Introduction
The carbon captured by marine living organisms has been termed “blue
carbon” (Nelleman et al., 2009). Among marine ecosystems, the organic carbon
(OC) accumulation rate of vegetated coastal systems such as seagrass meadows,
mangrove forests, and salt marshes is estimated to be higher than that of
terrestrial forests (Mcleod et al., 2011). The global total OC stock
contained in the top 1 m of sediment and in the plant biomass in these
vegetated ecosystems is estimated to be 0.63–8.54 Pg C (Pendleton et al.,
2012). Thus, vegetated ecosystems are expected to contribute greatly to the
mitigation of global warming. In this regard, seagrass meadows have attracted
particular attention because they are one of the most dominant blue carbon
sinks (Kennedy et al., 2010; Fourqurean et al., 2012). However, the OC stock
of a seagrass meadow is highly variable, depending on geographical region
(Miyajima et al., 2015), seagrass species (Lavery et al., 2013),
microlocation within a seagrass patch (Ricart et al., 2015), and the patch
scale (Miyajima et al., 2017). Hence, to develop a precise methodology of OC
estimation and reduce the uncertainty of the global estimate, it is necessary
to understand the factors controlling OC stocks in seagrass meadows (Duarte
et al., 2013).
Seagrass meadows enhance the accumulation of sedimentary OC by directly
supplying abundant OC from their high production (Duarte et al., 2010), by
reducing sediment resuspension, and by promoting sedimentation of
autochthonous and allochthonous OC in the water column (Agawin and Duarte,
2002; Gacia and Duarte, 2001; Gacia et al., 2003; Hendriks et al., 2008).
However, our knowledge of the factors that mediate the sequestration of
sedimentary OC by seagrass meadows is limited. For example, the chemical
recalcitrance of the supplied organic matter (Trevathan-Tackett et al., 2017;
Watanabe and Kuwae, 2015) and the specific surface area of the sediment
(Miyajima et al., 2017) are factors that control the sedimentary OC stock in
seagrass meadows. Recent studies have also shown that, in addition to
chemical and physical factors, biological factors such as primary
productivity, seagrass shoot density, and the amount of leaf material (as
indicated by the leaf area index) also affect the sedimentary OC stock
(Samper-Villarreal et al., 2016; Serrano et al., 2014, 2016b). In addition,
an increase in the amount of leaf material may enhance the trapping of
suspended OC and, thus, the accumulation of sedimentary OC (Dahl et al.,
2016; Gacia et al., 1999). An increase in seagrass density may also cause an
increase in seagrass production per unit area and thus enhance the direct
supply of seagrass-derived OC. However, few previous studies have analyzed
the controlling factors and provenance of sedimentary OC along a seagrass
biomass gradient (Kennedy et al., 2004, 2010; Samper-Villarreal et al., 2016;
Howard et al., 2018). Kennedy et al. (2004, 2010) and Howard et al. (2018)
found no significant relationship between seagrass biomass and sedimentary
OC, whereas Samper-Villarreal et al. (2016) concluded that autochthonous
sedimentary OC increased as the leaf area index increased. However, they did
not show the mechanism (pathway) by which seagrass-derived OC became
sedimentary OC; that is, they did not show whether the seagrass trapped
seagrass-derived OC suspended in the water column or directly supplied
seagrass-derived carbon to the sediments.
To assess the effect of seagrass on the sedimentary OC stock, it is important
to examine all stock components, including live and dead aboveground and
belowground biomass in the sediment column, and their origins. For this
reason, it is necessary to retrieve intact cores, because both macroscopic
plant materials (Miyajima et al., 1998) and OC derived from calcareous
organisms such as corals, foraminifera, molluscs, and coralline algae
(Ingalls et al., 2003; Versteegh et al., 2011) occur in the coarse sediment
fraction (sand and gravels), especially in tropical seagrass meadows around
coral reefs (Suzuki, 2005). However, to our knowledge, all previous studies
have only examined some of the stock components, for example, the fine
sediment fraction (< 1–2 mm diameter) (Hemminga et al., 1994;
Miyajima et al., 2015; Kennedy et al., 2004; Ricart et al., 2015), dead plant
structures (Cebrian et al., 2000), surface sediment (Barron et al., 2004),
and small subsamples from a core (Dahl et al., 2016).
In this study, to investigate the pathways of sedimentary OC accumulation in
seagrass meadows, especially the direct supply of belowground seagrass
detritus, we used intact cores that included all live and dead seagrasses
and sediments and then performed the OC mass and stable carbon isotope
analyses of all components of the cores to examine the origin of the OC
along a seagrass biomass gradient.
