DSpace at VNU: Sources and Exchange of Particulate Organic Matter in an Estuarine Mangrove Ecosystem of Xuan Thuy National Park, Vietnam

Sources and Exchange of Particulate Organic Matterin an Estuarine Mangrove Ecosystem of Xuan Thuy National Park, Vietnam Nguyen Tai Tue&Tran Dang Quy&Hideki Hamaoka& Mai Trong Nhuan&Koji

Sources and Exchange of Particulate Organic Matter

in an Estuarine Mangrove Ecosystem of Xuan Thuy National

Park, Vietnam

Nguyen Tai Tue&Tran Dang Quy&Hideki Hamaoka&

Mai Trong Nhuan&Koji Omori

Received: 20 June 2011 / Revised: 9 February 2012 / Accepted: 11 February 2012 / Published online: 29 February 2012

# Coastal and Estuarine Research Federation 2012

Abstract The spatio-temporal variations in stable isotope

signatures (δ13

C and δ15

N) and C/N ratios of particulate organic matter (POM), and physicochemical parameters in

a creek water column were examined in an estuarine

man-grove ecosystem of Xuan Thuy National Park, Vietnam The

objective was to examine the factors influencing creek water

properties, and the sources and exchange of POM in this

important mangrove ecosystem The diel and seasonal

var-iations in water temperature, flow velocity, pH, dissolved

oxygen, and salinity demonstrated that tidal level, season,

and biological factors affected the creek water properties

Mangroves had relatively low δ15

N and very low δ13

C values, with respective average values of 1.5±0.9‰ and

−28.1±1.4‰ The low mangrove leaf δ15

N indicated minor anthropogenic nitrogen loading to the mangrove forests A

significant positive correlation between POM–δ13

C and sa-linity along the axis of Ba Lat Estuary, Red River, indicated

that marine phytoplankton (δ13

C value, −21.4±0.5‰) was the predominant source of POM at the estuary mouth Based

on the co-variation of δ13

C and C/N ratios, marine phyto-plankton and mangrove detritus were predominant in POM

of major creeks and small creeks, respectively During the diurnal tidal cycle, the dynamics of POM were affected by sources of organic matter, tidal energy, and seasonal factors The contribution of mangrove detritus to POM reached a maximum at the low tide and was enhanced during the rainy season, whereas marine phytoplankton contribution was highest at high tide

Keywords Particulate organic matter Mangrove ecosystem Stable isotopes Xuan Thuy National Park Vietnam

Introduction

Mangrove ecosystems are highly productive coastal sys-tems, forming both a critical boundary between the sea and land and providing essential habitats for plants and animals They are traditionally considered to play a critical role in the biogeochemical carbon cycle in (sub)trophic regions, being an important source of particulate organic matter (POM) and dissolved organic carbon in coastal waters (Jennerjahn and Ittekkot2002) The export of mangrove de-tritus is correlated with total litter production (Mfilinge et al

2005), particularly during the rainy season (Dittmar and Lara

2001a; Alongi2009)

Mangrove-derived POM has been recognized as a major food source for a variety of invertebrate species (Camilleri

1992) and contributes up to 84% of the diet of commercially important juvenile prawns (Chong et al.2001) Therefore, the dynamics of POM are likely to influence the whole food web

in the mangrove ecosystem (Chong et al.2001; Bouillon et al

2002) However, the factors influencing the sources and

Electronic supplementary material The online version of this article

(doi:10.1007/s12237-012-9487-x) contains supplementary material,

which is available to authorized users.

N T Tue

Graduate School of Science and Engineering, Ehime University,

2-5 Bunkyo-cho,

Matsuyama, Japan

N T Tue (*):H Hamaoka:K Omori

Center for Marine Environmental Studies, Ehime University,

2-5 Bunkyo-cho,

790-8577, Matsuyama, Japan

e-mail: tuenguyentai@gmail.com

T D Quy:M T Nhuan

Faculty of Geology, Hanoi University of Science,

334 Nguyen Trai, Thanh Xuan,

Hanoi, Vietnam

DOI 10.1007/s12237-012-9487-x

exchange of POM have not yet been fully understood in

mangrove ecosystems, particularly in developing countries

such as Vietnam

Mangrove creeks act as a major route for transport of

mangrove detritus to adjacent environments (Alongi2009)

