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Journal of Oceanography, Vol. 57, pp. 421 to 431, 2001

Keywords:⋅ δ13C,⋅ δ15N,⋅ southwesternThailand,

⋅ mangrove,⋅ seagrass,⋅ POM,⋅ sediment.

* Corresponding author. E-mail: t-kura@ees.hokudai.ac.jp

Copyright © The Oceanographic Society of Japan.

Stable Carbon and Nitrogen Isotopic Characterizationof Organic Matter in a Mangrove Ecosystem on theSouthwestern Coast of Thailand

TOSHIKATSU KURAMOTO* and MASAO MINAGAWA

Graduate School of Environmental Earth Science, Hokkaido University, Sapporo 060-0810, 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 carbonand nitrogen isotopic analyze, conducted on various organic samples, including landand mangrove plants, soils, particulate organic matter (POM), and sea and riversediments along the southwestern coast of Thailand. The δ13C values of land plantsand POM in river water can be explained in terms of a greater influence of C3 plantsthan C4 plants in this area. The POM and sediments from the Trang River and KoTalibong area showed systematically higher δ15N values than those from Ko Muk andother coastal areas. Organic matter in the Trang River might be influenced by nitro-gen released from agricultural or human waste, which could affect the isotopic com-position of POM and sediments in the Trang River estuary and along the coast nearthe river mouth. We used a stochastic method to estimate the contributions of fourorganic end-members, identifiable by their δ13C and δ15N values. The results impliedthat 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 ofcoastal POM to sediments was estimated to be only 13% in Ko Muk and 19% in KoTalibong. Mangrove plants contributed approximately 23% in both areas. It was con-cluded that seagrasses are an important source of sedimentary organic matter in thiscoastal region of southwestern Thailand.

trial C4 plants, which use PEP carboxylation, have asmaller δ13C fractionation of about –12‰ (O’Leary,1988). Seagrasses, which are marine vascular C3 plants,have δ13C values ranging from –15 to –3‰ (Zieman etal., 1984; Wada et al., 1990; Yamamuro, 1999). It is alsoknown that δ13C of phytoplankton changes with environ-mental factors, such as water temperature and ambientpCO2, as well as plankton growth rates (Rau et al., 1992;Goerike and Fry, 1994; Yoshioka, 1997).

The nitrogen isotope ratio (15N/14N, hereafter pre-sented as δ15N on the atmospheric nitrogen scale) ofparticulate organic matter (POM) depends on the frac-tions of organic (phytoplankton) and inorganic (availableas substrates for photosynthesis) nitrogen in the watercolumn (e.g., Miyake and Wada, 1971; Wada and Hattori,1978; Montoya and McCarthy, 1995). Previous work hasreported that nitrogen uptake processes often induce iso-topic fractionation in primary producers. For example,biological N2 fixation introduces 15N-depleted nitrogenof atmospheric origin to suspended matter by introduc-ing the low δ15N characteristic of atmospheric N2 (Sainoand Hattori, 1980). By contrast, denitrification increases

1. IntroductionCoastal seas occupy roughly 10% of the ocean and

sustain levels of primary production comparable to thoseof the world’s open oceans (e.g., Walsh, 1991). Duringthe last decade, many studies have focused on the poten-tial contribution of coastal seas to global carbon cycling.In coastal regions, high production is mainly due to thesupply of nutrients from rivers and the transport of or-ganic debris from land. We need to analyze organic sub-stances in coastal regions in order to evaluate the role ofterrestrial materials in marine biogeochemical systems.Stable carbon and nitrogen isotope ratios have been usedas indicators to estimate the proportion of terrestrial or-ganic material that contributes to marine organic matter(Peters et al., 1978). Terrestrial C3 plants, which useRubisco carboxylation during photosynthesis, have a car-bon isotope ratio (13C/12C, hereafter presented as δ13C onthe PDB scale) of approximately –27‰, while the terres-

422 T. Kuramoto and M. Minagawa

the δ15N of nitrate due to reduction of the nitrate to N2 orN2O, both of which processes induce large isotopicfractionation (e.g., Cline and Kaplan, 1975). Recently, ithas been suggested that nitrogen isotopes of nitrate andPOM vary systematically in watersheds, including bothpristine forested and agricultural catchments. For exam-ple, the nitrate in water draining from agricultural sitestends to contain high concentrations of 15N (Harringtonet al., 1998). Previous work has also reported thatparticulate organic nitrogen (PON) from estuarine waterssurrounded by densely populated areas tends to have aδ15N higher than 8‰ because organic waste from humanresidential areas often enhances denitrification in drain-age systems. Consequently, the nitrate is enriched in 15N,and this affects the δ15N of organic matter in the ecosys-tem (Mariotti et al., 1984; Cifuentes et al., 1988). Fur-thermore, nitrogen isotopes have been effectively usedin research to evaluate food web structures in ecosystems.Animals accumulate 15N in the body due to preferentialexcretion of 14N, thus causing a stepwise enrichment of

15N within food chains (DeNiro and Epstein, 1981;Minagawa and Wada, 1986).

