Food sources of macro invertebrates in an important mangrove ecosystem of vietnam determined by dual stable isotope signatures

Food sources of macro-invertebrates in an important mangrove ecosystem of Vietnam determined by dual stable isotope signatures Nguyen Tai Tuea,b,⁎ , Hideki Hamaokab, Atsushi Sogabec, Tra

Food sources of macro-invertebrates in an important mangrove ecosystem of Vietnam determined by dual stable isotope signatures

Nguyen Tai Tuea,b,⁎ , Hideki Hamaokab, Atsushi Sogabec, Tran Dang Quyd, Mai Trong Nhuand, Koji Omorib

aGraduate School of Science and Engineering, Ehime University, 2‐5 Bunkyo-cho, Matsuyama, Japan

bCenter for Marine Environmental Studies, Ehime University, 2‐5 Bunkyo-cho, Matsuyama, Japan

cGraduate School of Biosphere Science, Hiroshima University, 1-4-4 Kagamiyama Higashi-Hiroshima 739‐8528, Japan

dFaculty 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 12 February 2012

Received in revised form 24 April 2012

Accepted 9 May 2012

Available online 16 May 2012

Keywords:

Stable Isotopes

Invertebrate

Food Sources

Mangrove Ecosystem

Vietnam

Dual stable isotope signatures (δ13C and δ15N) were applied to determine the contribution of mangrove ma-terials and other organic carbon sources to the invertebrate community in an ecologically important man-grove ecosystem of Vietnam We have analyzed 181 specimens of 30 invertebrate species and found δ13C and δ15N ranging from −14.5 to −26.8‰ and from 1.3 to 12.1‰, respectively From taxa measured for stable isotopes, polychaete, gastropods, bivalves, and grapsid crabs living in mangrove forest showed relative low

δ13C values, while fiddler crabs inhabiting in the land–water ecotone showed the highest δ13C values The

δ13C showed that just a few mangrove inhabitants directly relied on the mangrove materials The wide ranges

of δ13C and δ15N signatures indicated that the invertebrates utilized heterogeneous diets, comprising benthic microalgae, marine phytoplankton, particulate organic matter, sediment organic matter, mangrove detritus, and meiofauna and rotten animal tissues as the supplemental nutrient food sources Moreover, the significant correlation between δ13C values and body sizes of invertebrates showed that snails Littoraria melanostoma and Terebralia sulcata, bivalve Glauconome virens, and portunid crab Scylla serrata exhibited ontogenetic shifts

in diets The present study showed that adjacent habitats such as tidal flat and mangrove creeks seem to con-tribute an important microalgal food resource for invertebrates and highlighted the need for conservations of mangrove forests and the adjacent habitats

© 2012 Elsevier B.V All rights reserved

1 Introduction

Mangrove forests have often been regarded as one of the most

productive ecosystems in the (sub)tropical coastal waters They are

characterized by the high biodiversity of invertebrates and fish

(Nagelkerken et al., 2008; Sasekumar et al., 1992) Several hypotheses

have been proposed for the explanation of the high biodiversity of

the invertebrates and fish in the mangrove ecosystem, including

(1) mangroves characterize by the structural complexity of

pneumat-ophores and/or prop roots which provide shelter from predators for

invertebrates and fish (Kon et al., 2009), and (2) mangroves produce

large amounts of organic matter that form the basis of the estuarine

Heald, 1972) Therein, the later hypothesis has long been debated

be-tween non-stable isotopic (i.e., stomach content analysis, fecal

analy-sis, and direct observation (Michener and Lajtha, 2007)) and isotopic

studies Based on stomach content analysis several studies have demon-strated that the mangrove detritus contributes a significant amount

of organic carbon fueling detrital-based food webs (Nanjo et al., 2008; Nordhaus et al., 2011; Odum and Heald, 1972) Nevertheless, the isoto-pic studies have failed to confirm the ingestion of mangrove materials in the estuarine food webs (France, 1998; Rodelli et al., 1984), apparently because the mangrove detritus is too refractory and the simple inges-tion of mangrove detritus does not indicate any direct assimilainges-tion of that material (Fry and Ewel, 2003)

Invertebrates that play important roles in the mangrove structure processes (Cannicci et al., 2008), organic carbon dynamics (Robertson and Daniel, 1989), biogeochemical processes (Kristensen, 2000), are preyed upon foraging fishes during high tides (Kruitwagen et al., 2010), and serve as important links between mangrove detritus and estuarine secondary production (Lee, 2008) The gastropods and brachyuran crabs are the most dominant invertebrate groups in the mangrove eco-system They play an important role in leaf litter turnover, for example, sesarmid crabs can consume approximately 70% of the total annual litter fall from the forest floor (Robertson and Daniel, 1989), significantly retaining mangrove organic production and reducing direct export Moreover, recent isotopic studies pointed out that several invertebrate species rely on the mangrove materials and such use of mangrove carbon

⁎ 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 (Cell); fax: +81 89 927 9643.

