Stable Carbon and Nitrogen Isotopic Characterization of Organic Matter in a Mangrove Ecosystem on the Southwestern Coast of Thailand

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1 Journal of Oceanography, Vol. 57, pp. 421 to 431, 2001 Stable Carbon and Nitrogen Isotopic Characterization of Organic Matter in a Mangrove Ecosystem on the Southwestern Coast of Thailand TOSHIKATSU KURAMOTO* and MASAO MINAGAWA Graduate School of Environmental Earth Science, Hokkaido University, Sapporo , Japan (Received 19 July 2000; in revised form 21 November 2000; accepted 24 November 2000) Organic matter in a tropical mangrove ecosystem was characterized by stable carbon and nitrogen isotopic analyze, conducted on various organic samples, including land and mangrove plants, soils, particulate organic matter (POM), and sea and river sediments along the southwestern coast of Thailand. The δ 13 C values of land plants and POM in river water can be explained in terms of a greater influence of C 3 plants than C 4 plants in this area. The POM and sediments from the Trang River and Ko Talibong area showed systematically higher δ 15 N values than those from Ko Muk and other coastal areas. Organic matter in the Trang River might be influenced by nitrogen released from agricultural or human waste, which could affect the isotopic composition of POM and sediments in the Trang River estuary and along the coast near the river mouth. We used a stochastic method to estimate the contributions of four organic end-members, identifiable by their δ 13 C and δ 15 N values. The results implied that seagrasses were a major source of sedimentary organic matter, contributing 42 ± 5% in the Ko Muk area and 36 ± 5% in the Ko Talibong area. The contribution of coastal POM to sediments was estimated to be only 13% in Ko Muk and 19% in Ko Talibong. Mangrove plants contributed approximately 23% in both areas. It was concluded that seagrasses are an important source of sedimentary organic matter in this coastal region of southwestern Thailand. Keywords: δ 13 C, δ 15 N, southwestern Thailand, mangrove, seagrass, POM, sediment. 1. Introduction Coastal seas occupy roughly 10% of the ocean and sustain levels of primary production comparable to those of the world s open oceans (e.g., Walsh, 1991). During the last decade, many studies have focused on the potential contribution of coastal seas to global carbon cycling. In coastal regions, high production is mainly due to the supply of nutrients from rivers and the transport of organic debris from land. We need to analyze organic substances in coastal regions in order to evaluate the role of terrestrial materials in marine biogeochemical systems. Stable carbon and nitrogen isotope ratios have been used as indicators to estimate the proportion of terrestrial organic material that contributes to marine organic matter (Peters et al., 1978). Terrestrial C 3 plants, which use Rubisco carboxylation during photosynthesis, have a carbon isotope ratio ( 13 C/ 12 C, hereafter presented as δ 13 C on the PDB scale) of approximately 27, while the terres- * Corresponding author. t-kura@ees.hokudai.ac.jp Copyright The Oceanographic Society of Japan. trial C 4 plants, which use PEP carboxylation, have a smaller δ 13 C fractionation of about 12 (O Leary, 1988). Seagrasses, which are marine vascular C 3 plants, have δ 13 C values ranging from 15 to 3 (Zieman et al., 1984; Wada et al., 1990; Yamamuro, 1999). It is also known that δ 13 C of phytoplankton changes with environmental factors, such as water temperature and ambient pco 2, as well as plankton growth rates (Rau et al., 1992; Goerike and Fry, 1994; Yoshioka, 1997). The nitrogen isotope ratio ( 15 N/ 14 N, hereafter presented as δ 15 N on the atmospheric nitrogen scale) of particulate organic matter (POM) depends on the fractions of organic (phytoplankton) and inorganic (available as substrates for photosynthesis) nitrogen in the water column (e.g., Miyake and Wada, 1971; Wada and Hattori, 1978; Montoya and McCarthy, 1995). Previous work has reported that nitrogen uptake processes often induce isotopic fractionation in primary producers. For example, biological N 2 fixation introduces 15 N-depleted nitrogen of atmospheric origin to suspended matter by introducing the low δ 15 N characteristic of atmospheric N 2 (Saino and Hattori, 1980). By contrast, denitrification increases 421