Based on knowledge from modern analogues, stablecarbon and nitrogen isotopes have also been used forpaleoenvironmental studies. For example, Altabet et al.(1995) studied the δ15N of sediment cores to reconstructpast denitrification changes in the Arabian Sea. However,it is not known how organic matter in the pelagic sedi-ment of coastal areas records environmental changes.

In this study, land plants, POM, and surface sedi-ment samples were collected at two locations along thesouthwestern coast of Thailand. One area is influencedby river runoff from a watershed with a large populationand the other is a coastal area with no major river inflow.The purpose of the study was to demonstrate how carbonand nitrogen isotope analyses describe the mixing of ter-restrial and marine organic matter in the coastal sedimentsof two areas with differing degrees of human activity. Iso-topic analyses were conducted on land and marine plantsgrowing 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 areunderlined.

Stable Carbon and Nitrogen Isotopic Characterization of Organic Matter on the Southwestern Coast of Thailand 423

acterize terrestrial and marine organic matter for futurepaleoenvironmental research using sediment cores in theAndaman Sea.

2. LocationLand plants, tidal plants, and POM in river water and

seawater were collected near Haad Chao Mai NationalPark (7°12′–26′ N, 99°16′–31′ E) in Thailand. The sam-pling locations and station codes used in this study areshown in Fig. 1. This region of Thailand, drained by sev-eral rivers, the Trang River being the largest. The easternside of Ko Muk receives discharge from only a few smallrivers, but the area around Ko Talibong, located at themouth of the Trang River, is strongly influenced by riverwater. The cities of Trang and Kantang are located alongthe Trang River, and there are many charcoal factories,shrimp ponds, and fish paste factories near the cities.Mangrove is the predominant vegetation along bothshorefronts and riversides in this area, and plantations ofrubber and palm trees are common.

3. Materials and MethodsFresh leaves were collected from heights of 1–2 m

on land and mangrove plants and were washed carefullywith deionized water as soon as possible after collection.Each sample was dried in an electric oven at 60°C, andwas then powdered and sieved through a 250 µm mesh.

River water and seawater samples were collected with aVan Dorn water sampler (1~3 l). The water samples werefiltered through precombusted (450°C, 2 h) WhatmanGF/F filters to collect particulate organic matter (POM).The filters were dried in an electric dryer and then placedin a desiccator with 12 N HCl for half a day to allowdecarbonization. Surface soil samples from a rubber treeplantation, a forest near the national park, and a dune nearthe coast were collected with a shovel and stored inprecombusted glass bottles. The soil samples were driedin an electric oven at 60°C, before crushing and sievingthrough a 250 µm mesh to remove large plant debris andcoarse sand. SCUBA divers collected river and coastalsurface sediments, which were also stored inprecombusted glass bottles. Dry sediments were alsocrushed and sieved. All powdered soil and sediment sam-ples were decarbonized using 1 N HCl, rinsed with dis-tilled water to remove salt components, and then driedagain. The carbon and nitrogen isotopic ratios of land andmangrove leaves, POM, soils, and sediments were deter-mined by the flow-injection method using a Finnigan MAT252 mass spectrometer connected with a Fisons NA1500elemental analyzer. The results are presented using theconventional delta value notation, and calibrated to PDBstandards or atmospheric N2. The analytical error wasestimated to be within 0.2‰ for both δ13C and δ15N basedon replicate runs of an amino acid reagent.

Table 1. Elemental and isotopic analytical results for land and mangrove plants.