E-mail address:tuenguyentai@gmail.com (N.T Tue).

1385-1101/$ – see front matter © 2012 Elsevier B.V All rights reserved.

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depends on the morphological characteristics of mangrove forests

(Bouillon et al., 2004) and/or microhabitats (Kon et al., 2007)

Stable isotopes have been used to track organic matter flows and

to measure the food web structures, and ontogenetic shifts in diets

(Michener and Lajtha, 2007) Stable isotopes of carbon (δ13C) and

nitrogen (δ15

N) have frequently applied for tracing the flow of organic

matters in lake (Post, 2002), estuarine (Peterson et al., 1985), and

ma-rine (Michener and Lajtha, 2007) food webs Therein, δ13C can be

used to evaluate the ultimate sources of carbon for an organism when

the isotopic signatures of the sources are different (Post, 2002) The

δ13C is enriched approximately 0.5–1‰ in the animal relative to its

diet The basic assumption for estimation of trophic position is based

on a conservative enrichment of 3–4‰ δ15N in a consumer relative to

its diet (Michener and Lajtha, 2007) Stable isotope signatures (δ13C

and δ15N) are therefore powerful methods for determining the ingested

food sources and the food assimilation over long period of a consumer

Stable isotope measurements have been applied to trace mangrove

detritus in the estuarine environment (Dittmar et al., 2001), to estimate

the assimilation of mangrove detritus by a specific species such as snail

(Penha-Lopes et al., 2009; Slim et al., 1997) and fiddler crabs (France,

1998) Yet, stable isotope analysis has been rarely applied to identify

food sources of a large range of invertebrate taxa in the mangrove

eco-system (i.e.,Bouillon et al., 2002a, 2004; Kon et al., 2007; Kruitwagen

et al., 2010), particularly in developing countries such as Vietnam

In the present study, we have analyzed dual stable isotope

signa-tures (δ13C and δ15N) of 181 specimens of 30 invertebrate species

for figuring out the question what degree of mangrove materials is

as-similated by invertebrates in an ecologically important mangrove

ecosystem of Vietnam The objectives of the present study are (1) to

determine the utilization of food sources by invertebrates and (2) to

examine the question of whether these invertebrates exhibit the

on-togenetic shifts in diets

2 Materials and methods

2.1 Study area

The present study was conducted in an estuarine mangrove

ecosys-tem of Xuan Thuy National Park (XTP) in northern Vietnam (Fig 1) The

detailed description for the XTP has been shown elsewhere (Tue et al.,

2012, 2012) Briefly, XTP covers a total wetland area of 12,000 ha, of

which about 3000 ha are covered by mangrove forests The most

abun-dant mangrove species are Sonneratia caseolaris, Kandelia obovata,

Aegiceras corniculatum, and Avicennia marina The mangrove ecosystem

provides a broad array of ecosystem services such as maintaining high biodiversity, storm protection, erosion mitigation, local climate regula-tion, fishery producregula-tion, and the subsistence livelihood The economic values of the mangrove ecosystem have been estimated from 31,565,720 to 34,620,100 VND/year (price in 2002, currency exchange rate, US$1= VND15,300) (Nhuan et al., 2003) Therein, invertebrate production contributes an important economic value and livelihood for local communities, estimating VND30,000 per local person/day by selling crabs, shrimps, and oysters (Nhuan et al., 2009) In the year

1989, XTP is declared as the first wetland of International importance

of Southeast Asia (http://www.ramsar.org) In the year 2004, through UNESCO XTP is designated to be the most important sub-zone of the Red River Delta Biosphere Reserve (http://www.unesco.org) The characteristics of seasons, tides, salinity, and other environmen-tal conditions are well described elsewhere (Tue et al., 2012) Briefly, the XTP has a distinct monsoon climate with a rainy season from June

to October, and a dry season from November to May The tides are char-acterized with tidal amplitudes ranging from 1.5 to 1.8 m, and the max-imum and minmax-imum tidal levels are 3.6 m and 0.5 m, respectively The salinity ranges from 12.5 to 25.6‰ for the rainy season and from 21.1 to 26.5‰ for the dry season

2.2 Field sampling

Field work was conducted from 28 January to 10 February 2008 Invertebrates were collected at two sites in the XTP (Fig 1a) The sampling transect is started at the tidal flat and creek bank toward the dense mangroves at sites 1 (Fig 1b) and 2 (Fig 1c), respectively The benthic invertebrate specimens were manually collected by hand during low tides Total invertebrate specimens were 181, belonging to

