DSpace at VNU: A cross-system analysis of sedimentary organic carbon in the mangrove ecosystems of Xuan Thuy National Park, Vietnam

Journal of Sea Research 67 (2012) 69–76 Contents lists available at SciVerse ScienceDirect Journal of Sea Research journal homepage: www.elsevier.com/locate/seares A cross-system analysis of sedimentary organic carbon in the mangrove ecosystems of Xuan Thuy National Park, Vietnam Nguyen Tai Tue a, b,⁎, Nguyen Thi Ngoc c, Tran Dang Quy d, Hideki Hamaoka b, Mai Trong Nhuan d, Koji Omori b a Graduate School of Science and Engineering, Ehime University, 2-5 Bunkyo-cho, Matsuyama, Japan Center for Marine Environmental Studies, Ehime University, 2-5 Bunkyo-cho, Matsuyama, Japan Graduate School of Science and Technology, Kumamoto University, Kumamoto 860-8555, Japan d Faculty of Geology, Hanoi University of Science, 334 Nguyen Trai, Thanh Xuan, Hanoi, Viet nam b c a r t i c l e i n f o Article history: Received 19 June 2011 Received in revised form 16 October 2011 Accepted 20 October 2011 Available online 12 November 2011 Keywords: Mangrove ecosystems Sediment Organic carbon C/N δ13C Xuan Thuy National Park a b s t r a c t A cross-system analysis of bulk sediment composition, total organic carbon (TOC), atomic C/N ratio, and carbon isotope composition (δ13C) in 82 surface sediment samples from natural and planted mangrove forests, bank and bottom of tidal creeks, tidal flat, and the subtidal habitat was conducted to examine the roles of mangroves in sedimentation and organic carbon (OC) accumulation processes, and to characterize sources of sedimentary OC of the mangrove ecosystem of Xuan Thuy National Park, Vietnam Sediment grain sizes varied widely from 5.4 to 170.2 μm (mean 71.5 μm), with the fine sediment grain size fraction (b 63 μm) ranging from 11 to 99.3% (mean 72.5%) Bulk sediment composition suggested that mangroves play an important role in trapping fine sediments from river outflows and tidal water by the mechanisms of tidal current attenuation by vegetation and the ability of fine roots to bind sediments The TOC content ranged from 0.08 to 2.18% (mean 0.78%), and was higher within mangrove forests compared to those of banks and bottoms of tidal creeks, tidal flat, and subtidal sediments The sedimentary δ13C ranged from −27.7 to −20.4‰ (mean −24.1‰), and mirrored the trend observed in TOC variation The TOC and δ13C relationship showed that the factors of microbial remineralization and OC sources controlled the TOC pool of mangrove sediments The comparison of δ13C and C/N ratio of sedimentary OC with those of mangrove and marine phytoplankton sources indicated that the sedimentary OC within mangrove forests and the subtidal habitat was mainly composed of mangrove and marine phytoplankton sources, respectively The application of a simple mixing model showed that the mangrove contribution to sedimentary OC decreased as follows: natural mangrove forest> planted mangrove forest> tidal flat> creek bank> creek bottom > subtidal habitat © 2011 Elsevier B.V All rights reserved Introduction Mangrove forests occur along ocean coastlines throughout the tropics and subtropics, and they support numerous ecosystem services such as nursery grounds for commercial and ecologically important fish, shrimp and shellfish, nesting and foraging habitat of migratory birds, and as a renewable resource of fuel (Alongi, 2011) The total net primary production of mangrove ecosystems has been estimated at 218±72 Tg C/year (Bouillon et al., 2008; Twilley et al., 1992), ranking as one of the most productive biomes on earth As result, the mangrove ecosystem is an important sink (Eong, 1993; Twilley et al., 1992) and source (Mfilinge et al., 2005; Rodelli et al., 1984) of organic carbon (OC) In term of the OC sink, Donato et al (2011) showed that whole-ecosystem carbon storage of the ⁎ Corresponding author at: 790-8577 Center for Marine Environmental Studies, Ehime University, 2-5 Bunkyo-cho, Matsuyama, Japan Tel.: + 81 89 927 9643, + 81 902 894 1610 (mobile); fax: + 81 89 927 9643 E-mail addresses: tuenguyentai@gmail.com, tuent@sci.ehime-u.ac.jp (N.T Tue) 1385-1101/$ – see front matter © 2011 Elsevier B.V All rights reserved doi:10.1016/j.seares.2011.10.006 Indo-Pacific mangrove forests consists of tree and detrital organic matter, and sedimentary OC containing on average 1023 Mg C/ha As a result mangroves are among the most carbon-rich forests in the tropics Therefore, mangrove OC is an important factor in the global and local OC budgets (Duarte et al., 2005) However, mangrove ecosystems are