424 T. Kuramoto and M. Minagawa

4. ResultsLand and mangrove plants had δ13C within the com-

mon range for C3 plants, with values ranging from –31.2to –23.5‰ (average –28.2 ± 2.2‰, n = 8) for the landplants and from –29.0 to –25.4‰ (average –27.0 ± 1.3‰,n = 6) for the mangrove plants (Table 1). Thus, no sig-nificant difference in δ13C was found between land andmangrove plants. In contrast, the δ15N of these plants didshow a statistically significant difference (p = 0.008), witha 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‰) forthe mangrove plants. One unusual data point was obtainedfrom a land plant sample collected in a charcoal factoryalong the bank of a small river, and the sample was sus-pected of containing artifacts, such as animal waste, whichcaused the large δ15N. Nypa fruiticans and Duabangagradiflora, both from the Trang River bank sample, wereenriched in 15N compared to the other mangrove sam-ples. The C/N atomic ratios of the land and mangroveplants averaged 22.8 and 35.1, respectively, indicating that

Table 2. Elemental and isotopic analytical results for particulate organic matter (POM).

Stable Carbon and Nitrogen Isotopic Characterization of Organic Matter on the Southwestern Coast of Thailand 425

mangrove leaves have a higher carbon content than landplant leaves, although the variation was large among eachleaf type.

The δ13C of river POM varied from –27.1 to –24.0‰,with no obvious difference between large and small riv-

ers (Table 2). On the other hand, the δ15N of the riverPOM showed a large variation, between 3.1 and 7.7‰,with the higher values in the Trang River samples. Theδ13C of coastal POM ranged from –27.3 to –20.6‰, gen-erally 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 (%) δ13C (‰) δ15N (‰) C/N (mol mol–1) Remarks

Land soils12-3 (1) 0.042 0.0039 –23.9 2.5 12.4 bank12-7 1.3 0.091 –25.9 2.6 17.0 rubber plantation12-9 0.79 0.046 –27.8 2.9 20.1 rubber plantation16-1 3.5 0.21 –25.2 0.7 19.1 near national park

Average 1.4 0.088 –25.7 2.2 17.2s.d. 1.3 0.078 1.4 0.9 2.9

River sedimentsTrang river

9-3.5 3.2 0.22 –26.4 3.9 17.1 Trang river

Other9-1 0.31 0.023 –24.4 3.4 15.5 river mouth12-1 0.027 0.0028 –22.8 1.6 11.512-2 0.058 0.0051 –23.2 1.9 13.412-5 0.63 0.033 –24.3 3.3 22.1 river mouth12-12 3.4 0.15 –27.4 1.0 26.8

Average 1.3 0.1 –24.7 2.5 17.7s.d. 1.5 0.1 1.6 1.0 5.2

Coastal sedimentsKo Taribong area

9-10 0.35 0.022 –25.4 4.0 18.5 Trang river mouth9-13 0.15 0.014 –20.5 3.6 12.115-1 0.21 0.026 –19.6 4.7 9.715-2 0.076 0.0087 –20.6 3.4 10.115-3 0.11 0.012 –20.0 3.2 11.615-4 0.16 0.017 –21.0 4.7 10.515-5 0.17 0.016 –21.2 3.8 12.215-7 1.2 0.076 –25.2 3.3 18.1

Ko Muk area6-1 0.048 0.0058 –19.0 2.3 9.76-2 0.051 0.0064 –19.3 2.3 9.412-3 (2) 0.023 0.0028 –21.5 0.7 9.5 near bank14-1 0.12 0.014 –21.1 1.6 10.314-2 0.14 0.016 –21.6 2.2 9.814-3 0.10 0.0070 –23.8 1.4 16.914-9 0.062 0.0082 –18.4 2.4 8.814-10 0.092 0.0089 –19.8 1.1 12.014-12 0.14 0.013 –19.9 3.3 11.9

Average 0.19 0.016 –21.0 2.8 11.8s.d. 0.26 0.016 2.0 1.2 3.0

426 T. Kuramoto and M. Minagawa

negative value of δ13C for coastal POM was found in theKo Talibong area, which is attributed to the contributionof terrestrial organic matter. The δ15N of coastal POMvaried between +3.1 and +8.4‰, which is similar to thatof river POM. Coastal POM at stations 9-10 and 15-7near the Trang River mouth had larger δ15N than all othersamples. A significant difference in the δ15N of POM wasalso found between the Trang River and other rivers ofthe coastal region (p = 0.04).