30 species (Table 2) The species of polychaete, bivalves (Geloina coaxans and Laternula truncata), gastropods, and grapsid crabs were collected

within mangrove forests The crabs of Leucosiidae, Ocypodidae families,

and portunid crab (Scylla serrata) were collected from fringe mangrove forests, tidal flat, and creek banks Prawns and portunid crab (Charybdis

helleri) were collected from Tra Creek by the gill nets during spring and

ebb tides The number of invertebrate samples is shown inTable 2 These invertebrates were selected in the present study because they are predominant and high economic values in the XTP (Cuong and Khoa,

2004)

In general, the benthic microalgae (BMA) production is low in the mangrove forests due to the high tannin concentration and low light

Fig 1 Sampling sites within the mangrove ecosystem of Xuan Thuy National Park, Vietnam (a), the sampling transect at site 1 (b) and at site 2 (c) In each sampling transect the

N.T Tue et al / Journal of Sea Research 72 (2012) 14–21

penetration (Alongi, 1994) However, the BMA production has been

considered as an important food source for invertebrates in the

man-grove ecosystem (Bouillon et al., 2002a; France, 1998) In present

study, the BMA samples were collected at five sites in the tidal flat

and creek bank to examine the contribution of this organic matter

source to the invertebrates The BMA samples were extracted from

conspicuous layers on surface sediments according to the procedure

described byBouillon et al (2002a)

The invertebrate and BMA samples were packed in labeled

poly-ethylene bags and immediately stored in iceboxes, transported to

the laboratory for further processing and were frozen at −20 °C

until analysis

2.3 Sample preparation and analysis

In the laboratory, the polychaete species were kept alive for 24 h

in filtered seawater to allow them to excrete their gut contents The

invertebrate specimens were washed by distilled water and wiped

with paper And then, the carapace width of crabs and prawns and

shell size of mollusks were measured The invertebrates were

dissect-ed and only their muscle tissues were taken for stable isotope

analy-sis The muscle tissues were dried at 60 °C for 24 h, and then ground

to a fine powder by an agate mortar and pestle

Post et al (2007)showed that δ13C can be altered by changes in

lipid contents of invertebrates In the present study, the lipid content

is extracted from the invertebrate tissues prior to stable isotope

anal-ysis The pulverized muscle tissue was then placed in an Eppendorf

tube and immersed in a 2:1 chloroform:methanol solution for 24 h

to extract lipids After lipid extraction, the muscle tissue was dried

in an electric oven at 60 °C for 24 h BMA was treated with 1 N HCl

for δ13C analysis according to the procedure for sediments described

byTue et al (2012), and sub-sample of BMA for δ15N analysis was

not treated with acid

The muscle tissues and BMA samples were packed in tin capsules

and analyzed for stable isotope signatures (δ13C and δ15N) by using a

stable isotope mass spectrometer (ANCA-GSL; Sercon Inc., UK) Stable

isotopic compositions are expressed by δ values, which are measured

as a ratio of the heavy to light isotopes in a sample relative to a standard

by an equation: δX ‰Þ ¼ Rsample=Rstandard−1Þ  1;000, where X is the

isotope value in permil (‰) and R is the ratio of the heavy to the light

isotope of the sample (Rsample) to a standard (Rstandard) For stable

carbon and nitrogen isotope ratios, R is13C/12C and15N/14N,

respective-ly The standards were Pee Dee Belemnite (PDB) limestone carbonate

for δ13C and atmospheric nitrogen for δ15N During analysis processes,

L-histidine was used as certified reference material Analytical errors

were 0.1‰ for δ13C and 0.2‰ for δ15N, respectively

Linear regression analysis was used to examine the size-specific

shifts in δ13C and δ15

N within each invertebrate species The signifi-cance level of linear regression was 0.05 (p b0.05) for each statistical

procedure

3 Results and discussion

3.1 Background data for potential organic carbon food sources of

invertebrates

The characteristics of major primary producers and other potential

organic carbon food sources of invertebrates in the XTP have been

reported elsewhere (Tue et al., 2012, 2012) The stable isotope

compositions of the potential food sources are sumarized inTable 1

The δ13C of the potential food sources ranged from −29.9± 0.5 to

−20.2± 0.6‰ The δ13C values were increased as follows: mangrove

leaves, mangrove sediments, tidal flats, creek bank and bottom

sedi-ments, particulate organic matter (POM), marine phytoplankton, and

BMA The δ15N values of the potential food sources ranged from 0.7 ±

0.6 to 3.9± 0.9‰ The δ15N values showed an increasing trend from

mangrove leaves, though to BMA, POM, phytoplankton, and to creek bank and mangrove sediments The stable isotope compositions of BMA from the present study were consistent with previous reports in mangrove ecosystems from Coringa Sanctuary, India (Bouillon et al., 2002a, 2004) and Sikao Creek, Thailand (Kon et al., 2007)