and likewise the import of phytoplankton from coastal

waters (Romigh et al 2006; Bouillon et al 2007) Thus,

the creek systems play a vital role in maintaining the

phys-ical and biologphys-ical structure of the mangrove ecosystem, as

well as the out-welling of nutrients and organic matter from

mangrove forests In addition, POM, nutrients and other

physicochemical properties of creek water are highly

influ-enced by tidal level (Bouillon et al.2007), season (Romigh

et al 2006), and biogeochemical processes in sediments

(Kristensen 2008) The POM pool often originates from

variable sources (e.g., mangrove detritus, marine

phyto-plankton, benthic microalgae, and terrestrially derived

or-ganic matter) (Rezende et al.1990; Bouillon et al 2007)

Therefore, to understand the dynamics of POM in mangrove

ecosystems, it is important to examine the contributions of

mangrove detritus under different tidal, spatial, and seasonal

scales

In the present study, we test the hypothesis that the

physicochemical properties of water column and the sources

and dynamics of POM in an estuarine mangrove ecosystem are controlled by the changes in tidal level, seasonal, and biological factors We investigate the spatio-temporal obser-vations of stable isotopes (δ13

C andδ15

N) and C/N ratios of POM and the physicochemical parameters (water level, flow velocity, water temperature, dissolved oxygen (DO), pH, salinity, and total suspended matter (TSM)) of creek water for (1) examining the factors influencing the physicochem-ical properties of the creek water and (2) determining the spatio-temporal variations in POM constituents in the estu-arine mangrove ecosystem

Materials and Methods

Study Area

The present study was conducted in an estuarine mangrove ecosystem of Xuan Thuy National Park (XTP) in Northern Vietnam The XTP is located along the south entrance of Ba Lat Estuary (BLE), Red River (Fig.1) A detailed descrip-tion of the XTP is shown in the Electronic supplementary material The XTP has two major creek systems (Tra and Vop Creeks) that meander through the dense mangrove

Fig 1 Sampling sites within

mangrove ecosystem of Xuan

Thuy National Park in northern

Vietnam Map numbers are

based on the order number of

spatial POM samples MS

denotes monitoring site, and E1

to E6 denote sampling sites

from the Ba Lat Estuary Site

E7 (15 km from the estuary

mouth) is not shown on the map

forests Tra Creek is the largest and is one of the major

connections between mangrove forests and the sea

There-fore, Tra Creek likely plays a vital role in the physical and

biological processes underlying the development and

suste-nance of the mangrove ecosystem

Field Sampling

Field sampling was carried out in both rainy (from 10

to 16 September, 2009) and dry seasons (from 9 to 26

March, 2010) During the rainy season, a monitoring

site (MS, 20° 14′ 15.2″ N, 106° 33′ 56.7″ E) was

established in the middle of Tra Creek for collecting

water samples for POM and physicochemical

measure-ments at 1-h intervals during a diurnal tidal cycle from

14:25, 11th to 14:25, 12th September, 2009 To assure

no site contamination by anthropogenic sources and the

examination of a natural system, the MS location was

chosen within the most developed mangroves and far

from human activity and settlements

During the dry season, water samples were likewise

collected during a diurnal tidal cycle from 10:40, 11th to

10:40, 12th March, 2010 for determination of POM and

physicochemical parameters, following the same method

and location (MS) as during the rainy season To examine

the potential marine POM end-member, estuarine water

samples were collected along a salinity gradient of the

BLE during flood tide, and the sampling sites were assigned

from E-1 to E-7 (Fig.1) In addition, creek water samples

were spatially collected during flood tide in order to

exam-ine the sources of POM in the geographical distribution

(Fig.1)

Water samples were all taken at 30 cm below the

surface by a Van Dorn bottle POM samples were

col-lected by filtering 0.5 L of water through pre-weighed

and pre-combusted (at 550°C) 47 mm Whatman GF/F

glass fiber filters After collection, POM samples were

lightly rinsed with distilled–deionized water to remove

salt residue, immediately placed on ice, and transported

to the field laboratory where the samples were dried at

60°C for 24 h

At the MS station, flow velocity and water temperature

were simultaneously recorded by a velocity–temperature

compact instrument (Compact–EM, model AEM–HR, Alec

electronics Co.ltd) during a diurnal tidal cycle from both the

rainy and dry season The basic parameters of pH, salinity,

and DO were measured on-site using HORIBA portable

instruments, which consisted of a pH electrode (model

9621–10D) with an accuracy of ±1%, DO electrode (model

OM–51) with an accuracy of ±1%, and salinity electrode

(model 9382–10D) with an accuracy of ±0.1%, respectively

The dominant sources of terrestrial organic matter in

the XTP are mangroves Fresh mangrove leaves were

collected by hand from three dominant mangrove spe-cies, consisting of Sonneratia caseolaris, Kandelia obo-vata, and Aegiceras corniculatum The leaves were carefully rinsed with distilled water after collection to remove any potential extraneous material and stored on ice and transported to the field laboratory for drying at 60°C for 36 h