The δ13C of soils ranged from –27.8 to –23.9‰ (Ta-ble 3). The largest δ13C was recorded for a sample fromStation 12-3 collected near a man-made seawall. Sincethis sample was composed of sandy soil, the total organiccarbon (TOC) content and the C/N ratio were both thelowest among all samples, at 0.04% and 12.4, respectively.The C/N ratios of the other soil samples ranged from 17.0to 20.1 and were lower than those of land and mangroveplants. The δ15N of soils ranged between 0.7 and 2.9‰,which is similar to the ranges found in previous reports(e.g., Heaton, 1986). The δ13C of the coastal sedimentsvaried between –25.4 and –19.0‰ (Table 3). Lower val-ues of δ13C (–25.4 and –25.2‰) were found at stations 9-10 and 15-7 near the Trang River estuary. The total or-ganic carbon content of river sediments varied widelyfrom 0.03 to 3.4%, while the samples from mangrove for-ests showed a relatively higher TOC of 3.2 to 3.5%. Theδ15N of river and coastal sediments varied from 1.0 to3.9‰ and from 0.7 to 4.3‰, respectively. Sediments andPOM from the Trang River and the Ko Talibong area haveδ15N values that higher than in the other samples (p <0.001).

5. Discussion

5.1 The δ13C of plants, POM and sedimentsThe analytical results for δ13C in the samples are pre-

sented in Fig. 2. The carbon isotopic compositions ob-tained in this study are consistent with previously pub-lished results in the other coastal regions. In this study,the average δ13C of mangrove plants was –27.0‰ with arange of –29.0 to –25.4‰. Previous studies have reportedthat δ13C values of mangrove plants vary between –33and –24‰ in southern Florida and Guadeloupe, FrenchWest Indies (Zieman et al., 1984; Lallier-Verges et al.,1998). Furthermore, Hayase et al. (1999) conducted iso-tope studies in the Matang mangrove forest in Malaysia,and reported that the δ13C values of mangrove leavesranged from –28.7 to –26.7‰.

It was speculated that C4 plants are widely distrib-uted in tropical areas of the world; therefore, we expectedthat C4 plants contribute to organic matter more or less.However, the average δ13C of land plants was –28.2‰,suggesting little contribution of C4 plants. Generally, Themean δ13C values of land soils and river POM were also

low (–25.7 and –26.1‰, respectively), except for onesandy soil sample. Given that C3 plants have a lower δ13Cthan C4 plants (O’Leary, 1988) and since much river POMis derived from terrestrial organic matter, such as land ormangrove plant debris, the low δ13C found in our soiland river POM samples probably reflects the dominanceof C3 plants in the studied region.

The one unusual land soil sample was collected at adune near a seawall that had been destroyed by a coastalwave (Table 3). Therefore, the organic fraction in this soilcould have originated from biological debris, such asmarine macrophytes, conveyed by the wave. The highδ13C value and high sand content of the sample also sup-port this explanation. The other soil samples showed val-ues similar to those of land plants.

River sediments from stations 9-3.5 and 12-12 hadrelatively low δ13C compared to the other samples andalso had high organic carbon contents. These stations werelocated by a river where mangrove plants are the predomi-nant vegetation. Tropical mangrove swamps are knownto cause high sedimentation rates in the surrounding ar-eas, due to their high production rates and the consequentaccumulation of organic carbon. Thus, the low δ13C val-ues of the river sediments from these stations presum-ably resulted from the contribution of mangrove plant

Fig. 2. The δ13C of organic matter from southwestern coast ofThailand. Vertical bars show average values and horizontalbars show 2σ range.

Stable Carbon and Nitrogen Isotopic Characterization of Organic Matter on the Southwestern Coast of Thailand 427

debris. The δ13C compositions of mangroves andsediments in this study are consistent with the previousobservation that mangrove swamp sediments have δ13Cvalues similar to or slightly larger than those of mangroveplants (Lallier-Verges et al., 1998).

The δ13C composition of coastal POM are consist-ent with the values of –23 to –17‰ previously reportedfor marine phytoplankton from an equatorial region (Rauet al., 1982). Particulate organic matter in coastal seawaterhad greater δ13C values than that in river water, with theexception of stations 9-10 and 9-12, both of which arelocated at the mouth of the Trang River. These two sta-tions are possibly strongly influenced by the Trang River,which is consistent with the indication from δ13C valuesthat coastal POM is composed of terrestrial organic mat-ter that originated mostly from C3 plants. The δ13C val-ues of surface sediments from the Trang River mouth (9-10 and 15-7) were also lower than those from sedimentsat the other locations, which also suggests a strong influ-ence by terrestrial organic matter.