2 the δ 15 N of nitrate due to reduction of the nitrate to N 2 or N 2 O, both of which processes induce large isotopic fractionation (e.g., Cline and Kaplan, 1975). Recently, it has been suggested that nitrogen isotopes of nitrate and POM vary systematically in watersheds, including both pristine forested and agricultural catchments. For example, the nitrate in water draining from agricultural sites tends to contain high concentrations of 15 N (Harrington et al., 1998). Previous work has also reported that particulate organic nitrogen (PON) from estuarine waters surrounded by densely populated areas tends to have a δ 15 N higher than 8 because organic waste from human residential areas often enhances denitrification in drainage systems. Consequently, the nitrate is enriched in 15 N, and this affects the δ 15 N of organic matter in the ecosystem (Mariotti et al., 1984; Cifuentes et al., 1988). Furthermore, nitrogen isotopes have been effectively used in research to evaluate food web structures in ecosystems. Animals accumulate 15 N in the body due to preferential excretion of 14 N, thus causing a stepwise enrichment of 15 N within food chains (DeNiro and Epstein, 1981; Minagawa and Wada, 1986). Based on knowledge from modern analogues, stable carbon and nitrogen isotopes have also been used for paleoenvironmental studies. For example, Altabet et al. (1995) studied the δ 15 N of sediment cores to reconstruct past denitrification changes in the Arabian Sea. However, it is not known how organic matter in the pelagic sediment of coastal areas records environmental changes. In this study, land plants, POM, and surface sediment samples were collected at two locations along the southwestern coast of Thailand. One area is influenced by river runoff from a watershed with a large population and the other is a coastal area with no major river inflow. The purpose of the study was to demonstrate how carbon and nitrogen isotope analyses describe the mixing of terrestrial and marine organic matter in the coastal sediments of two areas with differing degrees of human activity. Isotopic analyses were conducted on land and marine plants growing in mangrove forests, and were then used to char- Fig. 1. Sampling locations in southwestern Thailand. The sample number in the Ko Talibong and the Trang River areas are underlined. 422 T. Kuramoto and M. Minagawa

3 acterize terrestrial and marine organic matter for future paleoenvironmental research using sediment cores in the Andaman Sea. 2. Location Land plants, tidal plants, and POM in river water and seawater were collected near Haad Chao Mai National Park ( N, E) in Thailand. The sampling locations and station codes used in this study are shown in Fig. 1. This region of Thailand, drained by several rivers, the Trang River being the largest. The eastern side of Ko Muk receives discharge from only a few small rivers, but the area around Ko Talibong, located at the mouth of the Trang River, is strongly influenced by river water. The cities of Trang and Kantang are located along the Trang River, and there are many charcoal factories, shrimp ponds, and fish paste factories near the cities. Mangrove is the predominant vegetation along both shorefronts and riversides in this area, and plantations of rubber and palm trees are common. 3. Materials and Methods Fresh leaves were collected from heights of 1 2 m on land and mangrove plants and were washed carefully with deionized water as soon as possible after collection. Each sample was dried in an electric oven at 60 C, and was then powdered and sieved through a 250 µm mesh. River water and seawater samples were collected with a Van Dorn water sampler (1~3 l). The water samples were filtered through precombusted (450 C, 2 h) Whatman GF/F filters to collect particulate organic matter (POM). The filters were dried in an electric dryer and then placed in a desiccator with 12 N HCl for half a day to allow decarbonization. Surface soil samples from a rubber tree plantation, a forest near the national park, and a dune near the coast were collected with a shovel and stored in precombusted glass bottles. The soil samples were dried in an electric oven at 60 C, before crushing and sieving through a 250 µm mesh to remove large plant