3.2 Food sources of invertebrates in the mangrove ecosystem 3.2.1 Stable isotope compositions of invertebrates

From the taxa measured for stable isotopes, the δ13C values of in-vertebrates ranged from −14.5 to −26.8‰ (Table 2,Figs 2 and 3) The lowest δ13C values were expressed in two polychaete species,

followed by bivalves L truncata and G coaxans, grapsid crabs Episesarma

versicolor and Metopograpsus messor, snails Cassidula aurisfelis, Cerithidea ornata, and Littoraria melanostoma, and grapsid crab Sesarma dehaani.

Highest δ13C values were expressed in ocypodid crabs Uca acuta,

Uca borealis, Uca flammula, grapsid crab Metaplax elegans, and followed

by ocypodid crab Uca urvillei, slug Onchidiidae spp., crab of family Leucosiidae, ocypodid crab Uca arcuata, the portunid crab C helleri, the grapsid crab Metaplax longipes, prawns, and the portunid crab S serrata

(Table 2,Fig 2)

The δ15N values ranged from 1.3 to 12.1‰, the grapsid crab M.

elegans expressed the lowest average δ15N value, followed by the snails

L melanostoma, C ornata, C aurisfelis, the grapsid crabs M messor and S dehaani, slug Onchidiidae spp., and bivalves L truncata and G coaxans.

Highest δ15N values were expressed in prawns of Palaemonidae and

Penaeidae families, and polychaete (Diopatra neapolitana), followed

by the portunid crabs C helleri and S serrata, polychaete (Nephthys

polybranchia), fiddler crabs, and tidal flat bivalves Glauconome virens

and Ensis magnus (Fig 2,Table 2)

3.2.2 Food sources of invertebrates in mangrove ecosystem 3.2.2.1 Polychaete The polychaete N polybranchia had the δ13C value

in the range of mangrove leaves (Fig 2) The δ13C value of polychaete

D neapolitana was similar to that of mangrove sediments or more

enriched 1–2‰ than that of mangrove leaves These results indicated that two polychaete species may obtain the carbon food sources from mangrove materials such as decomposed leaves on the forest floor and/

or scavenge mangrove detritus in the mangrove sediments δ13C

signa-tures of polychaete confirmed that the feeding guild of D neapolitana is

herbivore and/or scavenger (Fauchald and Jumars, 1979), and suggested

the feeding guilds of N polybranchia may be a motile subsurface

deposit-feeder More depleted in13C of polychaete (i.e., N polybranchia) compare

Table 1 Stable carbon and nitrogen ratios of the potential organic carbon food sources in the mangrove ecosystem of Xuan Thuy National Park, Vietnam.

Organic matter sources ACR δ13C δ15N n References

Kandelia obovata Kao − 27.2 0.9 1.6 0.9 9 Tue et al.

(2012)

Aegiceras corniculatum Aec − 27.3 0.6 0.7 0.6 9

Sonneratia caseolaris Soc − 29.9 0.5 2.4 0.2 8 Marine phytoplankton Phyto − 21.2 0.5 3.6 0.3 3

Benthic microalgae BMA −20.2 0.6 2.3 0.6 5 This study Mangrove sediments Msed − 25.9 1.4 4.3 0.4 12 Tue et al.

(2012) Adjacent habitat

sediments

Ased

Creek bank sediments Cbs − 24.1 1.2 3.9 0.9 5 Tue et al.

(2012) Bottom creek

sediments

Tidal flat sediments Tfs − 24.2 1.0 3.4 1.2 13 ACR: acronym; n: number of samples; SD: 1 standard deviation.