Sample Preparation and Analysis

TSM was obtained after re-drying at 60°C until a con-stant weight The POM samples were first decarbonated

by fumigating with HCl (12 N) within a contained desiccator for 12 h The POM samples were then dried

at 60°C for 24 h to evaporate the HCl and were sub-sequently ground to a fine powder using an agate mor-tar and pestle for analysis of stable isotopes (δ13

C and

δ15

N) and C/N ratios

Stable isotope signatures (δ13

C andδ15

N) and C/N ratios

of POM, and mangrove leaves were analyzed using a gas chromatograph combustion isotope ratio mass spectrometry (PDZ Europa Ltd., ANCA-SL) Stable isotope signatures were expressed in permil (‰) deviations from the standard value by the following Eq.1:

ð1Þ where R013

C/12C or 15N/14N, Rsample is the isotope ratio

of the sample, and Rstandard is the isotope ratio of a standard referenced to Pee Dee Belemnite limestone carbonate for 13C/12C and to atmospheric nitrogen for

15

N/14N During analysis processes, L-histidine was used

as certified reference material The precision of analyt-ical methods were ±0.1‰ and ±0.2‰ for δ13

C and

δ15

N, respectively

Statistical Analyses Pearson’s correlation was used to determine relation-ships between various physicochemical parameters and stable isotope signatures (δ13

C and δ15

N) and C/N ratios

of POM from the BLE and during the diurnal tidal cycles of the mangrove creek The significance level

of the Pearson’s correlation was 0.05 (p<0.05) for each statistical procedure The correlation results were used

to understand factors influencing physicochemical prop-erties of creek water and spatio-temporal variations in POM constituents A one-factor ANOVA was applied to determine the seasonal effects (rainy and dry) on the physicochemical parameters and stable isotope signa-tures (δ13

C and δ15

N) and C/N ratio All statistical

analyses were performed using a SPSS statistical

soft-ware package 17 (SPSS 17.0)

Results and Discussion

Factors Influencing the Physicochemical Properties

of Creek Water Column

The seasonal pattern in range (min, max) and average value

of physicochemical parameters of diurnal tidal cycles in a

mangrove creek of XTP are shown in Table1 The water

temperature represented typical regional climate variability

during the sampling periods The flow velocity closely

followed with the water level pH decreased to a minimum

during the onset of flood tides in both seasons DO values

reached a minimum during the night in both seasons

Salin-ity decreased during ebb tide and reached a minimum at the

onset of flood tide in both seasons (Electronic

supplemen-tary material, Figs S1 and 2) The tidal level had

signifi-cantly positive correlations with flow velocity and salinity

during the rainy season, whereas it was negatively

correlat-ed with water temperature In addition, the flow velocity

showed a significantly positive correlation with salinity and

a negative correlation with pH During the dry season, the

tidal level was positively correlated with flow velocity and

water temperature, whereas it was negatively correlated with

DO (Table2)

The significant correlations among physicochemical

parameters showed that tidal level was one of major factors

affecting the creek water properties Numerous studies have

reported a similar relationship (Dittmar and Lara2001b), but

porewater seepage (Bouillon et al.2007) and groundwater

flow from the mangrove forests (Akamatsu et al.2009) may

also have contributed During the rainy season, the water

column at low tide expressed higher temperatures and lower

salinities, indicating that warm and lower-salinity porewater

was seeping into the mangrove creek A decrease in salinity during ebb tide in the rainy season has been observed in other mangrove forests as a result of low salinity porewater seepage (Dittmar and Lara2001b)