5.2 The δ15N of plants, POM and sedimentsThe analytical results for δ15N in the samples are

presented in Fig. 3. The δ15N values for mangroves wereabout 1.8‰ higher than for land plants. Excluding theexceptionally large value of δ15N from the one samplementioned above, the difference in δ15N between man-groves and land plants is statistically significant (p <0.008). Since mangrove is a tropical shrub that grows onmuddy banks where river water and seawater mix, thenutrient sources for mangroves are different from thosefor land plants, and this could explain the observed dif-ference in δ15N. If riverine water or seawater were en-riched in 15N, mangroves would be expected to havehigher δ15N than land plants. Furthermore, specific ni-trogen diagenesis, such as denitrification, which occursnear the water-sediment interface, may be a possible causeof this enrichment. The δ15N values of mangroves (Nypafruiticans and Duabanga gradiflora) from the Trang Riverbanks were higher than in the other mangrove samples,suggesting that the nitrate of the Trang River is enrichedin 15N (Table 1). The δ15N of the Trang River POM wasalso higher than in the other rivers, suggesting that δ15Nof nitrate in Trang River water is greater than in seawateror water from the other small rivers.

The δ15N of coastal POM showed values similar topreviously reported values from the Indian Ocean, wherePOM ranged from +3.2 to +10.1‰ (Saino and Hattori,1980). The contribution made by N2 fixation was negli-gible in the area. Larger δ15N values were obtained atstations 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 δ15N between the Trang River area andthe Ko Muk area was also observed for coastal POM, river

sediments, and coastal sediments. These results thereforesuggest that nitrogen in the Trang River watershed mightbe enriched in 15N, and that its influence extends as far asthe coastal sea.

Harrington et al. (1998) reported that nitrate in thestreams draining agricultural areas had high δ15N valuesrelative to the streams flowing through pristine forestedareas, because the former has an anthropogenic influencessuch as the input of human and domesticated animalwastes (Kreitler and Jones, 1975). Our study area con-tains many plantations of rubber and palm trees. How-ever, the plant and soil samples from these plantationsshowed relatively low δ15N values (–2.4 to +2.9‰), whichmight reflect the use of commercial fertilizer (δ15N of –4to +4‰; Heaton, 1986) or atmospheric precipitation(Fogel and Paerl, 1993). Although the loss of volatileammonia from commercial fertilizers and manure canraise the δ15N of the residual nitrate locally (Kreitler,1979), this cannot explain the high δ15N observed in theentire Trang River system.

A possible cause of the high δ15N for organic matterin the Trang River is the input of domestic wastewaterfrom Trang and Kantang Cities. It is known that POMfrom estuarine waters located near high population areasis characterized by heavy δ15N (Mariotti et al., 1984;

Fig. 3. The δ15N of organic matter from southwestern coast ofThailand. Vertical bars show average values and horizontalbars show 2σ range.

428 T. Kuramoto and M. Minagawa

Cifuentes et al., 1988). The recent urbanization may haveenhanced denitrification within the drainage system, andconsequently raised the δ15N of nitrate in the Trang Riverwater.

Mangrove estuaries have long been recognized asmajor nursery areas of commercial importance (Odum andHeald, 1975). For example, there are many shrimp pondsin the Trang River watershed. Therefore, an additionalpossibility is that the drainage from fish cultivation fa-cilities is contributing inorganic nitrogen and fragmentsof zooplankton, both enriched with 15N, to the organicmaterial in the Trang River.

5.3 Source analysis of sedimentary organic matterIn general, δ13C, δ15N, and C/N ratios have been used

as indices of terrestrial and marine organic matter becausethese measures are independently useful in discriminat-ing between land plants and marine phytoplankton(Peters et al., 1978; Wada et al., 1987; Jasper andGagosian, 1990). Figure 4 shows the observed relation-ship (a) between δ13C and the C/N ratio, and (b) betweenδ15N and δ13C for each category of organic samples. Itappears that the coastal sediments from southwest Thai-land were not simply controlled by the mixing of terres-trial and marine organic material, because δ13C values ofcoastal sediments in both the Ko Muk and Ko Talibongareas were enriched in 13C compared to the coastal POM(Fig. 4(b)). It is known that the diagenetic alteration oforganic matter causes both δ13C and δ15N to increase.However, the δ15N results of POM and sediments showedopposite trends to the δ13C results. The coastal sedimentmay be influenced by the other organic matter with highδ13C and low δ15N.