N.T Tue et al / Journal of Sea Research 72 (2012) 14–21

to that of other benthic fauna was also reported inBouillon et al (2002b), apparently a preferential food source of polychaete living in dense mangrove forests was the mangrove detritus The polychaete

δ15N values were high (Table 2,Fig 2), indicating that they occupied higher trophic position in the benthic food web The trophic position suggested these polychaete species were also nourished on the enriched

15N food sources (i.e., animal tissues) The Nephtyids can feed on small invertebrates, including mollusks, crustaceans, and other polychaete The Onuphids may nourish by the rotten animal tissues as the supple-mental nutrient food sources (Fauchald and Jumars, 1979) Nevertheless, this explanation needs to be confirmed by an experimental determina-tion of the variadetermina-tion of isotopic composidetermina-tions of polychaete

3.2.2.2 Gastropods Two individuals of slug Onchidium spp had an

aver-age δ13C value (−17.7‰) close to that of the BMA, and δ15N values were higher from 3 to 4‰ than that of BMA (Fig 2) The isotopic

com-positions obviously indicated that Onchidium spp grazed on the

con-spicuous layer of BMA on surface sediments

The mangrove snails showed a wide range of δ13C, from −23.7 to

−16.4‰ (Fig 3a) The wide range of δ13C signatures indicated the heterogeneous diets of snails (Penha-Lopes et al., 2009; Rodelli et al.,

1984) that can be explained by several factors, such as (1) feeding guilds (Guest et al., 2004), (2) ontogeny (Fratini et al., 2004; Slim et al., 1997), and (3) microhabitats (Fratini et al., 2004; Penha-Lopes et al., 2009)

As shown inFig 3a, the δ13C values of the snails were within the ranges of the BMA, marine phytoplankton, POM, and sediment organic matter of adjacent habitats or slightly higher than the mangrove sedi-ments The δ13C values from the present study indicated the preferential BMA, marine phytoplankton, and POM over the mangrove leaves of the mangrove snails These findings are consistent with the results of diets

of Terebralia sulcata and L melanostoma from mangrove ecosystems of

Okinawa, Japan (Meziane and Tsuchiya, 2000) and Coringa Wildlife Sanctuary, India (Bouillon et al., 2002a), respectively The food sources

of snail T sulcata from the present study were contradictory with the

stomach content analysis that reported for mangrove snails of genus

Terebralia such as T palustris (Fratini et al., 2004; Penha-Lopes et al., 2009; Slim et al., 1997).Slim et al (1997)showed that the stomach

of adult and juvenile snail T palustris contained up to 62.5% and 19% of

mangrove materials, respectively The contradictory results between the two methods can be explained by the snails that ingested the man-grove litters but not always assimilated (Fry and Ewel, 2003; Lee et al., 2001) Hence, the mangrove materials probably did not form an important contribution to energy intake by these mangrove snails

Table 2

Stable isotope signatures (δ 13 C and δ 15 N) of invertebrates in Xuan Thuy National Park,

Vietnam.

Polychaete

Diopatra neapolitana Dn 52.7 2.1 11.3 0.8 − 25.5 0.1 3

Gastropods

Cassidula aurisfelis Ca 25.2 2.2 5.6 0.7 − 22.8 1.1 3

Dostia violecea Dv 16.6 1.9 7.3 3.5 − 19.2 1.5 11

Littoraria melanostoma Lm 20.6 3.6 4.1 2.8 − 22.1 0.6 10

Terebralia sulcata Ts 26.1 4.9 9.3 0.9 − 19.6 2.5 18

Cerithidea ornata Co 23.0 1.4 6.8 1.3 − 22.4 0.7 5

Bivalves

Glauconome virens Gv 30.3 4.2 9.2 0.5 − 21.3 0.5 11

Laternula truncata Lt 49.7 1.2 5.7 0.8 − 24.6 0.7 3

Geloina coaxans Gc 28.3 1.9 7.1 0.3 − 24.3 0.3 4

Crabs

Grapsidae

Metopograpsus messor Mm 11.8 0.7 5.0 1.0 − 22.7 0.9 6

Sesarma dehaani Sd 17.7 1.2 7.8 0.9 − 22.6 0.8 3

Metaplax elegans Me 7.9 0.5 3.3 0.7 − 16.7 1.3 6

Metaplax longipes Ml 16.0 3.2 9.9 0.3 − 18.5 1.1 11

Leucosiidae

Ebalia malefactrix Ebm 18.2 0.7 9.3 0.6 − 17.6 1.1 6

Ocypodidae

Uca urvillei Uu 13.7 2.1 8.0 0.6 − 17.1 0.5 3

Uca arcuata Uar 25.7 2.7 8.8 0.5 − 19.0 0.9 7

Portunidae

Scylla serrata Ss 49.5 18.0 10.0 0.5 − 19.4 1.8 17

Charybdis helleri Ch 51.8 14.4 10.6 0.6 − 17.6 1.0 6

Prawns

Palaemonidae

Macrobrachium rosenbergii Mr 74.9 9.3 10.9 0.3 − 19.5 1.2 8

Penaeidae

Metapenaeus joyneri Mj 81.6 6.5 11.0 0.3 − 19.8 0.3 6

Penaeus merguensis Pm 62.6 14.1 11.1 0.6 − 19.0 1.2 11

Penaeus monodon Pmo 57.1 4.3 10.4 0.3 − 17.9 0.6 5

ACR: acronym; n is number of samples; mean, and mean and SD values are given

where n = 2, and n ≥ 3, respectively; L: body length, for crab is carapace length, and

other species is total body length.