Materials and methods
Study sites
To assess the relationship between seagrass and the sedimentary OC stock, we
chose tropical Indo-Pacific seagrass meadow sites. Globally, the tropical
Indo-Pacific region is the world's largest bioregion and contains the highest
diversity of seagrasses, which are distributed predominantly on coral reef
flats (Short et al., 2007). Globally, the total documented seagrass area is
164 000 km2 (Green and Short, 2003), and the total seagrass area in
the Indo-Pacific region, excluding Australia, where both tropical and
temperate seagrasses are distributed, is around 32 400 km2, or about
20 % of the total area. Furthermore, given that about half of the
documented seagrass habitat in Australia is composed of tropical seagrasses
(Kirkman, 1997), the total area of tropical Indo-Pacific seagrass habitat
reaches approximately 116 000 km2, accounting for 70 % of the
global seagrass area. Thus, accurate estimation of the blue carbon stock of
seagrasses in the tropical Indo-Pacific region is important for the
estimation of the global seagrass carbon stock. However, in spite of the
geographical importance of this region, reports on seagrass OC stocks there
are limited (Lavery et al., 2013; Miyajima et al., 2015).
We obtained cores from two Indo-Pacific tropical seagrass meadow sites from
13 to 23 August 2014. The two sites, a back-reef site (Shiraho reef) and an
estuarine site (Fukido estuary), both located around Ishigaki, Okinawa,
southwestern Japan (Fig. 1), have different allochthonous carbon input
amounts. The back-reef site is situated on a well-developed reef flat about
1 km wide, where seagrass meadows, dominated by Thalassia hemprichii, are distributed between 100 and 300 m from the shoreline. The
site is about 2 km south of the mouth of the Todoroki River, and most
sediments transported by the river accumulate on its northern side (Mitsumoto
et al., 2000) because the prevailing current, which is controlled by large
channels in the reef, is northward (Tamura et al., 2007). Therefore,
terrestrial sediment input to the back-reef site is low. The mud
(silt + clay) content of the surface sediment of the seagrass meadows at
the site ranges from 1.2 to 3.9 % (mean 2.3 %) (Tanaka and Kayanne,
2007). The estuarine site is located near the mouth of a small river, which
is bordered by small mangrove forests. The freshwater inflow is low, so water
exchange between the river and estuary is controlled mainly by tidal motion
(Terada et al., 2007). The dominant seagrass species at the site is
Enhalus acoroides. The mud content of the surface sediment in the
seagrass meadows at the estuarine site ranges from 0.9 to 6.4 % (mean
3.6 %) (Tanaka and Kayanne, 2007).
Study sites. (a, b) Study site location on Ishigaki,
Japan. Sampling points at (c) the back-reef site and
(d) the estuarine site. At the back-reef site, the circle indicating
the southernmost vegetated sampling point actually represents a cluster of
six sampling points.
Core sampling
We developed a new box corer to facilitate the retrieval of intact cores that
preserve sedimentary structures as well as aboveground and belowground live
and dead seagrasses (Fig. 2). The box corer is 15 cm wide, 15 cm deep, and
17 cm high and is made of stainless steel so that it can cut through roots
and rhizomes. A shutter 1 cm above the bottom of the corer is designed to
cut through the relatively hard belowground seagrasses, making it possible to
obtain intact cores. The corer also has a lid to prevent the loss of
surficial sediments from the core during underwater sampling. The corer is
large enough to retrieve all components of the OC stock whole: shoots, live
and dead aboveground and belowground seagrasses, and old skeletal OC in sand
and gravel derived from calcareous organisms such as corals, foraminifera,
molluscs, and coralline algae. Most cores obtained with the corer were about
15 cm long, but we were not able to insert the core to its full length at
three sampling points because of the presence of large gravels in the
sediment. We were able to collect all of the seagrass biomass at these
points, however.
The newly developed box corer and a sampled core. Schematic diagrams
of (a) a cross section of a core and (b) the design of the
corer. (c) Photograph of a core from the back-reef site. The
dominant seagrass species is Thalassia hemprichii.