Seagrasses are a likely source of organic materialcontributing to sediments because they have high δ13Cvalues 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 inδ13C and δ15N during decomposition. Chirapart andYamamuro (1999) reported that the seagrasses are com-mon in southwestern Thailand, and have δ13C and δ15Nvalues ranging from –13 to –7‰ and 0 to +5‰, respec-tively. Therefore, the most likely explanation for the largeδ13C and low δ15N of the coastal sediments in our studyis that they received a significant contribution of organicmatter from seagrasses. Although a specific feature suchas seagrass is distinctive in this field, the results can alsobe interpreted as the typical mixing of major end-mem-bers from land and marine sources. Land plants andcoastal POM represent two end-members characterizingthe distribution of sedimentary organic matter from riv-ers to the coastal sea. Since coastal POM is mainly influ-enced by phytoplankton, it is used as an end-member forthe water column from the surface water to the bottom

sediment. Mangrove plants and seagrasses also supplyorganic matter, each with a specific δ13C and δ15N.Particulate organic matter in the Ko Talibong area had asignificantly higher 15N than that in the Ko Muk area.This isotopic peculiarity seems to extend into the coastalsediments in both areas. From these results, we concludethat the sedimentary organic matter along the southwest-ern coast of Thailand is composed of four isotopic end-members: coastal POM, land plants, seagrasses, and man-grove plants. We have attempted to estimate the masscontribution of each of these organic sources to the coastalsediments.

Since at least four end-members were identified, theordinary mass balance analysis based on a double tracersystem could not be applied. Instead, we used thestochastic method (a Monte Carlo simulation) that wasdeveloped to estimate the contribution by each organicsource in a multi-component mixture (Minagawa, 1992).

Fig. 4. (a) Carbon isotopic composition and carbon to nitrogenatomic ratio (C/N ratio) for organic matter from along south-western coast of Thailand. (b) Carbon and nitrogen isotopiccomposition of organic matter along southwestern coast ofThailand. Vertical and horizontal bars show 2σ range.

Stable Carbon and Nitrogen Isotopic Characterization of Organic Matter on the Southwestern Coast of Thailand 429

Since the Monte Carlo method generates randomly mixedproportions of each source and then checks whether eachcase satisfies the isotopic condition, this approach canprovide a possible range for the contributions of morethan three sources. We assumed that the land plants andseagrasses had common isotopic features at Ko Muk andKo Talibong, while coastal POM and mangrove were dis-tinct between these two areas (Fig. 4). Since the stochasticmethod can give a number of possible combinations ofprobability for each end-member, the results are presentedusing a box-hinge plot to include the mean values ± the25% probability range (Fig. 5). The schematic configu-ration 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 playa significant role in the biogeochemical systems at bothKo Muk and Ko Talibong. If the assumption underlyingthe model is appropriate in this case, the results suggestthat 42% and 36% of sedimentary organic matter origi-nated from seagrasses at Ko Muk and Ko Talibong, re-spectively. Mangroves and land plants each seem to con-tribute about 23% of the organic matter in both areas.Coastal POM may supply more organic matter at KoTalibong than at Ko Muk, but the proportion appears tobe less than 20% of the total organic matter in both areas.These results imply that along the southwestern coast ofThailand, vascular plants, including land and mangroveplants, are the source of about 45% of the sedimentaryorganic matter. In addition, the contribution of seagrassesseems larger in mangrove vegetated area along the shore-line than in coastal seas under the influence of river dis-charge.

We used carbon and nitrogen content to calculate theisotopic mass balance for the mixed organic matter. If we

calculate the C/N ratio by the mass balance estimation,the C/N atomic ratio of the mixture is higher (approxi-mately 20) than that of the measured coastal sediments,suggesting that the carbon contribution of the seagrassfraction might be overestimated in the model. One rea-son may be the effect of diagenic degradation of sourcematerials in the sediments. Furthermore, the C/N ratiosof all end-members, except POM, are greater than 22 andsuch artifacts may require correction in order to estimatethe real contributions of land plants, mangroves, andseagrasses, with a consequent decrease in the relativecontribution by POM. To evaluate this quantitatively, weneed basic information on isotopic change during the al-teration of organic matter, but this information was notavailable to us in the present study. Benner et al. (1991)reported the results of isotopic fractionation occurringduring experimental alteration of the seagrass Spartinaalterniflora, and concluded that both 12C and 14N couldbe slightly enriched in any organic matter remaining dur-ing decomposition in the sediments. If the same isotopicshift occurs in our case, the model estimation should berevised to decrease the contribution of plants and POMand increase the contribution of seagrasses. We empha-size that further research is needed to evaluate such aneffect due to the alteration of organic matter.

Fig. 6. Schematic configuration of contribution for each or-ganic source estimated using stochastic method.

Fig. 5. A box-hinge plot of probability for each organic sourceend-member along southwestern coast of Thailand. Eachbox and lateral bar shows 25% and 75% probability range.Vertical line in the box shows the median value.