debris and coarse sand. SCUBA divers collected river and coastal surface sediments, which were also stored in precombusted glass bottles. Dry sediments were also crushed and sieved. All powdered soil and sediment samples were decarbonized using 1 N HCl, rinsed with distilled water to remove salt components, and then dried again. The carbon and nitrogen isotopic ratios of land and mangrove leaves, POM, soils, and sediments were determined by the flow-injection method using a Finnigan MAT 252 mass spectrometer connected with a Fisons NA1500 elemental analyzer. The results are presented using the conventional delta value notation, and calibrated to PDB standards or atmospheric N 2. The analytical error was estimated to be within 0.2 for both δ 13 C and δ 15 N based on replicate runs of an amino acid reagent. Table 1. Elemental and isotopic analytical results for land and mangrove plants. Stable Carbon and Nitrogen Isotopic Characterization of Organic Matter on the Southwestern Coast of Thailand 423

4 4. Results Land and mangrove plants had δ 13 C within the common range for C 3 plants, with values ranging from 31.2 to 23.5 (average 28.2 ± 2.2, n = 8) for the land plants and from 29.0 to 25.4 (average 27.0 ± 1.3, n = 6) for the mangrove plants (Table 1). Thus, no significant difference in δ 13 C was found between land and mangrove plants. In contrast, the δ 15 N of these plants did show a statistically significant difference (p = 0.008), with a range of 2.4 to +0.8 (average 0.5 ± 1.1 ) for the land plants and 0.4 to +6.3 (average 2.7 ± 2.1 ) for the mangrove plants. One unusual data point was obtained from a land plant sample collected in a charcoal factory along the bank of a small river, and the sample was suspected of containing artifacts, such as animal waste, which caused the large δ 15 N. Nypa fruiticans and Duabanga gradiflora, both from the Trang River bank sample, were enriched in 15 N compared to the other mangrove samples. The C/N atomic ratios of the land and mangrove plants averaged 22.8 and 35.1, respectively, indicating that Table 2. Elemental and isotopic analytical results for particulate organic matter (POM). 424 T. Kuramoto and M. Minagawa

5 mangrove leaves have a higher carbon content than land plant leaves, although the variation was large among each leaf type. The δ 13 C of river POM varied from 27.1 to 24.0, with no obvious difference between large and small rivers (Table 2). On the other hand, the δ 15 N of the river POM showed a large variation, between 3.1 and 7.7, with the higher values in the Trang River samples. The δ 13 C of coastal POM ranged from 27.3 to 20.6, generally showing larger values than river POM. An extreme Table 3. Elemental and isotopic analytical results for land soils, river sediments, and coastal sediments. Sample name TOC (%) TN (%) δ 13 C ( ) δ 15 N ( ) C/N (mol mol 1 ) Remarks Land soils 12-3 (1) bank rubber plantation rubber plantation near national park Average s.d River sediments Trang river Trang river Other river mouth river mouth Average s.d Coastal sediments Ko Taribong area Trang river mouth Ko Muk area (2) near bank Average s.d Stable Carbon and Nitrogen Isotopic Characterization of Organic Matter on the Southwestern Coast of Thailand 425

6 negative value of δ 13 C for coastal POM was found in the Ko Talibong area, which is attributed to the contribution of terrestrial organic matter. The δ 15 N of coastal POM varied between +3.1 and +8.4, which is similar to that of river POM. Coastal POM at stations 9-10 and 15-7 near the Trang River mouth had larger δ 15 N than all other samples. A significant difference in the δ 15 N of POM was also found between the Trang River and other rivers of the coastal region (p = 0.04). The δ 13 C of soils ranged from 27.8 to 23.9 (Table 3). The largest δ 13 C was recorded for a sample from Station 12-3 collected near a man-made seawall. Since this sample was composed of sandy soil, the total organic carbon (TOC) content and the C/N ratio were both the lowest among all samples, at 0.04% and 12.4, respectively. The C/N ratios of the other soil samples ranged from 17.0 to 20.1 and were lower than those of land and mangrove plants. The δ 15 N of soils ranged between 0.7 and 2.9, which is similar to the ranges found in previous reports (e.g., Heaton, 1986). The δ 13 C of the coastal sediments varied between 25.4 and 19.0 (Table 3). Lower values of δ 13 C ( 25.4 and 25.2 ) were found at stations 9-10 and 15-7 near the Trang River estuary. The total organic carbon content of river sediments varied widely from 0.03 to 3.4%, while the samples from mangrove forests showed a relatively higher TOC of 3.2 to 3.5%. The δ 15 N of river and coastal sediments varied from 1.0 to 3.9 and from 0.7 to 4.3, respectively. Sediments and POM from the Trang River and the Ko Talibong area have δ 15 N values that higher than in the other samples (p < 0.001). 