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Journal of Oceanography, Vol. 57, pp. 421 to 431, 2001
Keywords: ⋅ δ13C, ⋅ δ15N, ⋅ southwestern Thailand,
⋅ mangrove, ⋅ seagrass, ⋅ POM, ⋅ sediment.
* Corresponding author. E-mail: firstname.lastname@example.org
Copyright © The Oceanographic Society of Japan.
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 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 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 δ13C values of land plants and POM in river water can be explained in terms of a greater influence of C3 plants than C4 plants in this area. The POM and sediments from the Trang River and Ko Talibong area showed systematically higher δ15N values than those from Ko Muk and other 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 near the river mouth. We used a stochastic method to estimate the contributions of four organic end-members, identifiable by their δ13C and δ15N 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 con- cluded that seagrasses are an important source of sedimentary organic matter in this coastal region of southwestern Thailand.
trial C4 plants, which use PEP carboxylation, have a smaller δ13C fractionation of about –12‰ (O’Leary, 1988). Seagrasses, which are marine vascular C3 plants, have δ13C values ranging from –15 to –3‰ (Zieman et al., 1984; Wada et al., 1990; Yamamuro, 1999). It is also known that δ13C of phytoplankton changes with environ- mental factors, such as water temperature and ambient pCO2, 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) of particulate organic matter (POM) depends on the frac- tions 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 iso- topic fractionation in primary producers. For example, biological N2 fixation introduces
15N-depleted nitrogen of atmospheric origin to suspended matter by introduc- ing the low δ15N characteristic of atmospheric N2 (Saino and Hattori, 1980). By contrast, denitrification increases
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 poten- tial 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 or- ganic debris from land. We need to analyze organic sub- stances 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 or- ganic material that contributes to marine organic matter (Peters et al., 1978). Terrestrial C3 plants, which use Rubisco carboxylation during photosynthesis, have a car- bon isotope ratio (13C/12C, hereafter presented as δ13C on the 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 or N2O, 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 exam- ple, the nitrate in water draining from agricultural sites tends to contain high concentrations of 15N (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 δ15N higher than 8‰ because organic waste from human residential 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 used in research to evaluate food web structures in ecosystems. Animals accumulate 15N in the body due to preferential excretion 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, stable carbon and nitrogen isotopes have also been used for paleoenvironmental studies. For example, Altabet et al. (1995) studied the δ15N of sediment cores to reconstruct past 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 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 ter- restrial and marine organic matter in the coastal sediments of two areas with differing degrees of human activity. Iso- topic 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.
Stable Carbon and Nitrogen Isotopic Characterization of Organic Matter on the Southwestern Coast of Thailand 423
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 (7°12′–26′ N, 99°16′–31′ E) in Thailand. The sam- pling locations and station codes used in this study are shown in Fig. 1. This region of Thailand, drained by sev- eral 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 sam- ples were decarbonized using 1 N HCl, rinsed with dis- tilled 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 deter- mined 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 N2. The analytical error was estimated to be within 0.2‰ for both δ13C and δ15N based on 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. Results Land and mangrove