To measure the total OC mass (OCtotal), we quantified three
components of the box corer samples (Fig. 3): (1) live seagrasses
(OCbio); (2) dead plant structures (> 2 mm in size:
dead seagrass leaves, sheaths, rhizomes, and roots detached from live
structures) (OCdead); and (3) OC in the coarse
(> 1 mm diameter) sediments (excluding dead plant structures
> 2 mm in size) (OCcsed). We also collected samples
with cylinder cores so that we could obtain depth profiles of OC in the fine
(< 1 mm diameter) sediments (including dead plant structures
< 1 mm in size) (OCfsed). It was technically impossible
to obtain these profiles with the box corer because of its large surface
sampling area and the high density of the belowground structures (Fig. 2c).
The samples retrieved by the box corer were immediately sieved through a
1 mm mesh sieve in situ to obtain the > 1 mm fractions of the
OCbio, OCdead, and OCcsed. Live
seagrasses have air-filled lacunae so that they float; thus, we considered
buoyant seagrasses captured by the sieve to be OCbio (Borum et
al., 2006). We merged any dead plant structures attached to live seagrasses
into OCbio because their mass was usually very small. We
collected a cylinder core 10–16 cm long with an acrylic pipe (internal
diameter 6.6 cm) from a point immediately adjacent to each box core. We
subdivided each cylinder core into 1 cm long subsamples from the surface to
the bottom of the core.
Calculation of total OC mass (OCtotal; g C m-2)
in the top 0.15 m layer. OCsed is sedimentary OC,
OCbio is OC in live seagrasses, OCdead is OC in dead
plant structures (> 2 mm in size: dead seagrass leaves, sheaths,
rhizomes, and roots detached from live structures), OCcsed is OC
in the coarse (> 1 mm diameter) sediments (excluding dead plant
structures > 2 mm in size), and OCfsed is OC in the
fine (< 1 mm diameter) sediments (including dead plant
structures < 1 mm in size).
We obtained 20 paired samples (one box and one cylinder core) from the
back-reef site and eight paired samples from the estuarine site. At the
back-reef site, we collected 16 paired samples from vegetated points in the
seagrass meadows, two from bare patches in the seagrass meadows, and two from
unvegetated areas (Fig. 1c). Similarly, at the estuarine site, we collected
five paired samples from vegetated points, one pair from a bare point, and
two paired samples from unvegetated areas near the river mouth (Fig. 1d).
Potential sources of sedimentary OC (OCsed) were also collected
at both sites and analyzed for δ13C. Samples of seagrass leaves
were collected from all dominant seagrass species at each site: T. hemprichii, Cymodocea rotundata, C. serrulata, and
Halodule uninervis at the back-reef site, and E. acoroides,
T. hemprichii, and C. serrulata at the estuarine site.
Samples for determining the δ13C of algae and corals were taken
from epiphytes, benthic microalgae, and the dominant coral species (mainly
Acropora spp. and Porites spp.) at the sites. Epiphytes
were collected from the seagrass leaves by using a stainless steel scraper,
and benthic microalgae were extracted from the surface sediment (up to
approximately 1 mm depth) by the method of Kuwae et al. (2008). All obtained
samples were stored in polyethylene bags at -20 ∘C until analysis.
We used the published δ13C data of suspended OC (collected about
1 km off the outer reef edge of Ishigaki) and of terrestrial particulate
organic matter (POM; collected from the Fukido River, Ishigaki) from Miyajima
et al. (2015). We assumed that the published δ13C data were
normally distributed.
OC and stable isotope analysis
We identified live seagrasses to the species level and separated aboveground
biomass (leaf blades) from belowground biomass (leaf sheathes, rhizomes, and
roots). Then we dried all parts at 60 ∘C and weighed them. Box corer
sediments were dried at 60 ∘C and sieved through a 2 mm mesh sieve,
and the included dead plant structures (> 2 mm in size) were
picked out and weighed. To ensure homogeneity of subsamples, the coarse
sediments (excluding the dead plant structures) were first crushed to
approximately 1 mm grains with a jaw crusher (Jaw Crusher PULVERISETTE 1
Model I classic line, FRITSCH, Ltd., Idar-Oberstein, Germany) and then
divided into 16 or 64 subsamples with a splitter (Simple microsplitter,
Iwamoto Mineral, Ltd., Tokyo, Japan). The cylinder core samples were
subdivided into surface (0–1 cm depth), intermediate (5–8 cm depth), and
bottom (9–16 cm depth) layers and dried at 60 ∘C. For the
OCfsed analysis, each layer was sieved through a 1 mm mesh sieve
and then subdivided into two or four subsamples with the splitter. All
subsamples used for chemical analyses were weighed and then powdered and
homogenized in an agate mill.