ecologically diverse and their carbon storage in sediments can vary widely between ecosystems, from under to b40%, with the global median value at 2.2% (Kristensen et al., 2008) The global carbon budget extrapolations are consequently biased (Bouillon et al., 2008) Therefore, the level and dynamics of OC storage in sediments of individual mangrove ecosystem are needed to better assess the global carbon budget This is particularly so in mangrove ecosystems of Vietnam where little data have been published (i.e., Tue et al., 2011a,b) The storage of OC in mangrove sediments is dependent on several factors such as the sources of OC (Bouillon et al., 2003) and the presence of mangroves (Donato et al., 2011) Sedimentary OC can originate from local production by mangroves and/or tidally suspended organic matter (Bouillon et al., 2003; Kristensen, et al., 2008) Therefore, the 70 N.T Tue et al / Journal of Sea Research 67 (2012) 69–76 contribution of each OC source is needed to assess the mangrove contribution Additionally, the presence of mangroves will increase the sedimentation rates, and consequently OC burial in sediments (Perry and Berkeley, 2009) Thus, it is necessary to understand the driving forces behind the OC accumulation in the mangrove ecosystem Mangrove ecosystems comprise of habitats, including intertidal mangroves, tidal creeks, creek banks, tidal flat, and subtidal zone The sedimentary OC pools in these habitats may be distinguished by differences in their respective OC sources (Tue et al., 2011a) and by tidal amplitude (Bouillon et al., 2003) Therefore, a cross-system analysis of sedimentary OC is needed to understand the biogeochemical cycling of OC through the whole mangrove ecosystem Furthermore, the amount and sources of sedimentary OC in these habitats should be determined for mangrove and coastal food web studies (Bouillon et al., 2002) and paleoenvironmental reconstruction (Tue et al., 2011a) In present study we examine the hypothesis of whether mangroves boost the sedimentation and OC accumulation processes in an estuarine mangrove ecosystem We investigate a cross-system analysis of bulk sediment composition, TOC, C/N ratio, and δ13C in 82 surface sediment samples from natural and planted mangrove forests, bank and bottom of tidal creeks, tidal flat, and subtidal habitat for (1) examining the roles of mangroves in sedimentation and OC accumulation processes, and (2) characterizing the sources of sedimentary OC in the mangrove ecosystem Materials and methods 2.1 Study area The present work was conducted in an estuarine mangrove ecosystem of Xuan Thuy National Park (XTP) in northern Vietnam (Fig 1) The XTP is located along the southern part of the Ba Lat Estuary of the Red River which is the largest river in northern Vietnam The XTP covers a total wetland area of 12,000 ha, of which about 3000 are covered by mangrove forests Generally, mangroves in the XTP can be classified into natural and planted mangrove forests The natural mangrove forests are mainly distributed in the northern area of the XTP, dominated by the mangrove species Sonneratia caseolaris, Kandelia obovata, Aegiceras corniculatum, and Avicennia marina The planted mangrove forests are mainly distributed in the southern part of the XTP, dominated by K obovata (Hong et al., 2004) The mangrove ecosystems of the XTP are a valuable ecological and economic resource, providing essential nursery grounds for many species of fishes, invertebrates, and waterfowl The mangrove ecosystems are therefore a major stopover for migratory birds between northern and southern Asia (Thuy, 2004) Additionally, the mangroves also provide renewable fuel, and directly support the livelihood for local communities The XTP is recognized as a hotspot of biodiversity in Vietnam, and as a result the Convention on Wetlands of International Importance declared XTP as the first Ramsar Site of Southeast Asia (http://www.ramsar.org) 2.2 Field sampling Field work was conducted from 10 to 25 June, 2010 in the XTP A total of 82 surface sediment samples were collected across a broad range of mangrove habitats, comprising natural and planted mangrove forests, creek bank, creek bottom, tidal flat, and subtidal habitat The respective numbers of sediment samples of each the habitats were 12, 20, 5, 5, 13, and 27, and their locations are shown in Fig These samples represented all major habitat types of the mangrove ecosystem in the XTP, and as such well reflected the OC sources, hydrodynamic conditions, as well as marine-mangrove Fig Map of Xuan Thuy National Park, and the