The lowest temperatures occurred at the onset of low tide

in the dry (winter) season (Electronic supplementary mate-rial, Fig S1b), suggesting flow-back of surface water into the intertidal and small creeks The slight decrease in salin-ity during ebb tide in the dry season can be interpreted by flow-back of creek water from the northern part of Tra Creek, where it is wider and the residence time of sea water

is longer and less influenced by porewater seepage One-factor ANOVA results showed that the season appeared to affect the physicochemical parameters (temper-ature, pH, DO, salinity) As shown in Fig.3, water temper-ature and DO were significantly higher, but other parameters (pH, salinity) were significantly lower during the rainy season The seasonal effects on the physicochemical prop-erties of creek water have been also observed in other mangrove ecosystems (see Romigh et al 2006; Koné and Borges 2008) The higher level of salinity during the dry season indicated the high evaporation rates (Koné and Borges 2008) Additionally, the relatively low pH during the rainy season referred to the effect of high inputs of rain and river waters in the creek water

The sudden decrease in DO level at the onset of flood tide during the rainy season can be interpreted as a combination of three factors: (1) seepage of low DO porewater from mangrove forests to creeks (Bouillon et

al 2007), (2) oxidation of organic matter in the surface sediments (Marchand et al 2004), and (3) aquatic respi-ration (Romigh et al.2006) In addition, the combination

of a significant positive correlation between DO and pH and low DO at night (high tide) during dry season (Electronic supplementary material, Fig S2d) suggested that aquatic respiration can be a major factor (Dittmar and Lara 2001b)

Table 1 Seasonal pattern in

range (max, min) and average

values of physicochemical

parameters of diurnal tidal cycles

in a mangrove creek of XTP

The Potential End-Member Characteristics of POM in the

Estuarine Mangrove Ecosystems of XTP

In the XTP, Tue et al (2012) reported that abundances of

aquatic macrophytes, seagrasses and benthic microalgae

are very low to absent As a result, marine phytoplankton

and mangrove detritus were major sources that

contrib-uted to the POM pool of the creeks (Fig 4) Thus, the

stable isotope signatures of these end-members were

measured for determination the spatio-temporal variations

in POM compositions

Mangrove Leaf End-Member

Mangroves had relatively lowδ15

N and very lowδ13

C, with respective average values (mean ± SD) of 1.5 ± 0.9‰ and

−28.1±1.4‰, respectively Leaf δ15

N values of the three dominant mangrove species were lower than that reported

on mangrove leaves from the Pearl River estuary, China

(Lee 2000) and of tropical estuarine-mangroves of India

(Bala Krishna Prasad and Ramanathan2009) However, leaf

δ15

N values were similar to those of white mangrove

(Laguncularia racemosa) from Florida and Belize (Wooller

et al.2003) and dwarf mangroves located at the ocean edge

from south Florida (Fry et al.2000) The relatively low leaf

δ15

N values from the three mangrove species may indicate

low anthropogenic influence in the XTP, and leaf δ15

N values were indicative of nitrogen derived from nitrogen

fixation (Fry et al.2000; Wooller et al.2003) The mangrove

leaf δ13

C values were consistent with previous reports on other mangrove species, which characterize by the C3 pho-tosynthetic pathway (Rodelli et al 1984; Lee 2000) Leaf C/N ratio had an average of 27.1±10.4, which is similar to previous reports on the true mangroves in tropical estuarine mangroves (i.e., Bala Krishna Prasad and Ramanathan

2009)

The Marine Phytoplankton End-Member

In the BLE,δ13

C gradually increased from the upper to the mouth of the estuary (Fig 2a) δ13

C showed a significant positive correlation with salinity (r200.5, p<0.05) and the distance from the sea of sampling sites (r200.58, p<0.05)

δ15

N was highest at the mouth and markedly decreased to-ward the upper estuary (Fig.2a).δ15

N showed a significant but weak positive correlation with salinity (r200.33, p<0.05) and the distance from the sea of sampling sites (r200.26, p<0.05) C/N ratio varied slightly from the upper to the mouth

of the estuary (Fig.2b) and showed no significant correlation with salinity or the locations of sampling sites

The δ13

C gradient indicated that there was a change in the constituents and sources of POM from the mouth to the upper estuary (Middelburg and Nieuwenhuize 1998) The highest δ13C value (−21.2±0.5‰) at the estuary mouth indicated an origin of marine phytoplankton This is similar

to the typical values of marine phytoplanktonδ13

C, which ranged from−18‰ to −22‰ (Rodelli et al.1984) From 1 to

5 km from the sea, the averageδ13C value (−24.0±0.7‰)