430 T. Kuramoto and M. Minagawa

6. ConclusionThe δ13C and δ15N of biogeochemical samples from

the southwestern coast of Thailand were measured for twodifferent areas: Ko Muk, a natural mangrove shore, andKo Talibong, which receives the discharge of a large river.The isotopic composition of POM, sediments, and man-grove plants was found to be significantly different be-tween the two areas. The δ15N values of POM, mangroves,and sediments were relatively enriched in Ko Talibongarea, suggesting that nitrogen from the Trang River isinfluenced by isotopic elements related to human activi-ties. We estimated the relative contributions of four end-members using a stochastic method based on the δ13Cand δ15N values of the organic matter. The results im-plied that seagrasses were a major source of sedimentaryorganic matter, contributing 42 ± 5% in the Ko Muk areaand 36 ± 5% in the Ko Talibong area. The contribution ofcoastal POM to the sediments was estimated to be only13% at Ko Muk and 19% at Ko Talibong. There were noobvious differences between the two areas with respectto mangroves, which contribute approximately 23% toorganic matter. It is concluded that seagrasses are an im-portant source of sedimentary organic matter in the coastalregions of southwestern Thailand.

AcknowledgementsWe 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 kindcooperation in collecting samples. Prof. Khan and her staffat Kasetsartthe University classified the land plants. Thestaff in the Department of Fisheries of Kasetsartthe Uni-versity and the Marine National Park Supporting Centerin Thailand also helped to collect samples, for which weare indebted to them. This study was supported by a Grant-in-Aid for international scientific research programs (No.09041147) from the Ministry of Education, Science,Sports and Culture, of Japan.

ReferencesAltabet, M. A., R. Francois, D. W. Murray and W. L. Prell

(1995): Climate-related variations in denitrification in theArabian Sea from sediment 15N/14N ratios. Nature, 373,506–509.

Benner, R., M. L. Fogel and E. K. Sprague (1991): Diagenesisof belowground biomass of Spartina alterniflora in salt-marsh sediments. Limnol. Oceanogr., 36, 1358–1374.

Chirapart, A. and M. Yamamuro (1999): Chemical composi-tions of the seagrasses species from Haad Chao Mai Na-tional Park. p. 122–141. In Effects of Grazing and Distur-bance by Dugons and Turtles on Tropical Seagrass Ecosys-tems, ed. by I. Koike, Ocean Research Institute, Universityof Tokyo.

Cifuentes, L. A., J. H. Sharp and M. L. Fogel (1988): Stablecarbon and nitrogen isotope biogeochemistry in the Dela-ware estuary. Limnol. Oceanogr., 33, 1102–1115.

Cline, J. D. and I. R. Kaplan (1975): Isotopic fractionation ofdissolved nitrate during denitrification in the eastern tropi-cal north Pacific Ocean. Mar. Chem., 3, 271–299.

DeNiro, M. J. and S. Epstein (1981): Influence of diet on thedistribution of nitrogen isotopes in animals. Geochim.Cosmochim. Acta, 45, 341–351.

Fogel, M. L. and H. W. Paerl (1993): Isotopic tracers of nitro-gen from atmospheric deposition to coastal waters. Chem.Geol., 107, 233–236.

Goerike, R. and B. Fry (1994): Variations of marine planktonδ13C with latitude, temperature, and dissolved CO2 in theworld ocean. Global Biogeochem. Cycle, 8, 85–90.

Harrington, R. R., B. P. Kennedy, C. P. Chamberlain, J. D. Blumand C. L. Folt (1998): 15N enrichment in agricultural catch-ments: field patterns and applications to tracking Atlanticsalmon (Salmo salar). Chem. Geol., 147, 281–294.

Hayase, S., T. Ichikawa and K. Tanaka (1999): Preliminary re-port on stable isotope ratio analysis for samples from Matangmangrove brackish water ecosystems. J. A. R. Q., 33, 215–221.

Heaton, T. H. E. (1986): Isotopic studies of nitrogen pollutionin the hydrosphere and atmosphere: a review. Chem. Geol.,59, 87–102.

Jasper, J. P. and R. B. Gagosian (1990): The source and deposi-tion of organic matter in the Late Quaternary Pigmy Basin,Gulf of Mexico. Geochim. Cosmochim. Acta, 54, 1117–1132.

Kreitler, C. W. (1979): Nitrogen-isotope studies of soil andgroundwater nitrate from alluvial fan aquifers in Texas. J.Hydrolog., 42, 147–170.