Fig 2 Dual isotope plot of the δ 15 N and δ 13 C (mean ± SD, ‰) values of invertebrates from the mangrove ecosystem of Xuan Thuy National Park, Vietnam The point denotes the mean value and error bar denotes 1 SD Boxes indicate the range of stable isotope compositions of the potential organic carbon food sources of invertebrates Refer to Tables 1

N.T Tue et al / Journal of Sea Research 72 (2012) 14–21

Subsequently, fecal materials of snails may contain large undigested

mangrove materials (Lee et al., 2001) In the ecological aspect, the

fecal materials can be quickly increased in bacteria biomass and nutrient

contents that can be easily assimilated by other deposit-feeders As a

result, snails play as an important linkage between mangrove primary

producer and other invertebrates (Lee, 2008)

Mangrove snails change their microhabitats with the life stages

(Fratini et al., 2004; Penha-Lopes et al., 2009) Juveniles prefer the

ad-jacent habitats such as creek bank and tidal flat and fringe mangrove

forest They will migrate into the landward zone after reaching to

adult stages (Fratini et al., 2004; Penha-Lopes et al., 2009) In addition,

many deposit feeders shift their diets during the development to meet

their nutrient demands (Hentschel, 1998) The ontogenetic shift in

diets was reported for the snail of genus Terebralia (i.e., T palustris)

from mangrove forests of Gazi Bay, Kenya (Slim et al., 1997), Inhaca

Island, Mozambique (Penha-Lopes et al., 2009) and Malindi, Kenya

(Fratini et al., 2004) These authors reported that the juvenile snails

are detritivorous and the adults are mainly leaf-litter consumers Our

re-sults showed that δ13

C was significantly negatively correlated with the

shell size of L melanostoma (regression line: δ13C =−0.1 × shell_size

−20.1; R²=0.39, p b0.05,Fig 4a) and of T sulcata (regression line:

δ13C =−0.44 × shell_size− 8.1; R² =0.73, p b 0.05,Fig 4b) These

re-sults demonstrated that the ontogenetic shift in diets may also occur

with these snail species The small snails may graze on enriched in13C

food sources such as the BMA, marine phytoplankton, and POM

They will shift to depleted13C food sources such as sediment organic

matter or mangrove materials when they reach to adult stages For

the L melanostoma species, they are generalist grazers, grazing on

sur-faces of the substrata non-selectively (Lee et al., 2001) In the present

study, the snails L melanostoma were found abundant on mangrove

roots and stems where they ingested the plant tissues and filamentous

algae on the plant surface Unfortunately, we could not measure the

sta-ble isotope compositions of the filamentous algae on mangrove roots

and stems However, the stable isotopic values of the filamentous algae

fromBouillon et al (2002a)suggested that the snail L melanostoma

fed on mixed diet that is probably composed of filamentous algae and

mangrove materials, with the latter sources increasing as with the size

of L melanostoma The trophic dimorphism between juveniles and adults

has been explained by a difference in radula morphology of gastropods

(Slim et al., 1997)

3.2.2.3 Bivalves Four filter feeding bivalve species can be separated by

the δ13C signatures (Table 2,Figs 2 and 3b) The δ13C values of two

bi-valves G coaxans and L truncate living in the dense mangrove forests

were in the range of POM and mangrove sediments (Table 2;Fig 2) The mangrove detritus proportion in POM was >50% during the flood tide in the small creeks of this mangrove ecosystem (Tue et al., 2012)

As a result, the mangrove detritus may contribute a significant propor-tion to diets of these mangrove bivalves This finding is consistent with

the report on the diet of bivalve G coaxans in the Okinawa mangrove

ecosystem (Bachok et al., 2003)

Fig 3 Frequency distribution of δ 13 C values of invertebrate groups from the mangrove ecosystem of Xuan Thuy National Park, Vietnam The gray bar indicates the ranges of the potential organic carbon food sources as referred in Table 1

Fig 4 Correlation between shell size and δ 13 C (pb0.05) of gastropod and bivalve species

