A cross-system analysis of sedimentary organic carbon in the mangrove ecosystemsof Xuan Thuy National Park, Vietnam a Graduate School of Science and Engineering, Ehime University, 2-5 Bu
Trang 1A cross-system analysis of sedimentary organic carbon in the mangrove ecosystems
of Xuan Thuy National Park, Vietnam
a
Graduate School of Science and Engineering, Ehime University, 2-5 Bunkyo-cho, Matsuyama, Japan
b
Center for Marine Environmental Studies, Ehime University, 2-5 Bunkyo-cho, Matsuyama, Japan
c
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
a b s t r a c t
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
δ 13
C
Xuan Thuy National Park
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, tidalflat, 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 170.2μm (mean 71.5 μm), with the fine sediment grain size fraction (b63 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 veg-etation and the ability offine 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, tidalflat, 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 andδ13C relationship showed that the factors of mi-crobial remineralization and OC sources controlled the TOC pool of mangrove sediments The comparison ofδ13C 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 ma-rine 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
1 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 importantfish, shrimp
and shellfish, nesting and foraging habitat of migratory birds, and as a
re-newable resource of fuel (Alongi, 2011) The total net primary
produc-tion 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
im-portant 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
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
wide-ly between ecosystems, from under 2 tob40%, with the global median value at 2.2% (Kristensen et al., 2008) The global carbon budget extrap-olations 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
⁎ 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.
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Trang 2contribution of each OC source is needed to assess the mangrove
contri-bution Additionally, the presence of mangroves will increase the
sedi-mentation 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, tidalflat, and subtidal zone The
sedimentary OC pools in these habitats may be distinguished by
differ-ences in their respective OC sources (Tue et al., 2011a) and by tidal
am-plitude (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, andδ13C in 82 surface sediment
samples from natural and planted mangrove forests, bank and bottom
of tidal creeks, tidalflat, 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
2 Materials and methods
2.1 Study area
The present work was conducted in an estuarine mangrove
ecosys-tem 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 ha are covered
by mangrove forests Generally, mangroves in the XTP can be classified into natural and planted mangrove forests The natural mangrove for-ests are mainly distributed in the northern area of the XTP,
dominat-ed 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 offishes, 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 live-lihood for local communities The XTP is recognized as a hotspot of biodi-versity in Vietnam, and as a result the Convention on Wetlands of International Importance declared XTP as thefirst Ramsar Site of South-east 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 man-grove forests, creek bank, creek bottom, tidalflat, and subtidal habi-tat The respective numbers of sediment samples of each the habitats were 12, 20, 5, 5, 13, and 27, and their locations are shown
inFig 1 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 1 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;
flat; and S: subtidal habitat The digit number assigns the order number of sediment samples in each habitat.
Trang 3interactions Surface sediments (0–2 cm) were collected by a
stain-less steel spade during low tide Sediment samples were packed in
la-beled 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 werefirstly 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 withfingers 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
sed-iment 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
com-pleted, 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
parti-cles 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, andb0.001 mm
2.3.2 Stable isotope and C/N analysis
Forδ13C, TOC, and C/N ratio analysis, sediment samples werefirst
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
pul-verized sediment sample was then placed in a microtube and
approx-imately 6 ml of 1 N HCl was added and repeated two times, mixed
thoroughly using a vibrating mixer, and then left at room
tempera-ture 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 isotopeδ13
C, TOC, and total nitrogen (TN) values were mea-sured by using a stable isotope mass spectrometer (ANCA-SL, PDZ