sampling sites The cross-system sampling sites are assigned as M: natural mangroves; P: planted mangroves; CB: creek bank; C: creek bottom; T: tidal flat; and S: subtidal habitat The digit number assigns the order number of sediment samples in each habitat N.T Tue et al / Journal of Sea Research 67 (2012) 69–76 interactions Surface sediments (0–2 cm) were collected by a stainless steel spade during low tide Sediment samples were packed in labeled polyethylene bags for further analysis Samples were immediately stored in iceboxes and transported to the laboratory where they were frozen at −20 °C until analysis 2.3 Sample preparation and analysis 2.3.1 Sediment grain size analysis Sediments were firstly dried in an electric oven at 105 °C for 48 h Sediment grain sizes were analyzed by sieve and pipette methods that are used for sand-rich and mud-rich samples, respectively For the sieve method, a total of 20 g of dried sand-rich sediment was wet-sieved using a mesh size of 0.063 mm to get rid of mud from the sediments Particles coarser than 0.063 mm were collected and dried in an electric oven at 105 °C for 24 h, and then were gently pounded with fingers The particles were passed through a series of sieves with mesh sizes of 0.50, 0.25, 0.125 and 0.063 mm by a sieve shaker (AS 200 Retsch, Germany) The particles retained on each sieve were weighed and converted into a percentage of the total sediment sample For pipette analysis, the fine particles passing through the 0.063 mm mesh were poured into a liter glass cylinder Distilled water was then added to bring it up exactly to 1000 ml The water column was vigorously stirred by a glass rod until all of the material was uniformly suspended throughout the water column Once completed, a pipette was inserted to a depth of 20 cm and exactly 25 ml of water was withdrawn at time intervals of 40 s, 16 min, 59 min, and 15 h The suspension samples were expelled into weighed 50 ml beakers, and then the water completely evaporated from the beakers using an electric oven at 105 °C The dried beakers and particles were re-weighed, and the particle weights were determined by subtracting the beakers from the dried beakers and particles The weights of these fractions were converted into a percentage of the total sediment sample These fractions corresponded to mesh sizes of 0.01, 0.005, 0.001, and b0.001 mm 2.3.2 Stable isotope and C/N analysis For δ 13C, TOC, and C/N ratio analysis, sediment samples were first completely dried in an electric oven at 60 °C, and then ground to a fine powder by an agate mortar and pestle A total of 200 mg of pulverized sediment sample was then placed in a microtube and approximately ml of N HCl was added and repeated two times, mixed thoroughly using a vibrating mixer, and then left at room temperature for 24 h to remove carbonates After acid treatment, the samples were thoroughly rinsed with milli-Q filtered distilled-deionized water and dried in an electric oven at 60 °C for 48 h Stable isotope δ 13C, TOC, and total nitrogen (TN) values were measured by using a stable isotope mass spectrometer (ANCA-SL, PDZ Europa, Ltd.) at the Center for Marine Environmental Studies, Ehime University, Japan δ 13C was expressed in ‰ (permil) deviations from the standard value by the following Eq (1): 13 Cị ẳ Rsample 1ị ì 1000 Rstandard 1ị where R = 13C/ 12C, Rsample is the isotope ratio of the sample, and Rstandard is the isotope ratio of a standard referenced to Pee Dee Belemnite (PDB) limestone carbonate The analytical error was± 0.1‰ for δ 13C 71 seagrasses, and other tidal organic matters Variations in the proportional contributions of these organic matter sources can cause a change in the sedimentary δ 13C values To determine the relative contribution of each organic matter source to sedimentary OC pool, a simple mixing model has been successfully used in mangrove ecosystems (i.e., Tue et al., 2011a), estuarine environments (i.e., He et al., 2010; Yu et al., 2010), and marine environments (i.e., Meksumpun et al., 2005) The simple mixing model can be written as below: 13 13 13 13 13 Csed ẳ f MG ì CMG ỵf MaA ì CMaA ỵf MiA ì CMiA ỵf S ì CS ỵf P ì 13 CP ỵỵf n ì 13 Cn f MG ỵf MaA ỵf MiA ỵf S ỵf P ỵf n ẳ 100% ð2Þ where fMG, fMaA, fMiA, fS, fP, and fn are the relative contribution of mangrove tissues, macroalgae, microalgae, seagrasses, marine phytoplankton, and n source (%), respectively; δ 13Csed, δ 13CMG, δ 13CMaA, δ 13CMiA, δ 13CS, δ 13CP, and δ 13Cn are the carbon stable isotope values of sedimentary OC, mangrove tissues, macroalgae, microalgae, seagrasses, marine