Table 2 Matrix showing

signif-icant (p<0.05) correlations

be-tween various physicochemical

parameters of the diurnal tides in

both rainy and dry seasons

Values from the dry season are

with bold emphasis

Tidal level Flow velocity Water temperature pH Salinity TSM δ 13

C

pH

δ 15

δ 13

Fig 2 Gradients in δ 13

C and

δ 15

N values ( ‰), salinity, and

C/N ratios from mouth to

upriver within the Ba Lat

Estuary Points denote mean

values with error bars as SD

(n03)

indicated that POM consisted of a mixture of C3

plant-derived organic matter (i.e., mangrove detritus) and

marine phytoplankton, with the latter being the

predom-inant source At a distance of 15 km from the sea, the

δ13C value (−25.4±0.7‰) indicated a dominance of C3

plant-derived organic matter (Middelburg and

Nieu-wenhuize 1998) These findings suggested that marine

phytoplankton production is transferred into the estuary

during flood tides

The Spatio-temporal Variations in POM Composition

The Spatial Variation in POM Composition

The variations ofδ13

C,δ15

N, and C/N ratios in POM from the Tra and Vop Creek systems are shown in Table3 POM

was more enriched in13C for Tra Creek (and its branches)

(p<0.05) POM was the most depleted in13C at the upper

part (sampling sites 15, 16, and 17), and the most enriched

in13C at the mouth of Vop Creek (sampling site 21) (see

Fig.1) POM-δ15

N showed no difference between Tra and

Vop Creek The C/N ratio was significantly higher in Vop Creek (p<0.05)

From Tra Creek and near the mouth of Vop Creek,δ15

N and C/N ratios of POM were similar to marine phytoplank-ton, whereas δ13C was lower by approximately 1–2‰ These results suggest that marine phytoplankton was a ma-jor source of the spatial POM pool (Fig.4) In some small creeks and the upper part of Vop Creek, POM was more depleted in13C, suggesting a high proportion of mangrove detritus or depletion in13C of the local phytoplankton com-munity (Bouillon et al 2000) However, the low δ15

N confirmed that mangrove detritus could be a significant source

A simple two-source mixing model was applied to cal-culate the relative contribution of mangrove detritus and marine phytoplankton to the POM pool Bouillon et al (2008) showed that mangrove leaf δ13

C values were not significantly altered during the decomposition processes Therefore, it is reasonable to assume that theδ13

C value of mangrove detritus was consistent with the mangrove leaf

δ13

C value

Table 3 Stable isotope ( δ 15

N and δ 13

C) values, C/N ratios, and the proportional contribution (%) of mangrove detritus and marine phytoplankton from spatial mangrove creek POM of the XTP

Creeks Sampling sites δ 15 N (‰) δ 13 C (‰) C/N ratio (mol mol−1) Source contribution (%)

Mangrove Phytoplankton

The spatial POM constituents indicated that, during the flood tide, the contribution by marine phytoplankton was

>50% from Tra Creek, reflecting the import of marine phytoplankton At the upper part of Vop Creek (at sites 15,

16, 17; see Table3), the mangrove contribution was >50%, indicating mangrove detritus could be transported to the upper creek during flood tide The high contribution of mangrove detritus was also found in many of the small creeks (i.e., at sites 2, 7, 14, 18, 22, 23, and 24; see Table3) This suggests that mangrove detritus was trapped within these small creeks, which are usually shallow, narrow, and have low-energy flow velocities during flood tide of the dry season

The Diel Variation in POM Composition

TSM showed a maximum level during ebb tide in both seasons δ15

N reached a maximum during high tide in the dry season and during ebb tide in the rainy season δ13

C

Fig 3 Box-and-whisker plots of physicochemical parameters from the

XTP mangrove ecosystem showing significant (p < 0.05) seasonal

effects The middle line of each plot is the median of the distribution.