N.T Tue et al / Journal of Sea Research 72 (2012) 14–21

The δ13C signatures of two bivalves G virens and E magnus living

in the tidal flat were in the ranges of marine phytoplankton and

BMA, and slightly enriched than that of POM Thus, they probably

had major diets from the POM, and the microalgae, consisting of

ma-rine phytoplankton (Rodelli et al., 1984) and BMA resuspension from

surface sediments by hydrodynamics (Cognie et al., 2001; Kang et al.,

1999) Interestingly, the measurements of δ13C values and shell sizes

were significantly negatively correlated for individuals of G virens

species (regression line: δ13C = −0.1 × shell_size − 18.6; R² = 0.57,

p b 0.05,Fig 4c), implying the ontogenetic shift in diets of the G virens

from the microalgal diets to POM containing high proportion of

man-grove detritus The difference in food resource utilization of juvenile

and adult G virens may relate to the mechanisms of selective feeding,

strength of inhalant flows, siphon sizes, and lipid contents of

filter-feeding bivalves (Kang et al., 1999)

3.2.2.4 Crabs The stable isotope compositions of these brachyurans

varied with the habitats, feeding behavior, and taxonomy (Table 2;

Fig 2) In general, the crabs fed on variety of food resources, consisting

of BMA, marine phytoplankton, POM, and sediment organic matters

from adjacent habitats and mangrove forests (Table 1;Figs 2 and 3c)

For grapsid crabs, the average δ13C value of M messor was −22.7 ±

0.9‰ (n= 6), indicated that the grapsid crab M messor did not directly

rely on the mangrove leaves The Metopograpsus spp has been reported

to be an omnivorous that nourished animals, plant materials, and

inor-ganic sediments (Poon et al., 2010) However, our results may not

sup-port the omnivorous feeding guild of the M messor in the XTP due to

the low δ15N values in the muscle tissues (Table 2) In addition, δ13C

was slightly higher than sediment organic matters of mangrove and

adjacent habitat sediments, and POM (Fig 2) The stable isotope

com-positions thus suggested that the grapsid crab M messor fed on a bulk

mixture of sediment organic matters with high proportion of mangrove

detritus

The δ13C of sesarmid crab S dehaani was slightly enriched relative

to that of the mangrove sediment organic matters (Table 2,Fig 2),

and the δ15N was 3.5‰ higher relative to these substrates The

isoto-pic compositions demonstrated that the sesarmid crab S dehaani fed

on the organic matters in the mangrove sediments, which composed

of the marine phytoplankton and mangrove detritus, with the later

sources being predominant (Tue et al., 2012)

Only one specimen of sesarmid E versicolor was collected in the

present study, thus it is not obvious to indicate the food sources of

this species However, the stable isotope compositions from the

pre-sent study are similar to those reported for the E versicolor from Indian

mangrove forest (Bouillon et al., 2002a) and Indonesian mangrove

for-est (Nordhaus et al., 2011) These patterns suggested that the sesarmid

E versicolor may have alike diets in different mangrove forests The diets

of E versicolor included sediment organic matter, other invertebrates,

carrion, and mangrove litter thereof (Nordhaus et al., 2011)

The δ13C values of two Metaplax species and the pebble crab Ebalia

malefactrix were higher than those of other sesarmid and grapsid

crabs (Table 2,Fig 2) The high δ13C values indicated that mangrove

organic carbon was not significantly contributed to diets of these crabs

The δ13C signatures of two Metaplax species showed that they nourished

the BMA, e.g., benthic diatoms and cyanobacteria (Bouillon et al., 2002a)

Moreover, the species M longipes had significantly higher δ15N than the

species M elegans (pb0.05) The δ15N values demonstrated that the

species M longipes fed on a higher trophic position than the species

M elegans The stable isotope compositions showed that the species

M longipes ingested the BMA, animal carrions, and juveniles of

gastro-pods and bivalves

For fiddler crabs (Uca spp.), δ13C values varied from −20.6 to

−14.3‰, with the most enriched in13C for U acuta species (δ13C values

of two individuals were −14.3‰ and −14.6‰) However, δ15N of the

Uca spp species had a small range, varying from 7.3 to 9.5‰ The

small range of δ15

N indicated that these fiddler crabs fed on the same

trophic level The fiddler crabs are conspicuous residents of the land– water ecotone in the mangrove ecosystem (France, 1998) They are de-posit feeders, ingesting organic matter from the exposed mud at low tide (Hogarth, 2007) Therefore, the δ13C values suggested that they nourished nutrients from surface sediments with a major food source from the BMA (Fig 2) Two species U arcuata and U urvillei collected

from the tidal flat at site 1 (Fig 1b) were enriched in13C compared to the BMA, referring to the preferential diets from the BMA The species

