20 % coverage (Figs. 18A,B).
There was a significant difference in % carbon of above ground plant material between species and with coverage (Table 1). The healthy populations had greater % carbon than the unhealthy. The highest average elemental carbon was measured in the healthy stands of Cymodocea serrulata at 37.40%. The smallest was 31.12 % in an Enhalus acoriodes unhealthy bed (Figs. 19A, B). There were significant differences in carbon production between above and below ground among sites, species and coverage (Table 1). The healthy seagrass had greater carbon production than the unhealthy seagrass. The highest above ground production at RBR was 67.44 g/m 2 in the healthy Enhalus bed and the smallest was a mere 0.67 g/m 2 in the unhealthy Halodule bed (Fig. 20A). The below ground production, however, was over 2 times greater at 151.16 g/m 2 (Fig. 20B), representing a more significant amount of carbon storage. There were significant differences in sediment grain size among sites, species and coverage (Table 2). The coverage itself, however, did not influence the sedimenttrapped fractions within the seagrass bed. Most of the trapped sediment size was > 63 μm, mostly between 63-125 μm. RBR had clearly different sediment fractions compared to HCM (Fig. 21). Larger sediment sizes were found at RBR. Fine clays were also present. There were a few bivalves observed within the bed. The proportion of < 63 μm particles at RBR was significantly greater than at HCM. The Thalassia hemprichii bed at HCM had a larger fractions than in other beds. There were significant differences in % carbon content in sediment among sites, species and coverage (Table 2). The < 63 μm sediment had much greater % carbon. RBR had much greater % carbon than the HCM (Fig. 22). The greatest average % carbon at 4.66% in the sediment was found in the Halodule uninervis bed at HCM. The living plant carbon storage is approximately 6.08 Mg C at RBR and 335.16 Mg C at HCM. These estimates do not include the additional source in soil carbon.
(A) (B) Figure 18. (A) Average above ground biomass and (B) below ground biomass at 20% and 80% coverages at HCM and RBR, n=3, error bar = SE; Th=Thalassia hemprichii, Ea=Enhalus acoriodes, Ho=Halophila ovalis, Cr=Cymodocea rotundata, Cs=Cymodocea serrulata, Si =Syringodium isoetifolium, Hu= =Halodule uninervis.
(A) (B) Figure 19. (A)The average % carbon element in the above ground biomass and (B) the % carbon in the below ground biomass at 20% and 80% coverages at HCM and RBR, n=3, error bar = SE; Th=Thalassia hemprichii, Ea=Enhalus acoriodes, Ho=Halophila ovalis, Cr=Cymodocea rotundata, Cs=Cymodocea serrulata, Si =Syringodium isoetifolium, Hu= =Halodule uninervis.
(A) (B) Figure 20. (A) The average above ground areal carbon production and (B) The below ground areal carbon production at 20% and 80% coverages HCM and RBR, n=3, error bar = SE; Th=Thalassia hemprichii, Ea=Enhalus acoriodes, Ho=Halophila ovalis, Cr=Cymodocea rotundata, Cs=Cymodocea serrulata, Si =Syringodium isoetifolium, Hu= =Halodule uninervis.
Table 1. The summary of the mean ± (SE) of above and below ground, % carbon and carbon content HCM and RBR; * is P <0.05 and ** is P <0.01 Parameters HCM Th Ea Ho Cr Cs Si Hu Cs 20% 80% 20% 80% 20% 80% 20% 80% 20% 80% 20% 80% 20% 20% 80% 20% Above ground biomass (g/m 2 ) 35.71 70.34 59.85 200.72 3.4 10.90 10.0 38.12 3.3 10.95 4.05 30.8 0.67 4.74 16.68 1.87 5.371 * 63.507 ** 20.335 ** 49.076 ** 43.185 ** (2.03) (12.70) (9.51) (20.79) (1.15) (0.43) (3.70) (21.84) (0.57) (0.68) (0.56) (4.80) (0.18) (0.43) (2.26) (0.59) Below ground biomass (g/m 2 ) 