For OC analysis, the homogenized samples were placed in silver containers (to
prevent the loss of acid-soluble OC in carbonate sediments) and pretreated
with hydrochloric acid to remove carbonates (Yamamuro and Kayanne, 1995).
First, each sample was weighed in a silver container and its weight was
adjusted to about 20 mg. Then, 1 N HCl was carefully and gradually added
until bubbles were no longer seen, and the sample was dried at 60 ∘C
overnight and at 105 ∘C for 1 h. The dried sample was then wrapped
in tin foil. We measured the total OC concentration and the stable carbon
isotopic ratio of each sample with an elemental analyzer-connected isotope
ratio mass spectrometer (FLASH EA 1112/DELTAplus Advantage,
Thermo Electron, Inc., Massachusetts, USA). The stable carbon isotope ratio
(δ13C) is reported as the relative per mil deviation from VPDB
(Vienna Pee Dee Belemnite). The analytical precision of the isotope ratio
mass spectrometer, based on the standard deviation of δ13C values
of internal reference replicates, was < 0.2 ‰.
Determination of the mass and δ13C of OC
We calculated OCtotal per unit area (g C m-2) at each
sampling point by summing the OCbio and OCsed
components in the top 0.15 m (Fig. 3) as follows:
OCtotal=OCbio+OCsed.OCbiowascalculatedasOCbio=∑i(aixi+biyi),
where ai and bi are the averaged OC concentrations
(g C g-1 DW) of the aboveground and belowground biomasses,
respectively, of the ith seagrass species collected at three different
sampling points (except C. serrulata, which was collected at only
one sampling point at the estuarine site), and xi and yi are the
aboveground and belowground biomasses (g m-2), respectively, of the
ith seagrass species. The biomasses of Syringodium isoetifolium,
Halophila ovalis, and an unidentified species at the back-reef site
accounted for < 0.1 % of the total biomass, so they were
excluded from this calculation. The averaged OC concentrations and
aboveground and belowground biomass dry weights are summarized in Table 1.
OCsed was calculated as follows:
OCsed=OCdead+OCcsed+OCfsed.
The terms of Eq. (3) were calculated by the following equations:
OCdead=1100(%OCleaf×ρleaf+%OCshrh×ρshrh+%OCroot×ρroot)×h,OCcsed=1100(%OCcsed×ρcsed)×h,OCfsed=13×1100(%OCfseds×ρfseds+%OCfsedm×ρfsedm+%OCfsedb×ρfsedb)×h,
where %OC is the concentration of OC (% DW) (n= 3); ρ is the
dry density (g DW m-3) of each component (indicated by subscripts:
leaf, dead leaf; shrh, dead sheath and rhizome; root, dead root; csed, coarse
sediment; fseds, fine sediment of the surface layer; fsedm, fine sediment of
the intermediate layer; fsedb, fine sediment of the bottom layer), and h is
the sample thickness (0.15 m). OCfsed is the averaged OC mass of
the three layers (surface, medium, and bottom) of fine sediment.
Organic carbon contents and dry
weights of each component of living biomass at the back-reef and estuarine
sites.
Back-reef
Estuary
%OC (% DW)
Dry weight (g m-2)
%OC (% DW)
Dry weight (g m-2)
mean ± SD (n)
mean ± SD (n)
mean ± SD (n)
mean ± SD (n)
Aboveground biomass
38.47 ± 3.06 (39)
74 ± 45 (16)
35.95 ± 2.28 (20)
70 ± 34 (5)
Belowground biomass
31.35 ± 2.93 (20)
675 ± 450 (16)
30.38 ± 2.55 (13)
1354 ± 847 (5)
δ13C of OCsed (δ13Csed) at each
sampling point was calculated as follows:
δ13Csed=1OCsedOCdead×δ13Cdead+OCcsed×δ13Ccsed+δ13Cfsed,
where δ13Cdead is the averaged δ13C value of
dead plant structures (sheath and rhizomes, and roots) at the back-reef and
estuarine sites. We did not include leaf detritus in the calculation of
δ13Csed because (1) the leaf fragments were so small
that we could not remove epiphytes from them, and (2) their mass was much
smaller than that of the sheath and rhizomes and roots, so we considered its
contribution to δ13Csed to be negligible. We used the
δ13Cdead value at each site for the calculation of
δ13Csed. The standard deviation (SD) of δ13Csed derived from the SD of δ13Cdead was
smaller than 0.1 ‰. δ13Ccsed is
the δ13C value of OCcsed. δ13Cfsed is
the averaged δ13C value of OCfsed multiplied by the OC mass
of each layer and was calculated as follows:
δ13Cfsed=13×1100(%OCfseds×ρfseds×δ13Cfseds+%OCfsedm×ρfsedm×δ13Cfsedm+%OCfsedb×ρfsedb×δ13Cfsedb)×h.