U borealis was collected from tidal flat and two species U flammula

and U acuta were collected from the creek bank (Fig 1c), had δ13C values higher from 2.6 to 4.6‰ than that of the BMA, suggesting that they may not rely only on the BMA food source Moreover, the δ15N values of these fiddler crabs were 5.0 to 7.2‰ greater than that of the BMA The15N enrichment has probably ruled out the substantial dietary contribution from the BMA, given the expected fractionation 3–4% en-richment in δ15N per trophic level (Post, 2002) Obviously, these fiddler crabs nourished other food sources with/without BMA such as bacteria (France, 1998), and ciliate protozoa and nematodes (Hogarth, 2007) In agreement with our study, the fiddler crabs in the mangrove ecosys-tems of Malaysia (Rodelli et al., 1984), Puerto Rico (France, 1998), and Coringa Wildlife Sanctuary, India (Bouillon et al., 2002a) were highly selective for the BMA and other food sources (i.e., bacteria, ciliate proto-zoa, and nematodes) than the mangrove detritus

The stable isotope compositions of two portunid crabs showed that they are among the top predators in the mangrove ecosystem (Fig 2) In Thailand mangrove forests, the portunid crabs feed on the slow moving benthic animals, including bivalves, snails, other crabs, and polychaete (Thimdee et al., 2004) Our results suggested that the portunid crabs probably relied on the various types of

inverte-brates (i.e., M longipes, T sulcata, Nassarius olivaceus, Dostia violecea, and G virens) Moreover, our findings showed that δ13C exhibited a sig-nificant negative correlation with carapace width >30 mm of portunid

crab S serrata (regression line: δ13C =−0.1 × carapace_width −14.4; R² =0.6, p b 0.05,Fig 5) This pattern suggested that the larger

individ-uals S serrata extensively fed on other invertebrates in the lower trophic levels (i.e., C ornata, S dehaani, and M messor) and microphytobenthos

for enough nutrient demand

3.2.2.5 Prawns Stable isotope composition for three penaeid shrimps

and Macrobrachium rosenbergii species were tightly clustered (Fig 2), which indicated of an assessment to the similar food resources As shown inFig 3d, the major food sources of these penaeid shrimps were BMA, marine phytoplankton, POM, and sediment organic matters

In addition, the δ15N signatures of the penaeid shrimps were highest that are indicative of the top predators among the invertebrates in the mangrove ecosystem of XTP The high δ15N signatures were indica-tive of a feeding on other small preys such as juveniles of crabs, gastro-pods, and bivalves In agreement with our study,Chong and Sasekumar (1981)showed that penaeid shrimps are opportunistic omnivorous and are known to feed on a variety of food — depending on the locality and availability of food items

Fig 5 Correlation between carapace width and δ 13C (pb 0.05) of portunid crab S serrata.

The square symbols are the values of individuals with carapace width b30 mm, and these individuals were not used to calculate the regression between carapace width and δ 13 C.

N.T Tue et al / Journal of Sea Research 72 (2012) 14–21

4 Conclusions

The dual stable isotope signatures (δ13C and δ15N) were applied to

identify the utilization of food sources by invertebrates in an

ecolog-ically important mangrove ecosystem of Vietnam The results showed

that the invertebrates had heterogeneous diets and just a few mangrove

inhabitants such as polychaete, gastropods, bivalves, and grapsid crabs

directly relied on the mangrove detritus In addition, fiddler crabs living

in the land-water ecotone were highly selective for the BMA and other

food sources (i.e., bacteria, ciliate protozoa, and nematodes) than the

mangrove detritus The present study supports other isotopic studies

that invertebrates assimilate a greater production of high nutrient food

sources such as BMA, marine phytoplankton, meiofauna, and animal

carrions in the mangrove ecosystem The ontogenetic shifts in diets

were identified for several invertebrate species, consisting of snails

L melanostoma and T sulcata, bivalve G virens, and portunid crab

S serrata On the basis of the δ13C and δ15N signatures, feeding guilds

of invertebrates could be divided into low trophic position, consisting

of snails, bivalves, grapsid crabs (exclude: M longipes) and high trophic

position (polychaete, grapsid crab M longipes, portunid crabs, and

prawns) This study revealed that the snails may play as a linkage

be-tween mangroves and other invertebrates in the high trophic positions,

and the estuarine food webs (Proffitt and Devlin, 2005) In addition, the

land–water ecotone such as tidal flats and creek banks seem to

contrib-ute an important microalgal food resource for invertebrates (e.g., fiddler

crabs and prawns) These results highlight the need for conservations of

mangrove forests and other habitats such as tidal flat and mangrove

creek systems

Acknowledgments

The authors are grateful to the staff of Hanoi University of Science,

Vietnam for supporting this research We express our sincere thanks

to anonymous reviewers for their critical reviews and comments

which significantly improved this manuscript This work was

supported by the “Global COE Program” from the Ministry of

Educa-tion, Culture, Sports, Science and Technology, Japan

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