105.88 153.60 128.28 419.08 3.9 6.45 25.0 59.74 1.0 12.78 1.56 16.5 4.26 12.81 26.82 4.31 0.859 90.290 ** 11.194 * 64.135 ** 55.621 ** (6.91) (22.43) (48.85) (22.56) (0.05) (0.33) (9.31) (25.68) (0.33) (4.28) (0.39) (4.55) (2.01) (1.96) (4.70) (1.18) Carbon element in above ground (%) 34.44 33.85 31.12 33.31 33.89 32.46 35.64 35.23 36.82 37.40 33.78 33.66 36.04 36.41 34.57 36.90 3.158 21.966 * 3.485 26.298 * 31.531 * (0.50) (1.01) (1.23) (1.52) (2.50) (1.20) (0.52) (1.25) (0.41) (0.37) (0.63) (0.36) (0.20) (0.61) (0.50) (1.12) Carbon element in below ground (%) 34.13 34.67 34.21 36.09 28.44 33.45 33.52 35.42 36.57 36.58 35.43 33.52 33.03 34.43 33.55 35.25 7.740 0.529 0.156 14.147 25.398 (0.10) (0.85) (0.63) (0.25) (1.83) (8.89) (1.33) (0.87) (0.28) (0.10) (0.42) (0.95) (0.33) (0.35) (0.71) (1.39) Above ground carbon (Mg/ha) 0.123 0.239 0.187 0.674 0.011 0.035 0.036 0.137 0.012 0.041 0.014 0.104 0.014 0.017 0.057 0.007 5.097 * 43.886 ** 20.336 ** 35.332 ** 31.079 ** (0.009) (0.046) (0.032) (0.096) (0.004) (0.003) (0.014) (0.078) (0.002) (0.002) (0.002) (0.016) (0.007) (0.002) (0.007) (0.002) Below ground carbon (Mg/ha) 0.361 0.536 0.442 1.512 0.011 0.021 0.086 0.216 0.004 0.047 0.006 0.056 0.014 0.044 0.090 0.016 0.637 90.323 ** 11.686 * 65.299 ** 56.536 ** (0.023) (0.091) (0.169) (0.072) (0.001) (0.006) (0.034) (0.097) (0.001) (0.016) (0.001) (0.017) (0.007) (0.007) (0.017) (0.005) RBR Hu Site F or Chisquare Species F or Chisquare Cover Species x Cover Site x Species xcover F or Chisquare F or Chi-square F or Chi-square
Table 2. The summary of the mean ± (SE) of sediment grain sizes and % carbon element in sediment at HCM and RBR; * is P <0.05 and ** is P <0.01 Parameters Sediment grain size < 63µm (%) Sediment grain size > 63µm (%) Carbon in Sediment grain size < 63µm (%) Carbon in Sediment grain size > 63µm (%) Th Ea Ho HCM Cr Cs Si Hu Cs 20% 80% 20% 80% 20% 80% 20% 80% 20% 80% 20% 80% 20% 20% 80% 20% Hu F or Chisquare F or Chisquare F or Chisquare F or Chi-square F or Chi-square 0.75 0.83 0.66 1.15 0.58 0.75 0.41 0.33 0.14 0.33 0.16 0.24 0.11 6.34 7.35 7.96 36.322 ** 13.499 * 0.001 14.583 44.660 ** (0.40) (0.37) (0.26) (0.40) (0.36) (0.22) (0.08) (0.22) (0.02) (0.06) (0.02) (0.08) (0.03) (2.30) (2.37) (2.04) 99.25 99.17 99.34 98.85 99.42 99.25 99.59 99.67 99.86 99.67 99.84 99.76 99.89 93.66 92.65 92.04 36.322 ** 33.294 ** 3.426 34.930 ** 44.704 ** (0.40) (0.37) (0.26) (0.40) (0.36) (0.22) (0.08) (0.22) (0.02) (0.06) (0.02) (0.08) (0.03) (2.30) (2.37) (2.04) 0.73 1.34 1.19 0.85 0.98 1.03 0.72 0.44 0.87 2.20 1.11 1.28 4.66 3.65 3.23 3.63 25.998 ** 37.036 ** 0.577 39.485 ** 48.143 ** (0.25) (0.19) (0.38) (0.22) (0.05) (0.09) (0.10) (0.12) (0.05) (0.11) (0.29) (0.17) (0.12) (0.22) (0.41) (0.15) 0.23 0.27 0.20 0.20 0.19 0.21 0.17 0.15 0.18 0.24 0.18 0.22 1.34 2.77 2.52 2.48 36.363 ** 31.971 ** 0.243 33.003 ** 47.572 ** (0.04) (0.02) (0.02) (0.02) (0.01) (0.01) (0.03) (0.02) (0.02) (0.03) (0.03) (0.02) (0.14) (0.17) (0.15) (0.21) RBR Site Species Cover Species x Cover Site x Species x Cover
Figure 21. The proportion of sediment grain sizes of soil at 20% and 80% coverages at HCM and RBR, n=3, error bar = SE; Th=Thalassia hemprichi, Ea=Enhalus acoriodes, Ho=Halophila ovalis, Cr=Cymodocea rotundata, Cs=Cymodocea serrulata, Si =Syringodium isoetifolium, Hu= =Halodulae uninervis. Figure 22. The percent elemental carbon in the soil at 20% and 80% coverages at HCM and RBR, n=3, error bar = SE; Th=Thalassia hemprichii, Ea=Enhalus acoriodes, Ho=Halophila ovalis, Cr=Cymodocea rotundata, Cs=Cymodocea serrulata, Si =Syringodium isoetifolium, Hu= =Halodule uninervis.