phytoplankton, and n sources, respectively The Ba Lat Estuary is a highly turbid estuary (van Maren, 2007), and as a result the local presence of aquatic macrophytes, seagrasses, and macroalgae is very low to absent (Tue et al., 2011a,b) In addition, the production of benthic microalgae within mangrove forests is usually very low, not only due to light limitation but also to inhibition by soluble tannins (Bouillon et al., 2000; Robertson and Alongi, 1992) When there are only two sources (e.g., mangrove litters and marine phytoplankton), substitution of fP= 100% - fMG in Eq (2), we have: f MG %ị ẳ 13 Csed 13 CP ì 100 13 CMG 13 CP 3ị Results 3.1 Bulk sediment composition Sediments in the present study were composed of fine sand, silt, and clay The fractions of these compositions decreased from silt, through clay, and to fine sand Plotting the percentages of these compositions on a ternary diagram shows that the main sediment facies can be classified into clayed silt, silt, sandy silt, silty sand, and fine sand (Fig 2A) The silt composition was predominant, ranging from 11 to 90% The silt fraction was greater than 60% in sediments of mangrove forests, banks and bottoms of creeks, but markedly dropped to less than 40% in subtidal sediments (Fig 2B) The clay fraction varied similarity to the variation pattern of silt (Fig 2B), with high fractions in sediments of mangrove forests, and the bank and bottom of creeks, whereas it was very low in subtidal sediments The sand fraction ranged from 0.7 to 89%, and displayed an inverse trend to that of silt and clay The sand fraction was very high in subtidal sediments, and decreased to less than 20% in sediments of mangrove forests, and in the banks and bottoms of creeks (Fig 2B) Sediment grain sizes varied widely from 5.4 to 170.2 μm, with a mean of 71.5 μm On average, the fine sediment grain size fraction (b63 μm, mean ± SD) decreased as follows: planted mangroves, creek bank, creek bottom, natural mangroves, tidal flat, and the subtidal habitat, with respective values of 93.7 ± 7.7, 95.4 ± 5.5, 92.6 ± 4.8, 88.7 ± 14.3, 81.8 ± 19, and 37.3 ± 34.7% (Figs 3A and 4A) 2.4 Fractional contribution of organic carbon sources 3.2 Total organic carbon (TOC) content, C/N ratios, and carbon isotope composition (δ 13C) The OC sources to mangrove sediments can originate from both autochthonous production such as mangroves, macroalgae and microalgae, and allochthonous sources such as phytoplankton, The TOC content (% dry weight) of sediments from natural and planted mangrove forests, creek bank, creek bottom, tidal flat, and the subtidal habitat are shown in Figs 3B and 4B Overall, the TOC 72 N.T Tue et al / Journal of Sea Research 67 (2012) 69–76 Fig The bulk sediment composition in mangrove ecosystems of the XTP (A) sediment facies; (B) sediment compositions content ranged from 0.08 to 2.18%, with a mean of 0.78% The TOC content (mean ± SD) showed a considerable decrease from 1.45 ± 0.45% (n = 12) to 1.09 ± 0.32% (n = 20) between natural and planted mangrove sediments, respectively The TOC content showed a decreasing trend from vegetated sediments, through creek bank, to creek bottom sediments The TOC contents were 0.81 ± 0.53% (n = 5) and 0.37 ± 0.29% (n = 5) for bank and bottom of creek sediments, respectively Toward the sea, the TOC content slightly increased to 0.86 ± 0.27% (n = 13) in sediments of tidal flat, but it was markedly lower at 0.24 ± 0.21% (n = 20) in sediments of subtidal habitat (Figs 3B, 4B) The relationship between TOC and the fine sediment grain size fraction (≤63 μm) is shown in Fig (Spearman correlation coefficient= −0.64, p b 0.0001), which was best described by a non-linear least- Fig Spatial distributions of fine sediment grain size fractions (%) (A); TOC (%) (B); C/N ratio (mol/mol) (C); and δ13C (‰) (D) in the surface sediments of the XTP N.T Tue et al / Journal of Sea Research 67 (2012) 69–76 73 Fig The variations of fine sediment grain size fractions (%) (A); TOC (%) (B); C/N ratio (mol/mol) (C); and δ13C (‰) (D) in various habitats as shown in Fig squares regression model The model showed TOC content increasing progressively as the fine sediment fraction increased in the range of 0–60%, then TOC content increased much faster in the range of the fine sediment fraction from 60 to 85% The TOC content then reached a plateau from 85 to 100%, indicating that the OC was readily absorbed into the fine sediment grain sizes There was a significant positive correlation between TOC and TN (TN = 0.09*TOC + 0.007, R = 0.89, p b 0.01, Fig 6), suggesting