5. Discussion 5.1 The δ 13 C of plants, POM and sediments The analytical results for δ 13 C in the samples are presented in Fig. 2. The carbon isotopic compositions obtained in this study are consistent with previously published results in the other coastal regions. In this study, the average δ 13 C of mangrove plants was 27.0 with a range of 29.0 to Previous studies have reported that δ 13 C values of mangrove plants vary between 33 and 24 in southern Florida and Guadeloupe, French West Indies (Zieman et al., 1984; Lallier-Verges et al., 1998). Furthermore, Hayase et al. (1999) conducted isotope studies in the Matang mangrove forest in Malaysia, and reported that the δ 13 C values of mangrove leaves ranged from 28.7 to It was speculated that C 4 plants are widely distributed in tropical areas of the world; therefore, we expected that C 4 plants contribute to organic matter more or less. However, the average δ 13 C of land plants was 28.2, suggesting little contribution of C 4 plants. Generally, The mean δ 13 C values of land soils and river POM were also Fig. 2. The δ 13 C of organic matter from southwestern coast of Thailand. Vertical bars show average values and horizontal bars show 2σ range. low ( 25.7 and 26.1, respectively), except for one sandy soil sample. Given that C 3 plants have a lower δ 13 C than C 4 plants (O Leary, 1988) and since much river POM is derived from terrestrial organic matter, such as land or mangrove plant debris, the low δ 13 C found in our soil and river POM samples probably reflects the dominance of C 3 plants in the studied region. The one unusual land soil sample was collected at a dune near a seawall that had been destroyed by a coastal wave (Table 3). Therefore, the organic fraction in this soil could have originated from biological debris, such as marine macrophytes, conveyed by the wave. The high δ 13 C value and high sand content of the sample also support this explanation. The other soil samples showed values similar to those of land plants. River sediments from stations and had relatively low δ 13 C compared to the other samples and also had high organic carbon contents. These stations were located by a river where mangrove plants are the predominant vegetation. Tropical mangrove swamps are known to cause high sedimentation rates in the surrounding areas, due to their high production rates and the consequent accumulation of organic carbon. Thus, the low δ 13 C values of the river sediments from these stations presumably resulted from the contribution of mangrove plant 426 T. Kuramoto and M. Minagawa

7 debris. The δ 13 C compositions of mangroves and sediments in this study are consistent with the previous observation that mangrove swamp sediments have δ 13 C values similar to or slightly larger than those of mangrove plants (Lallier-Verges et al., 1998). The δ 13 C composition of coastal POM are consistent with the values of 23 to 17 previously reported for marine phytoplankton from an equatorial region (Rau et al., 1982). Particulate organic matter in coastal seawater had greater δ 13 C values than that in river water, with the exception of stations 9-10 and 9-12, both of which are located at the mouth of the Trang River. These two stations are possibly strongly influenced by the Trang River, which is consistent with the indication from δ 13 C values that coastal POM is composed of terrestrial organic matter that originated mostly from C 3 plants. The δ 13 C values of surface sediments from the Trang River mouth (9-10 and 15-7) were also lower than those from sediments at the other locations, which also suggests a strong influence by terrestrial organic matter. 