The bottom and top lines of each box are the 25th and 75th percentiles and the bottom and top lines of the whiskers indicate the tenth and 90th percentiles, respectively

Fig 4 Bi-plot of C/N ratio (moles per mole) and δ 13

C (permil) of POM of diurnal tidal cycles during the rainy and dry seasons, and the

spatial POM of flood tide during the dry season from mangrove

ecosystem of the XTP Open and filled triangles and error bars denote

means and SD for marine phytoplankton (n 03) and mangrove leaves

(n 026), respectively

decreased at low tide in the rainy season and at the onset of

flood tide in the dry season, and increased at high tide in

both seasons C/N ratios showed a large range, and reached

a maximum during the rainy season, but slightly varied

during the dry season (Electronic supplementary material,

Figs S2and3)

TSM and stable isotope signatures showed significant

correlations with tidal level and flow velocities (Table 2),

suggesting that the tidal level and tidal energy may affect the

sources and exchange of POM in this mangrove ecosystem

The flow velocity was strong during ebb and flood tides in

both seasons, suggesting that tidal flows could be a

signif-icant factor in the resuspension of materials (e.g., mangrove

litter, POM, and sediments) from the forest floor and

subse-quent transport to adjacent waters (Furukawa et al.1997)

The flow velocity dropped to the lowest values at low and

high tides and caused accumulation of organic matter on

surface sediments (MS, Fig S1c, d) The tidal velocity

patterns indicated that tidal energy may play an important

role in the exchange of organic matter between mangrove

forests and adjacent waters (Lee 1995) TSM tended to

increase during the high tidal energy of ebb and flood tides,

especially during the rainy season (Table1, Fig.3),

indicat-ing that the surface sediments were disturbed Consequently,

TSM appears to be a major mechanism of organic and

inorganic matter transports in this system

Higher POM-δ15

N values during the rainy season (Fig.3) are likely caused by the contribution of high nutrient water

sources (e.g., ground water) to mangrove creeks and/or high

fractionation of nitrogen species Accordingly, Wösten et al

(2003) observed increased ground water nutrient

concentra-tions (NH4 and NO3 −) during the rainy season in the Ba Lat

Estuary Therefore, denitrification and nitrification

process-es and/or the predominance of the14N-DIN of marine

phy-toplankton origin in the creek water led to increasingδ15

N

in the POM pool (Ralison et al.2008)

One-factor ANOVA results showed that lowerδ13

C and higher C/N ratios were observed during the rainy season

(Fig 3) The seasonal effects on δ13

C and C/N ratios re-ferred to a higher contribution of mangrove detritus to the

POM pool during the rainy season The marked increase in

C/N ratios during the rainy season from 10.5 during ebb tide

to >16 at low tide indicated that the sources of the POM

pool suddenly changed The simultaneous increase in C/N

ratios and decrease inδ13

C values indicated that mangrove detritus was the major source of the POM pool during low

tide (Fig 4) The less low-tide decrease in δ13

C values during the dry season corresponded to a slight change in

C/N ratios This may be caused by the additional

rich-nitrogen organic matter input to the POM pool For

exam-ple, during mangrove litter decomposition, the bacterial

incorporation of organic nitrogen can cause a decrease in

the C/N ratios of POM (Dehairs et al.2000) During flood

tide, δ13

C gradually increased and reached a maximum of

−23.0‰ and −22.7‰ for rainy and dry seasons, respectively (Electronic supplementary material, Fig S3c, d), and C/N ratios only ranged from 7.4 to 10.6 These patterns showed that marine phytoplankton was a predominant source of the POM pool

The diel variations in C/N ratio andδ13

C during the rainy and dry seasons suggested that the POM pool at any time was a mixture of mangrove detritus and marine phytoplank-ton (Fig 4) The mangrove contribution was up to 75.7% and 55.8% during low tides, whereas, at high tides, it was only 26.5% and 21.9% for the rainy and dry seasons, re-spectively Thus, mangroves exported detritus to the creeks during ebb tides The dynamics of POM in this mangrove ecosystem are similar to the organic matter out-welling functions from other mangrove ecosystems (Dittmar and Lara2001a; Romigh et al.2006) Therefore, the mangrove ecosystem of XTP probably plays an important role for the local and global marine carbon budgets

These findings have significant implications for (1) un-derstanding the functioning of mangrove ecosystems, (2) examining organic carbon sources in mangrove sediments, and (3) explaining the diets of various aquatic animal spe-cies and food web structure in this important mangrove ecosystem in future studies

Acknowledgments The authors are grateful to staff of Xuan Thuy National Park, Vietnam, for their help with sampling We express our sincere thanks to anonymous reviewers and Dr Todd W Miller for their critical reviews 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 Technol-ogy, Japan.

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