the same origin of TOC and TN The regression line of TOC and TN passed very close to the origin (0,0), suggesting that the inorganic nitrogen content was insignificant and the most of the nitrogen content measured by the method of this study was related to organic nitrogen In present study, the TN can be used instead of organic nitrogen to calculate the atomic C/N ratio and ascertain the origins of sedimentary OC (Andrews et al., 1998; Tue et al., 2011a) The atomic C/N ratio ranged from 4.5 to 19.5, with a mean of 11.0 The C/N ratio trend was unclear with variation in TOC content (Figs 3C, 4C) The C/N ratios (mean ± SD) slightly increased from 11.6 ± 2.5 (n = 12) to 12.3 ± 3.1 (n = 20) between natural and planted mangrove forests From vegetated sediments to creek bottoms, the C/N ratios slightly decreased from 11.2 ± 2.0 (n = 5) in creek bank to 10.4 ± 1.3 (n = 5) in creek bottom sediments The C/N Fig Non-linear regression of the TOC (%) and fine sediment grain size fraction (%) in surface sediments of the XTP ratios slightly increased to 12.8 ± 3.3 (n = 13) in tidal flat sediments, however, this markedly dropped off to the lowest values of 8.8 ± 2.9 (n = 27) in subtidal sediments The lowest C/N ratios were generally associated with sediments poor in OC, which were mainly composed of the coarser grained sediments (Figs 3A,C) The sedimentary δ 13C values ranged from − 27.7 to − 20.4‰, with an average of − 24.1‰ In general, the sedimentary δ 13C values showed an inverse relationship with TOC content (Figs 3D, 4D) The sedimentary δ 13C values (mean ± SD) increased from −25.9 ± 1.4 (n = 12) to −24.6 ± 1.1‰ (n = 20) between natural and planted mangroves From vegetated sediments to creek bottoms, the δ13C values slightly increased to −24.1 ± 1.3‰ (n= 5) in creek bank sediments, and to −24.0 ±0.9‰ (n=5) in creek bottom sediments Toward the sea, the δ13C values slightly decreased to −24.2 ± 1.0‰ (n= 13) in tidal flat sediments, then markedly increased to −22.8 ± 1.0‰ (n= 27) in subtidal sediments The more decrease in δ13C values were associated with sediments rich in OC of vegetated mangroves, and more enriched δ13C values in the low OC content in the subtidal sediments The cross-system analysis of TOC and δ 13C showed an inverse relationship and was described by a non-linear least-squares regression model (Spearman correlation coefficient = −0.91, p b 0.0001, Fig 7) Fig Linear regression of the TOC (%) and TN (%) for the surface sediments of the XTP, the regression line runs very close to the origin (0,0), suggesting the N-inorganic is very small compared with the TN, and TN can therefore be used instead of organic nitrogen for determining the sources of sedimentary OC 74 N.T Tue et al / Journal of Sea Research 67 (2012) 69–76 Fig The inverse relationship between TOC (%) and δ13C (‰) in the surface sediments of the XTP The core sediment data were reported by Tue et al (2011a) The TOC and δ 13C relationship indicated that the δ 13C values was relatively higher when TOC content was b0.5% The TOC content (b0.5%) consisted of sediments from the subtidal habitat and creek bottom The δ 13C values decreased much faster (nearly 5‰) in the range of TOC content from 0.5–2.0% As seen, belonging to this range the sediments were mainly from tidal flat, creek bank, planted mangrove, and natural mangrove habitats When the TOC content was >2%, the δ 13C values reached to valley of the least-squares line and approached to those of the local mangrove tissues (− 28.06 ± 1.4‰, Tue et al., unpublished data), and sediments were only from natural mangrove forest Discussion 4.1 Mangroves enhanced the fine sediment grained sizes accumulation Mangrove forests play an important role in sedimentation processes of coastal environments (Robertson and Alongi, 1992), which are controlled by both biotic (e.g., tree densities, pneumatophore, prop and fine root systems), and abiotic factors (e.g., hydrodynamic processes, sediment supplied sources, sediment particle sizes, and geomorphological characteristics) Our results from bulk sediment composition were similar to the observations by Van Santen et al (2007) from the XTP mangrove ecosystem Additionally, this finding is consistent with the mangrove sediment characteristics from the Gulf of Papue (Walsh and Nittrouer, 2004), where mangrove sediments consist of clayed silts >30% and the sand fraction generally b10% The higher fractions of clay and silt in the mangrove sediments of our study compared to those of the tidal flat and subtidal sediments indicated that fine grained sediments were