5.2 The δ 15 N of plants, POM and sediments The analytical results for δ 15 N in the samples are presented in Fig. 3. The δ 15 N values for mangroves were about 1.8 higher than for land plants. Excluding the exceptionally large value of δ 15 N from the one sample mentioned above, the difference in δ 15 N between mangroves and land plants is statistically significant (p < 0.008). Since mangrove is a tropical shrub that grows on muddy banks where river water and seawater mix, the nutrient sources for mangroves are different from those for land plants, and this could explain the observed difference in δ 15 N. If riverine water or seawater were enriched in 15 N, mangroves would be expected to have higher δ 15 N than land plants. Furthermore, specific nitrogen diagenesis, such as denitrification, which occurs near the water-sediment interface, may be a possible cause of this enrichment. The δ 15 N values of mangroves (Nypa fruiticans and Duabanga gradiflora) from the Trang River banks were higher than in the other mangrove samples, suggesting that the nitrate of the Trang River is enriched in 15 N (Table 1). The δ 15 N of the Trang River POM was also higher than in the other rivers, suggesting that δ 15 N of nitrate in Trang River water is greater than in seawater or water from the other small rivers. The δ 15 N of coastal POM showed values similar to previously reported values from the Indian Ocean, where POM ranged from +3.2 to (Saino and Hattori, 1980). The contribution made by N 2 fixation was negligible in the area. Larger δ 15 N values were obtained at stations 9-10, 15-2, and 15-7 near the Trang River mouth, showing ranges similar to those of the Trang River POM. The difference in δ 15 N between the Trang River area and the Ko Muk area was also observed for coastal POM, river Fig. 3. The δ 15 N of organic matter from southwestern coast of Thailand. Vertical bars show average values and horizontal bars show 2σ range. sediments, and coastal sediments. These results therefore suggest that nitrogen in the Trang River watershed might be enriched in 15 N, and that its influence extends as far as the coastal sea. Harrington et al. (1998) reported that nitrate in the streams draining agricultural areas had high δ 15 N values relative to the streams flowing through pristine forested areas, because the former has an anthropogenic influences such as the input of human and domesticated animal wastes (Kreitler and Jones, 1975). Our study area contains many plantations of rubber and palm trees. However, the plant and soil samples from these plantations showed relatively low δ 15 N values ( 2.4 to +2.9 ), which might reflect the use of commercial fertilizer (δ 15 N of 4 to +4 ; Heaton, 1986) or atmospheric precipitation (Fogel and Paerl, 1993). Although the loss of volatile ammonia from commercial fertilizers and manure can raise the δ 15 N of the residual nitrate locally (Kreitler, 1979), this cannot explain the high δ 15 N observed in the entire Trang River system. A possible cause of the high δ 15 N for organic matter in the Trang River is the input of domestic wastewater from Trang and Kantang Cities. It is known that POM from estuarine waters located near high population areas is characterized by heavy δ 15 N (Mariotti et al., 1984; Stable Carbon and Nitrogen Isotopic Characterization of Organic Matter on the Southwestern Coast of Thailand 427

8 Cifuentes et al., 1988). The recent urbanization may have enhanced denitrification within the drainage system, and consequently raised the δ 15 N of nitrate in the Trang River water. Mangrove estuaries have long been recognized as major nursery areas of commercial importance (Odum and Heald, 1975). For example, there are many shrimp ponds in the Trang River watershed. Therefore, an additional possibility is that the drainage from fish cultivation facilities is contributing inorganic nitrogen and fragments of zooplankton, both enriched with 15 N, to the organic material in the Trang River. 