Eu-ropa, Ltd.) at the Center for Marine Environmental Studies, Ehime
University, Japan.δ13
C was expressed in‰ (permil) deviations from the standard value by the following Eq.(1):
δ13
Cð‰Þ ¼ ð Rsample
Rstandard1Þ×1000 ð1Þ
where R =13C/12C, Rsampleis 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‰
forδ13C
2.4 Fractional contribution of organic carbon sources
The OC sources to mangrove sediments can originate from both
autochthonous production such as mangroves, macroalgae and
microalgae, and allochthonous sources such as phytoplankton,
seagrasses, and other tidal organic matters Variations in the propor-tional contributions of these organic matter sources can cause a change in the sedimentaryδ13C values To determine the relative con-tribution of each organic matter source to sedimentary OC pool, a simple mixing model has been successfully used in mangrove ecosys-tems (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:
δ13
Csed¼ fMG×δ13
CMGþfMaA×δ13
CMaAþfMiA×δ13
CMiAþfS×δ13
CS
þfP×δ13
CPþ…þfn×δ13
CnfMGþfMaAþfMiAþfSþfPþfn¼100%
ð2Þ where fMG, fMaA, fMiA, fS, fP, and fnare the relative contribution of man-grove tissues, macroalgae, microalgae, seagrasses, marine phyto-plankton, and n source (%), respectively;δ13Csed,δ13CMG,δ13CMaA,
δ13
CMiA,δ13
CS,δ13
CP, andδ13
Cnare the carbon stable isotope values
of sedimentary OC, mangrove tissues, macroalgae, microalgae, sea-grasses, 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 usu-ally 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% - fMGin Eq.(2), we have:
fMGð%Þ ¼δ
13
Csedδ13
CP
3 Results 3.1 Bulk sediment composition Sediments in the present study were composed offine sand, silt, and clay The fractions of these compositions decreased from silt, through clay, and tofine sand Plotting the percentages of these com-positions 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 man-grove 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 ran-ged 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 de-creased 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 170.2μm, with a mean of 71.5μm On average, the fine sediment grain size fraction (b63 μm, mean±SD) decreased as follows: planted mangroves, creek bank, creek bottom, natural mangroves, tidalflat, and the sub-tidal 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 and4A) 3.2 Total organic carbon (TOC) content, C/N ratios, and carbon isotope composition (δ13C)
The TOC content (% dry weight) of sediments from natural and planted mangrove forests, creek bank, creek bottom, tidalflat, and the subtidal habitat are shown inFigs 3B and4B Overall, the TOC
Trang 4content ranged from 0.08 to 2.18%, with a mean of 0.78% The TOC
content (mean ± 1 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
de-creasing 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 in-creased to 0.86 ± 0.27% (n = 13) in sediments of tidalflat, 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 thefine sediment grain size frac-tion (≤63 μm) is shown inFig 5(Spearman correlation coefficient=
−0.64, pb0.0001), which was best described by a non-linear
least-fine sediment grain size fractions (%) (A); TOC (%) (B); C/N ratio (mol/mol) (C); and δ 13 C (‰) (D) in the surface sediments of the XTP Fig 2 The bulk sediment composition in mangrove ecosystems of the XTP (A) sediment facies; (B) sediment compositions.
Trang 5squares regression model The model showed TOC content increasing
progressively as thefine 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 thefine sediment grain sizes
There was a significant positive correlation between TOC and TN
(TN = 0.09*TOC + 0.007, R2= 0.89, pb0.01, 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
mea-sured 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
sedimenta-ry 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 ± 1 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
bot-toms, 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
ratios slightly increased to 12.8 ± 3.3 (n = 13) in tidalflat 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 sedimentaryδ13C values ranged from−27.7 to −20.4‰, with
an average of −24.1‰ In general, the sedimentary δ13C values showed an inverse relationship with TOC content (Figs 3D,4D) The sedimentaryδ13C values (mean ± 1 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 δ13
C values slightly increased to−24.1±1.3‰ (n=5) in creek bank sedi-ments, and to−24.0±0.9‰ (n=5) in creek bottom sediments Toward the sea, theδ13C values slightly decreased to−24.2±1.0‰ (n=13) in tidalflat sediments, then markedly increased to −22.8±1.0‰ (n=27)
in subtidal sediments The more decrease inδ13C values were associated with sediments rich in OC of vegetated mangroves, and more enriched
δ13
C values in the low OC content in the subtidal sediments
The cross-system analysis of TOC andδ13C showed an inverse re-lationship and was described by a non-linear least-squares regression model (Spearman correlation coefficient=−0.91, pb0.0001,Fig 7)