transported further up into the mangrove forest zone Van Santen et al (2007) reported that within the dense mangrove forests of the XTP the water levels were as high as 0.9 m, and 0.2 m during spring and neap tides, respectively Thus, the tidal inundation play as a pump preferentially transporting suspended sediments from the coastal waters to mangrove forests In addition, Van Santen et al (2007) also reported that tidal currents decreased from bare flats, to through the pioneer vegetation, and to dense mangrove trees Because the settling velocity of suspended sediments increases with increasing grain sizes (Sternberg et al., 1999), the reduction of tidal currents is one of multiple mechanisms for the dispersal and accumulation of fine sediments in mangrove forests (Furukawa and Wolanski, 1996) In addition to the decreasing current velocity, the friction by vegetation such as tree and aerial root densities, and the ability of fine roots in binding sediments which can be mainly attributed to the hydrodynamic attenuation, consequently causes the settling of fine suspended particles (Furukawa et al., 1997) The dense mangrove trees of K obovata, A corniculatum, S caseolaris, and A marina in the XTP can effectively reduce tidal flows and capture silts and clays Due to the vegetated friction the flow through the mangroves forms the turbulence zones, including jets, eddies, and stagnation zones (Furukawa and Wolanski, 1996; Furukawa et al., 1997) The high level of turbulence maintains in suspension the flocs of fine cohesive sediments, subsequently, the fine sediments can be transported further in the mangrove forests, and become accumulated at the time of high slack tide, as flow currents approach zero (Furukawa and Wolanski, 1996; Furukawa et al., 1997) This pattern indicated that the mangroves appear to have an important physical effect in actively trapping the fine sediments Furthermore, Alongi (2009) showed that most sediment imported into mangrove forests occurs during the wet season, which is a period when riverine sediment inflow is at its highest In the mangrove forests of XTP, sedimentation rates in the rainy season are higher than from the dry season (Van Santen et al., 2007), which is due to the high sediment loads from the Red River in the rainy season The median grain sizes of suspended sediments from the Red River varied from to μm (van Maren, 2007) The fine sediment grain sizes within the mangrove forests from this study were within the range to those of riverine-suspended sediments, which strongly suggest that the mangrove forests receive a large fraction of fine grained sediments from the adjoining Red River 4.2 Cross-system analysis of total organic carbon In this study, the cross-system analysis of TOC indicated much lower values than those in the surface sediments (0–1 cm) of mangrove forest from Gazi Bay (Kenya) and Pambala (Sri Lanka) but close to those found in sediments of mangrove forest from Tana Estuary (Kenya) (Bouillon and Boschker, 2006) In addition, TOC content showed a decreasing trend from vegetated mangroves, through to creek systems, and to the subtidal habitat There are several possible reasons for this trend, including the fine grained sediments, sources of OC, and microbial remineralization The TOC content and fine sediment grain sizes relationship showed that sediment grain sizes directly influenced the TOC content (Fig 5) The sediments with >85% fine grain sizes contained higher TOC content, referring that finer sediments may provide more reactive surface area that can gather organic matter In addition, surface area association can protect OC from remineralization and thus may provide a control on OC preservation in sediments (Bergamaschi et al., 1997) The inverse relationship between TOC and δ 13C is similar to those reported in sediment core of this mangrove ecosystem (Tue et al., 2011a) and other OC rich sediments of mangrove forests from Chunnambar and Pichavaram (India), Pambala (Sri Lanka), Gazi Bay and Tana delta (Kenya) (Bouillon and Boschker, 2006) These studies showed that the shift in δ 13C values in relation to TOC content can be explained by the mechanisms of microbial remineralization and variable inputs of OC sources During the microbial remineralization processes, Kristensen et al (2008) reviewed that eumycotes (fungi) and oomycotes (prototista) are highly-adapted for the capture of cellulose-rich vascular plant litter by pervasion and digestion from within organic matter The rapid growth of bacteria and fungi on mangrove organic matter will cause an