5.3 Source analysis of sedimentary organic matter In general, δ 13 C, δ 15 N, and C/N ratios have been used as indices of terrestrial and marine organic matter because these measures are independently useful in discriminating between land plants and marine phytoplankton (Peters et al., 1978; Wada et al., 1987; Jasper and Gagosian, 1990). Figure 4 shows the observed relationship (a) between δ 13 C and the C/N ratio, and (b) between δ 15 N and δ 13 C for each category of organic samples. It appears that the coastal sediments from southwest Thailand were not simply controlled by the mixing of terrestrial and marine organic material, because δ 13 C values of coastal sediments in both the Ko Muk and Ko Talibong areas were enriched in 13 C compared to the coastal POM (Fig. 4(b)). It is known that the diagenetic alteration of organic matter causes both δ 13 C and δ 15 N to increase. However, the δ 15 N results of POM and sediments showed opposite trends to the δ 13 C results. The coastal sediment may be influenced by the other organic matter with high δ 13 C and low δ 15 N. Seagrasses are a likely source of organic material contributing to sediments because they have high δ 13 C values of 15 to 3 (Zieman et al., 1984; Wada et al., 1990; Yamamuro, 1999). Furthermore, Zieman et al. (1984) reported that the seagrasses show little change in δ 13 C and δ 15 N during decomposition. Chirapart and Yamamuro (1999) reported that the seagrasses are common in southwestern Thailand, and have δ 13 C and δ 15 N values ranging from 13 to 7 and 0 to +5, respectively. Therefore, the most likely explanation for the large δ 13 C and low δ 15 N of the coastal sediments in our study is that they received a significant contribution of organic matter from seagrasses. Although a specific feature such as seagrass is distinctive in this field, the results can also be interpreted as the typical mixing of major end-members from land and marine sources. Land plants and coastal POM represent two end-members characterizing the distribution of sedimentary organic matter from rivers to the coastal sea. Since coastal POM is mainly influenced by phytoplankton, it is used as an end-member for the water column from the surface water to the bottom Fig. 4. (a) Carbon isotopic composition and carbon to nitrogen atomic ratio (C/N ratio) for organic matter from along southwestern coast of Thailand. (b) Carbon and nitrogen isotopic composition of organic matter along southwestern coast of Thailand. Vertical and horizontal bars show 2σ range. sediment. Mangrove plants and seagrasses also supply organic matter, each with a specific δ 13 C and δ 15 N. Particulate organic matter in the Ko Talibong area had a significantly higher 15 N than that in the Ko Muk area. This isotopic peculiarity seems to extend into the coastal sediments in both areas. From these results, we conclude that the sedimentary organic matter along the southwestern coast of Thailand is composed of four isotopic endmembers: coastal POM, land plants, seagrasses, and mangrove plants. We have attempted to estimate the mass contribution of each of these organic sources to the coastal sediments. Since at least four end-members were identified, the ordinary mass balance analysis based on a double tracer system could not be applied. Instead, we used the stochastic method (a Monte Carlo simulation) that was developed to estimate the contribution by each organic source in a multi-component mixture (Minagawa, 1992). 428 T. Kuramoto and M. Minagawa

9 Fig. 5. A box-hinge plot of probability for each organic source end-member along southwestern coast of Thailand. Each box and lateral bar shows 25% and 75% probability range. Vertical line in the box shows the median value. Since the Monte Carlo method generates randomly mixed proportions of each source and then checks whether each case satisfies the isotopic condition, this approach can provide a possible range for the contributions of more than three sources. We assumed that the land plants and seagrasses had common isotopic features at Ko Muk and Ko Talibong, while coastal POM and mangrove were distinct between these two areas (Fig. 4). Since the stochastic method can give a number of possible combinations of probability for each end-member, the results are presented using a box-hinge plot to include the mean values ± the 25% probability range (Fig. 5). The schematic configuration of this model and the results are presented for (a) Ko Muk and (b) Ko Talibong areas in Fig. 6. The estimations obtained suggest that seagrasses play a significant role in the biogeochemical systems at both Ko Muk and Ko Talibong. If the assumption underlying the model is appropriate in this case, the results suggest that 42% and 36% of sedimentary