The averaged values of the organic carbon concentration, δ13C, and
dry density of sediment and dead plant structures are summarized in Table 2.
Organic carbon content, δ13C, and dry density of each
of sediment and dead plant component at the back-reef and estuarine sites.
Back-reef
Estuary
Organic carbon
Dry density
Organic carbon
Dry density
(mg cm-3)
(mg cm-3)
%OC (% DW)
δ13C
mean ± SD (n)
%OC (% DW)
δ13C
mean ± SD (n)
mean ± SD (n)
(‰ vs. VPDB)
mean ± SD (n)
(‰ vs. VPDB)
mean ± SD (n)
mean ± SD (n)
Fine sediment
0.37 ± 0.13 (60)
-12.8 ± 0.8 (60)
893 ± 303 (60)
0.42 ± 0.20 (24)
-17.4 ± 3.6 (24)
760 ± 294 (24)
Coarse sediment
0.32 ± 0.13 (20)
-12.8 ± 1.1 (20)
292 ± 152 (20)
0.26 ± 0.08 (8)
-15.9 ± 1.5 (8)
475 ± 142 (8)
Dead leaf
24.80 ± 3.07 (3)
-8.9 ± 0.6a (5)
0.05 ± 0.04 (20)
23.31 ± 3.86b (3)
-9.3 ± 0.2a (5)
0.03 ± 0.04 (8)
Dead sheath and rhizome
21.29 ± 4.07 (3)
-8.9 ± 0.6a (5)
0.55 ± 0.63 (20)
27.52 ± 1.75b (3)
-9.3 ± 0.2a (5)
1.44 ± 1.86 (8)
Dead root
19.25 ± 1.67 (3)
-8.9 ± 0.6a (5)
0.26 ± 0.25 (20)
19.94 ± 5.89b (3)
-9.3 ± 0.2a (5)
0.31 ± 0.35 (8)
a Total of sheath and rhizomes, and root.
b At one sampling point (FS1) where the dominant species
was different, the values were dead leaf, 25.77 %; dead sheath and
rhizome, 19.05 %; and dead root, 19.21 %.
Results
Seagrass biomass and species composition at each site
At the back-reef site, the average (±SD) aboveground and belowground
biomass values were 74 ± 45 g DW m-2 (n= 16) and
675 ± 450 g DW m-2 (n= 16), respectively (Table 1). The
dominant species was T. hemprichii, accounting for 76.7 % of the
total biomass; C. rotundata (18.0 %), C. serrulata
(3.3 %), H. uninervis (1.7 %), H. ovalis
(< 0.1 %), S. isoetifolium (< 0.1 %), and
an unidentified species (< 0.1 %) were minor components at the
back-reef site. At the estuarine site, the average aboveground and
belowground biomasses were 70 ± 34 g DW m-2 (n= 5) and
1354 ± 847 g DW m-2 (n= 5), respectively (Table 1). The
dominant species was E. acoroides, accounting for 92.3 % of the
total biomass; T. hemprichii (7.0 %), C. serrulata
(0.6 %), and H. uninervis (< 0.1 %) were minor
components.