increase in δ 13C values In addition, the easy degradation of 13C-depleted organic compounds (i.e., polysaccharide and other phenolic polymers) of mangrove leaves during decomposing processes will also increase in δ 13C values in sedimentary OC (Benner et al., 1987, 1990) Furthermore, the TOC and δ 13C relationship could result from the admixture of mangrove litters and allochthonous sources, which can be described by a simple two source mixing model (Bouillon and Boschker, 2006; Bouillon et al., 2003; Middelburg et al., 1997) As seen in Fig 7, the sediments from mangrove forests mainly characterized by δ 13C b −26‰, and TOC content >1% Particularly, the core and N.T Tue et al / Journal of Sea Research 67 (2012) 69–76 surface sediments from natural mangrove forests had TOC content >2% and the δ 13C values approached those of mangrove tissues (−28.06 ± 1.4‰, Tue et al., unpublished data) The most likely explanation for the higher TOC content and more depleted in δ 13C of core sediments than those of surface sediments of this natural mangrove forest could be the contribution of below-ground biomass (mangrove root systems) rather than the from litterfall and tidal deposition This observation helps support the hypothesis that the natural mangrove forests are important in the OC sequestration in sediments, and consequently the sedimentary δ 13C values approached those of mangrove tissues In contrast, the δ 13C values were quite variable (from −24 to −20‰) and approached those of marine phytoplankton (−21.18 ± 0.45‰, Tue et al., unpublished data) in the subtidal sediments (with TOC content b0.5%) This result showed that marine phytoplankton is a dominant OC source in subtidal sediments Therefore, the major factor controlling the δ13C and TOC relationship was the OC mass balance of sources from mangrove litters and marine phytoplankton The decrease in TOC content meant concomitant increase in δ13C values from mangrove forests, through to bank and bottom of creeks, and to the subtidal habitat Overall, the mangrove OC mainly accumulated in the mangrove forest sediments, whereas, subtidal, creek bottom, and creek bank sediments received relatively more marine phytoplankton source 4.3 Sources of organic carbon in sediments In the XTP, mangroves are indicative of C3 plant photosynthesis, with mangrove leaves expressing a mean δ 13C value of − 28.06 ± 1.4‰, whereas marine phytoplankton has a mean δ 13C value of −21.18 ± 0.45‰ (Tue et al., unpublished data) In addition, due to their high cellulose content, mangrove tissues have a mean C/N ratio of 27.1 ± 10.4, whereas marine phytoplankton, which tends to be more nitrogen-rich, has relatively low C/N ratios, with a mean of 9.8 ± 1.2 (Tue et al., unpublished data) Therefore, comparison of δ 13C and C/N ratios of sedimentary OC with those of two OC sources (mangroves and marine phytoplankton) makes it possible to identify the sources of sedimentary OC As seen in Fig 8, the bi-plot of C/N ratio and δ 13C shows that sedimentary OC originated from marine phytoplankton and mangrove sources The highest δ 13C values and lowest C/N ratios were observed in sedimentary OC from the subtidal habitat The sedimentary δ 13C values and C/N ratios were very similar to those of marine phytoplankton, indicating that the marine phytoplankton supplied a significant OC contribution to subtidal sediments However, higher C/N ratios were also observed in some coarser subtidal sediment samples This pattern can be explained by the low TN absorption capacities of coarser sediment grain sizes (Bergamaschi et al., 1997) The gradual Fig Comparison of C/N ratios (mol/mol) and δ13C values in the surface sediments from various habitats to those of OC sources Open and filled diamonds and error bars denote means and standard deviation for the marine phytoplankton (n = 3) and mangrove leaves (n = 26) sources, respectively (Tue et al., unpublished data) 75 increase in C/N ratios, consistent with a progressive decrease in δ 13C values of sediments from the subtidal habitat to bottoms and banks of creeks (Figs 4C, D and 8), suggests a decrease in the contribution of marine phytoplankton to these sediments From bottoms to banks of creeks, a gradual increase in the C/N ratios and a concomitant decrease in δ 13C values indicated that sedimentary OC decreased in the marine phytoplankton constituent and increased in mangrove OC The tidal flat sedimentary OC slightly elevated the C/N ratio and decreased in δ 13C compared to those of the creek banks, suggesting that sedimentary OC consisted more of mangrove-derived