Discussion The average above and below ground biomass measured in our study was much less than reported by Durate and Chiscano (1999). The Thalassia hemprichii above and below ground biomass, for example, was 20% and 40% less than previously reported. The Halophila ovalis the above ground biomass presented only12.5% and the below ground was only 25%, also less. Enhalus acoriodes was the only species in which the biomass was higher than reported by Durate and Chiscano (1999). Therefore, it is possible that E. acoroides is resistant to change and that our seagrass beds have become less healthy over the last 20 years. The overall biomass at RBR was greater than at HCM which may be correlated with the fact that RBR is much more pristine and less disturbed by human activities. We rarely observed human activities at RBR during our field collections, but we often found shell collectors trampling on the seagrasses during our field work at HCM. There is more coastal development and tourism at the HCM sites. The decline of biomass could be a result from anthropogenic activities: coastal developments increase the sediment loading and more broadly the increase of temperature due to global climate change. Further investigations are needed to clarify what the major threats to the decrease of seagrass biomass at these sties are. There is little variation in % carbon content among seagrass leaves worldwide. According to Duarte (1999) the average (± SE) carbon was 33.6 ± 0.31 % DW, similar to the measurements in our studies in which the highest % carbon content was found in Cymodocea serrulata. The below ground % carbon content showed no differences from one species to another. The lowest % carbon content was the below ground tissue of Halophila ovalis, 28.44 ± 1.83 % DW from the unhealthy stand. H. ovalis has a different rhizome and root structure from other seagrasses. Its thin rhizomes are anchored shallower in the sediments. The plants are younger compared to the other species. In a Bolinao, The Philippines, study the PI was 2.2 leaves/day (Vermaat et al., 1995). The plants grow faster and expand their meadows rapidly, which even if it does not allow them to accumulate much carbon promotes productivity. They produced 165.9 leaves/shoot/ year, while the E. acoriodes produced only 11.5 leaves/shoot/year. The longevity of H. ovalis was less, only 27 ±
4.2 day compared to 787 ± 125 days of E. acoriodes, suggesting a very high turn over rate. H.ovalis, thus, is less of a carbon sink than other seagrass species. The carbon content of above and below ground biomass of living seagrasses parallels their above and below ground biomass. The below ground biomass stored 2 times carbon/ha than the above ground. It ranged between 0.011 1.512 Mg/ha to 0.007-0.674 Mg/ha respectively. E. acoroides contributed the greatest living carbon storage due to its high biomass. Thus, the healthy and dense seagrass beds are important and contribute to the greatest storage of carbon in the seagrass ecosystems. The fine sediment < 63 μm was significant greater at RBR due to the location near the mangrove forests by the enclosed bay, which allows the fine sediment to settle rather than being washed away at the open coasts as at HCM. The greater proportion of >63 μm in RBR might also be caused by filtering bivalves in the beds. Such beds are well known habitats for animals. We observed various fishes, crabs, shrimp and sea cucumbers at our collection sites. Fine clay, < 63 μm, has a greater % carbon content than the larger sized sediments. According to Fourqurean et al (2012), the average C org of seagrass soils was 1.4%, ranging between 0-48.2. Our sediment % carbon at RBR ranged between 2.48-3.65 and at HCM ranged between 0.15-4.66 %. Halodule uninervis soils provided a high % carbon at both sites. This might be because plants materials were buried. H. uninervis has fine long, narrow leaves, which normally lie close onto the substrate when the tide is out. Plants are easily buried by the sediment when there is high sediment loading from a terrestrial system or if broken off and buried. It is clear that further investigations are important to better understand the carbon reserves that buried materials provide. Acknowledgements I am grateful for the 2012 MAB Young Scientists Award, MAB Program, UNESCO for support. The Cluster and Program management Office, National Science and Technology Development Agency of Thailand provided assistance for the extensive dataset used in this report. I am grateful for Seaweed and Seagrass Research Unit, Excellence Centre for Biodiversity Peninsular Thailand, Department of Biology, Faculty of Science, Prince of Songkla University for extensive help
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