organic matter originated from seagrasses at Ko Muk and Ko Talibong, respectively. Mangroves and land plants each seem to contribute about 23% of the organic matter in both areas. Coastal POM may supply more organic matter at Ko Talibong than at Ko Muk, but the proportion appears to be less than 20% of the total organic matter in both areas. These results imply that along the southwestern coast of Thailand, vascular plants, including land and mangrove plants, are the source of about 45% of the sedimentary organic matter. In addition, the contribution of seagrasses seems larger in mangrove vegetated area along the shoreline than in coastal seas under the influence of river discharge. We used carbon and nitrogen content to calculate the isotopic mass balance for the mixed organic matter. If we Fig. 6. Schematic configuration of contribution for each organic source estimated using stochastic method. calculate the C/N ratio by the mass balance estimation, the C/N atomic ratio of the mixture is higher (approximately 20) than that of the measured coastal sediments, suggesting that the carbon contribution of the seagrass fraction might be overestimated in the model. One reason may be the effect of diagenic degradation of source materials in the sediments. Furthermore, the C/N ratios of all end-members, except POM, are greater than 22 and such artifacts may require correction in order to estimate the real contributions of land plants, mangroves, and seagrasses, with a consequent decrease in the relative contribution by POM. To evaluate this quantitatively, we need basic information on isotopic change during the alteration of organic matter, but this information was not available to us in the present study. Benner et al. (1991) reported the results of isotopic fractionation occurring during experimental alteration of the seagrass Spartina alterniflora, and concluded that both 12 C and 14 N could be slightly enriched in any organic matter remaining during decomposition in the sediments. If the same isotopic shift occurs in our case, the model estimation should be revised to decrease the contribution of plants and POM and increase the contribution of seagrasses. We emphasize that further research is needed to evaluate such an effect due to the alteration of organic matter. Stable Carbon and Nitrogen Isotopic Characterization of Organic Matter on the Southwestern Coast of Thailand 429

10 6. Conclusion The δ 13 C and δ 15 N of biogeochemical samples from the southwestern coast of Thailand were measured for two different areas: Ko Muk, a natural mangrove shore, and Ko Talibong, which receives the discharge of a large river. The isotopic composition of POM, sediments, and mangrove plants was found to be significantly different between the two areas. The δ 15 N values of POM, mangroves, and sediments were relatively enriched in Ko Talibong area, suggesting that nitrogen from the Trang River is influenced by isotopic elements related to human activities. We estimated the relative contributions of four endmembers using a stochastic method based on the δ 13 C and δ 15 N values of the organic matter. The results implied that seagrasses were a major source of sedimentary organic matter, contributing 42 ± 5% in the Ko Muk area and 36 ± 5% in the Ko Talibong area. The contribution of coastal POM to the sediments was estimated to be only 13% at Ko Muk and 19% at Ko Talibong. There were no obvious differences between the two areas with respect to mangroves, which contribute approximately 23% to organic matter. It is concluded that seagrasses are an important source of sedimentary organic matter in the coastal regions of southwestern Thailand. Acknowledgements We thank Professor I. Koike at the University of Tokyo for his excellent coordination of this field study. Thanks are also due to Drs. M. Nakaoka, H. Iizumi, M. Yamamuro, K. Kogure, and T. Komatsu and Mr. Y. Umezawa, all in the same research group, for their kind cooperation in collecting samples. Prof. Khan and her staff at Kasetsartthe University classified the land plants. The staff in the Department of Fisheries of Kasetsartthe University and the Marine National Park Supporting Center in Thailand also helped to collect samples, for which we are indebted to them. 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