Fig 4 The variations of fine sediment grain size fractions (%) (A); TOC (%) (B); C/N ratio (mol/mol) (C); and δ 13 C (‰) (D) in various habitats as shown in Fig 1
Fig 5 Non-linear regression of the TOC (%) and fine sediment grain size fraction (%) in
Fig 6 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
Trang 6ni-The TOC andδ13C relationship indicated that theδ13C values was
rel-atively higher when TOC content wasb0.5% The TOC content (b0.5%)
consisted of sediments from the subtidal habitat and creek bottom
Theδ13C values decreased much faster (nearly 5‰) in the range of
TOC content from 0.5–2.0% As seen, belonging to this range the
sed-iments were mainly from tidalflat, creek bank, planted mangrove,
and natural mangrove habitats When the TOC content was >2%,
the δ13
C 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
4 Discussion
4.1 Mangroves enhanced thefine sediment grained sizes accumulation
Mangrove forests play an important role in sedimentation
pro-cesses of coastal environments (Robertson and Alongi, 1992), which
are controlled by both biotic (e.g., tree densities, pneumatophore,
prop andfine 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 byVan Santen et al (2007)from the XTP mangrove
eco-system Additionally, thisfinding 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 generallyb10% 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 bareflats, to through the pioneer vegetation, and to dense
man-grove trees Because the settling velocity of suspended sediments
in-creases with increasing grain sizes (Sternberg et al., 1999), the
reduction of tidal currents is one of multiple mechanisms for the
dis-persal and accumulation of fine sediments in mangrove forests
(Furukawa and Wolanski, 1996)
In addition to the decreasing current velocity, the friction by
vegeta-tion such as tree and aerial root densities, and the ability offine roots in
binding sediments which can be mainly attributed to the hydrodynamic
attenuation, consequently causes the settling offine suspended
parti-cles (Furukawa et al., 1997) The dense mangrove trees of K obovata,
A corniculatum, S caseolaris, and A marina in the XTP can effectively
reduce tidalflows and capture silts and clays Due to the vegetated fric-tion theflow through the mangroves forms the turbulence zones, in-cluding jets, eddies, and stagnation zones (Furukawa and Wolanski, 1996; Furukawa et al., 1997) The high level of turbulence maintains
in suspension theflocs 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, asflow 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 thefine 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 for-ests 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 me-dian grain sizes of suspended sediments from the Red River varied from 4 to 8μm (van Maren, 2007) Thefine sediment grain sizes
with-in the mangrove forests from this study were withwith-in the range to those of riverine-suspended sediments, which strongly suggest that the mangrove forests receive a large fraction offine grained sedi-ments 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 man-grove 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 con-tent showed a decreasing trend from vegetated mangroves, through
to creek systems, and to the subtidal habitat There are several possi-ble 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 thatfiner sediments may provide more reac-tive 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 andδ13C is similar to those reported in sediment core of this mangrove ecosystem (Tue et al.,
Chunnambar and Pichavaram (India), Pambala (Sri Lanka), Gazi Bay and Tana delta (Kenya) (Bouillon and Boschker, 2006) These studies showed that the shift inδ13
C values in relation to TOC content can be explained by the mechanisms of microbial remineralization and vari-able 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 lit-ter by pervasion and digestion from within organic matlit-ter The rapid growth of bacteria and fungi on mangrove organic matter will cause
an increase inδ13C values In addition, the easy degradation of13 C-de-pleted organic compounds (i.e., polysaccharide and other phenolic polymers) of mangrove leaves during decomposing processes will also increase inδ13C values in sedimentary OC (Benner et al., 1987,
1990)
Furthermore, the TOC andδ13
C 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 inFig 7, the sediments from mangrove forests mainly character-ized byδ13Cb−26‰, and TOC content >1% Particularly, the core and
Fig 7 The inverse relationship between TOC (%) and δ 13
C (‰) in the surface sediments
of the XTP The core sediment data were reported by Tue et al (2011a)
Trang 7surface sediments from natural mangrove forests had TOC content
>2% and the δ13C values approached those of mangrove tissues
(−28.06±1.4‰, Tue et al., unpublished data) The most likely