Kreitler, C. W. and D. C. Jones (1975): Natural soil nitrate: thecause of the nitrate contamination of ground water in Run-nels County, Texas. Ground Water, 13, 53–61.

Lallier-Verges, E., B. P. Perrussel., J. Disnar and F. Baltzer(1998): Relationships between environmental conditionsand the diagenetic evolution of organic matter derived fromhigher plants in a modern mangrove swamp system(Guadeloupe, French West Indies). Org. Geochem., 29,1663–1686.

Mariotti, A., C. Lancelot and G. Billen (1984): Natural isotopiccomposition of nitrogen as a tracer of origin for suspendedorganic matter in the Scheldt estuary. Geochim. Cosmochim.Acta, 48, 549–555.

Minagawa, M. (1992): Reconstruction of human diet from δ13Cand δ15N in contemporary Japanese hair: a stochastic methodfor estimating multi-source contribution by double isotopictracers. Appl. Geochem., 7, 145–158.

Minagawa, M. and E. Wada (1986): Nitrogen isotope ratios ofred tide organisms in the East China Sea: a characterizationof biological nitrogen fixation. Mar. Chem., 19, 245–259.

Miyake, Y. and E. Wada (1971): The isotopic effect on the ni-trogen in biochemical, oxidation-reduction reactions.Records of Oceanographic Works in Japan, 11, 1–6.

Montoya, J. P. and J. J. McCarthy (1995): Nitrogen isotopefractionation during uptake by marine phytoplankton incontinuous culture. J. Plankton Res., 17, 439–464.

Stable Carbon and Nitrogen Isotopic Characterization of Organic Matter on the Southwestern Coast of Thailand 431

Odum, W. E. and E. J. Heald (1975): The detritus-based foodweb on an estuarine mangrove community. p. 265–286. InEstuarine Research, Vol. 1, Chemistry and Biology and theEstuarine System, ed. by L. E. Cronin, Academic Press, Inc.,New York.

O’Leary, M. H. (1988): Carbon isotopes in photosynthesis.Biosci., 38, 328–336.

Peters, K. E., R. E. Sweeney and I. R. Kaplan (1978): Correla-tion of carbon and nitrogen stable isotope ratios in sedi-mentary organic matter. Limnol. Oceanogr., 23, 598–604.

Rau, G. H., R. E. Sweeney and I. R. Kaplan (1982): Plankton13C:12C ratio changes with latitude: difference betweennorthern and southern oceans. Deep-Sea Res., 29, 1035–1039.

Rau, G. H., T. Takahashi, D. J. Des Marais, D. J. Repeta and J.H. Martin (1992): The relationship between organic matterδ13C and [CO2(aq)] in ocean surface water; data from aJGOFS site in Northeast Atlantic Ocean and a model.Geochim. Cosmochim. Acta, 56, 1413–1419.

Saino, T. and A. Hattori (1980): 15N abundance in oceanic sus-pended particulate matter. Nature, 283, 752–754.

Wada, E. and A. Hattori (1978): Nitrogen isotope effects in theassimilation of inorganic nitrogenous compounds by ma-

rine diatoms. Geomicrobiol. J., 1, 85–101.Wada, E., M. Minagawa, H. Mizutani, T. Tsuji, R. Imaizumi

and K. Karasawa (1987): Biogeochemical studies of thetransport of organic matter along the Otsuchi river water-shed, Japan. Estuar. Coast. Shelf Sci., 25, 321–336.

Wada, E., Y. Kabaya, K. Tsuru and R. Ishiwatari (1990): 13Cand 15N abundance of sedimentary organic matter in estua-rine areas of Tokyo Bay, Japan. Mass Spectroscopy, 38, 307–318.

Walsh, J. J. (1991): Importance of continental margins in themarine biogeochemical cycling of carbon and nitrogen.Nature, 350, 53–55.

Yamamuro, M. (1999): Importance of epiphytic cyanobacteriaas food sources for heterotrophs in a tropical seagrass bed.Coral Reefs, 18, 122–141.

Yoshioka, T. (1997): Phytoplanktonic carbon isotopefractionation: equations accounting for CO2-concentratingmechanisms. J. Plankton Res., 19, 1455–1497.

Zieman, J. C., S. A. Macko and A. L. Mills (1984): Role ofseagrasses and mangroves in estuarine food webs: tempo-ral and spatial changes in stable isotope composition andamino acid content during decomposition. Bull. Mar. Sci.,35, 380–392.