material In the planted mangrove forest, the δ 13C values decreased from seaward and creek edges (often with low mangrove tree densities) to the center of the mangrove forest (often dense mangrove trees) (Fig 3D), indicating that marine phytoplankton was the predominant in sediments from seaward and the creek edge of mangrove forests In natural mangrove forest, the δ 13C values approached the carbon isotope composition of mangrove tissues, thus, the C/N ratios were also expected to increase to that of mangrove leaves (27.1 ± 10.4, Tue et al., unpublished data) However, sedimentary C/N ratios were low relative to mangrove tissues, suggesting that there were rich nitrogen inputs (Middelburg et al., 1996) The mechanisms controlling nitrogen in sedimentary OC consist of below-ground input of nitrogen-rich mangrove material, nitrogen fixing-bacteria, uptake of available dissolved nitrogen compounds by benthic bacteria, and the import nitrogen rich materials (Middelburg et al., 1996; Muzuka and Shunula, 2006) From previous discussion and the references therein, Eq (3) was applied to calculate the relative contribution of mangrove litters to the sedimentary OC of the mangrove ecosystem from the XTP A spatial distribution map of mangrove contribution has been generated showing the general contribution by mangroves decreased as follows: natural mangrove forest > planted mangrove forest > tidal flat > creek bank > creek bottom > subtidal habitat (Fig 9A and B) The spatial distribution map also showed that the natural mangrove forest and the densely planted mangrove forest (i.e., at the sampling sites P13, P14, P16, P18, P19) contributed more than 80% of the OC in their sediments (Fig 9B) In addition, the mangrove contribution increased slightly with distance from seaward and creek edges (i.e., P7 b P6b P5 b M7) This pattern showed that the residence time of tidal water in the seaward and Fig Spatial distribution of mangrove contribution (%) in sedimentary OC of the XTP The small figure (A) shows the variation of mangrove contribution (%) in surface sediments of various habitats as shown in Fig The map shows spatial mangrove contributions in surface sediments of the XTP 76 N.T Tue et al / Journal of Sea Research 67 (2012) 69–76 creek edges or the succession of mangroves can play an important factor in the accumulation of mangrove OC Conclusions A cross-system analysis of bulk sediment composition, TOC, C/N ratio, and δ 13C in surface sediments from the XTP, Vietnam showed that (1) the silt and clay proportions were generally higher in the vegetated mangroves, banks and bottoms of creeks, and the tidal flat compared to that of subtidal sediments; (2) the TOC content was higher in the natural and planted mangrove forests compared to that of bank and bottom of creeks, through to the tidal flat, and to subtidal sediments The inverse relationship between TOC and δ 13C showed that the mechanisms of microbial remineralization and differences in OC sources (mangroves and marine phytoplankton) controlled the OC accumulation in sediments of the mangrove ecosystem; (3) the comparison of δ 13C and C/N ratio of sedimentary OC with those of mangrove and marine phytoplankton sources showed that the sedimentary OC of subtidal habitat was mainly composed of marine phytoplankton, whereas, sedimentary OC of natural mangrove forest was mainly originated from mangrove litters A simple mixing model was applied to calculate the relative contributions of mangrove and marine phytoplankton sources to sedimentary OC, with results showing the contribution of mangrove material decreased as follows: natural mangrove forest> planted mangrove>tidal flat >creek bank > creek bottom >subtidal habitat These results have presented evidence suggesting that mangroves act as important sinks to fine sediment grain sizes and OC in the estuarine mangrove ecosystems Especially, the natural mangroves are 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Sonneratia caseolaris, Kandelia obovata, Aegiceras corniculatum, and Avicennia marina The planted mangrove forests are mainly distributed in the southern part of the XTP, dominated by K obovata (Hong... Study area The present work was conducted in an estuarine mangrove ecosystem of Xuan Thuy National Park (XTP) in northern Vietnam (Fig 1) The XTP is located along the southern part of the Ba Lat... reflected the OC sources, hydrodynamic conditions, as well as marine -mangrove Fig Map of Xuan Thuy National Park, and the sampling sites The cross-system sampling sites are assigned as M: natural mangroves;