expla-nation for the higher TOC content and more depleted inδ13C 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
con-sequently the sedimentary δ13C values approached those of
man-grove tissues In contrast, theδ13C 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
sedi-ments (with TOC contentb0.5%) This result showed that marine
phy-toplankton is a dominant OC source in subtidal sediments Therefore, the
major factor controlling theδ13C and TOC relationship was the OC mass
balance of sources from mangrove litters and marine phytoplankton
The decrease in TOC content meant concomitant increase inδ13C 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 C3plant photosynthesis,
with mangrove leaves expressing a meanδ13C value of−28.06±
1.4‰, whereas marine phytoplankton has a mean δ13C 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
δ13C 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 inFig 8, the bi-plot of C/N ratio andδ13C shows that
sed-imentary OC originated from marine phytoplankton and mangrove
sources The highestδ13
C values and lowest C/N ratios were observed
in sedimentary OC from the subtidal habitat The sedimentaryδ13C
values and C/N ratios were very similar to those of marine
phyto-plankton, indicating that the marine phytoplankton supplied a
signif-icant 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
increase in C/N ratios, consistent with a progressive decrease inδ13C values of sediments from the subtidal habitat to bottoms and banks
of creeks (Figs 4C, D and8), 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 concomi-tant decrease inδ13C values indicated that sedimentary OC decreased
in the marine phytoplankton constituent and increased in mangrove
OC The tidalflat sedimentary OC slightly elevated the C/N ratio and decreased inδ13
C compared to those of the creek banks, suggesting that sedimentary OC consisted more of mangrove-derived material
In the planted mangrove forest, theδ13C values decreased from sea-ward 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, theδ13C values approached the carbon iso-tope 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 ni-trogen inputs (Middelburg et al., 1996) The mechanisms controlling nitrogen in sedimentary OC consist of below-ground input of nitrogen-rich mangrove material, nitrogenfixing-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 spa-tial distribution map of mangrove contribution has been generated showing the general contribution by mangroves decreased as follows: natural mangrove forest > planted mangrove forest > tidalflat>creek bank > creek bottom > subtidal habitat (Fig 9A and B) The spatial dis-tribution 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., P7bP6bP5bM7) This pat-tern showed that the residence time of tidal water in the seaward and
Fig 8 Comparison of C/N ratios (mol/mol) and δ 13 C 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
Fig 9 Spatial distribution of mangrove contribution (%) in sedimentary OC of the XTP The small figure (A) shows the variation of mangrove contribution (%) in surface sed-iments of various habitats as shown in Fig 1 The map shows spatial mangrove
Trang 8contri-creek edges or the succession of mangroves can play an important
factor in the accumulation of mangrove OC
5 Conclusions
A cross-system analysis of bulk sediment composition, TOC, C/N
ratio, andδ13C 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 tidalflat, and
to subtidal sediments The inverse relationship between TOC and
δ13C 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
ecosys-tem; (3) the comparison ofδ13C 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
ma-rine 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>tidalflat>creek bank>
creek bottom >subtidal habitat
These results have presented evidence suggesting that mangroves act
as important sinks tofine sediment grain sizes and OC in the estuarine
mangrove ecosystems Especially, the natural mangroves are very
impor-tant for the OC sequestration in the sediments These results highlight the
need for mangrove conservation, particularly natural mangrove forests
in tropical coastal systems In addition, the varieties ofδ13C, C/N ratio,
TOC, and bulk sediment grain sizes in the cross-system of natural and
planted mangrove forests, bank and bottom of tidal creeks, tidalflat,
and the subtidal habitat of mangrove ecosystems can be used to examine
the mangrove food web structures, and as well as indicators of
paleoen-vironmental change in the future studies
Acknowledgments
The authors are grateful to staff of Hanoi University of Science,
Vietnam, for their help with sampling We express our sincere thanks
to anonymous reviewers and Dr Todd W Miller for their critical
re-views and comments which significantly improved this manuscript
This work was supported by the“Global COE Program” from the Ministry
of Education, Culture, Sports, Science and Technology, Japan
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