OC mass
The average OC density (g C cm-3) did not differ significantly among
the fine sediment layers at either the back-reef site (paired t-test,
Bonferroni adjusted P > 0.05) or the estuarine site (Wilcoxon
signed rank test, Bonferroni adjusted P > 0.05). The averaged
OCbio, OCdead, OCfsed, OCsed,
and OCtotal values were significantly higher at points with
vegetation than at points without vegetation at both the back-reef site
(OCbio, t= -6.23, d.f. = 15, P < 0.001;
OCdead, W= 0, P < 0.001; OCfsed, t= -2.61, d.f. = 18, P= 0.018; OCsed, t= -2.85,
d.f. = 18, P= 0.011; OCtotal, t= -3.44,
d.f. = 18, P= 0.003) (Fig. 4a and b) and the estuarine site
(OCbio, t= -3.61, d.f. = 4, P= 0.022;
OCdead, W= 0, P= 0.036; OCfsed, t= -2.59, d.f. = 6, P= 0.041; OCsed, t= -3.33,
d.f. = 6, P= 0.016; OCtotal, t= -4.24,
d.f. = 6, P= 0.005) (Fig. 4c and d). At points with vegetation, the
averaged OCfsed and OCsed values did not
significantly differ between the sites (OCfsed, t= 0.33,
d.f. = 19, P > 0.05; OCsed, t= -1.53,
d.f. = 19, P > 0.05), whereas the averaged
OCbio, OCdead, OCcsed and
OCtotal values were significantly higher at the estuarine site
than at the back-reef site (OCbio, t= -2.25, d.f. = 19,
P= 0.036; OCdead, W= 11, P= 0.015;
OCcsed, t= -4.86, d.f. = 19, P < 0.001;
OCtotal, t= -2.34, d.f. = 19, P= 0.030) (Fig. 4b
and d). At points with vegetation, OCtotal ranged from 531 to
1785 g C m-2 across both sites. OCsed, which ranged from
433 to 1147 g C m-2 and was the main component of
OCtotal, accounted for 75.1 ± 13.1 % DW of
OCtotal. Hence, the contribution of the live seagrass itself
(OCbio) was minor (24.9 ± 13.1 % DW).
OCfsed was the major component of OCsed, accounting
for 55.6 ± 12.5 % DW of OCtotal; OCcsed and
OCdead were minor components, accounting for
15.0 ± 7.2 % DW and 4.5 ± 4.1 % DW of
OCtotal, respectively.
OC mass (OCbio, OCdead, OCcsed,
OCfsed, OCsed, and OCtotal) at (a) no-vegetation
(bare and unvegetated) points at the back-reef site,
(b) vegetated points at the back-reef site, (c) no-vegetation
points at the estuarine site, and (d) vegetated points
at the estuarine site. Boxes show the 25 and 75 % quantiles;
horizontal bands inside the boxes are median values; whiskers show maximum
and minimum values; and the open circles are outliers.
The average aboveground and belowground biomasses in OCbio did
not differ significantly between the sites (Fig. 5a) (aboveground biomass, t= 0.30, d.f. = 19, P > 0.05; belowground biomass, t= -1.75, d.f. = 4.67, P > 0.05). Belowground biomass
accounted for 89.1 ± 4.4 % DW of OCbio (Fig. 5b). The
averaged biomasses of aboveground (i.e., leaf) and belowground (i.e., sheath
and rhizome, and root) detritus in OCdead did not differ
significantly between the sites (aboveground detritus, t= 0.60,
d.f. = 7.82, P > 0.05; belowground detritus, W= 28,
P > 0.05) (Fig. 5c). The biomass of belowground detritus
accounted for 90.8 ± 12.0 % DW of OCdead (Fig. 5d). The
biomasses of sheath and rhizome, and root, accounted for
65.5 ± 19.2 % DW and 25.3 ± 16.0 % DW of
OCdead, respectively.
δ13C of OC
The average δ13Csed at the back-reef site (-12.6 ± 0.7 ‰)
was significantly higher than that of the estuarine site (-16.6 ± 3.1 ‰) (t= 3.61,
d.f. = 7, P= 0.008), and it was also significantly higher than the δ13C values of algae and corals (-15.2 ± 1.9 ‰)
(W= 2753, P < 0.001), suspended POM (-21.9 ± 1.6 ‰)
(t= 15.45, d.f. = 8, P < 0.001),
and terrestrial POM (-28.7 ± 1.5 ‰) (t= 29.25,
d.f. = 8, P < 0.001). However, average δ13Csed at the back-reef site was significantly lower than
δ13C of seagrass (-9.2 ± 1.3 ‰) (t= -12.64, d.f. = 57,
P < 0.001) (Fig. 6). Average δ13Csed at the estuarine site did not differ significantly
from δ13C of algae and corals (W= 457, P > 0.05),
but it was significantly higher than δ13C of both suspended POM
(t= 4.36, d.f. = 14, P < 0.001) and terrestrial POM (t= 10.05,
d.f. = 14, P < 0.001), and significantly lower than
δ13C of seagrass (t= -6.66, d.f. = 8, P < 0.001).
The average δ13C among fine sediment layers did not differ
significantly at either the back-reef site (Wilcoxon signed rank test,
Bonferroni adjusted P > 0.05) or the estuarine site (paired t-test,
Bonferroni adjusted P > 0.05).
Relationships among biomass, OC mass, and δ13C
At the back-reef site, we found significant correlations between
OCsed and DW-based (not carbon-based) biomass (F1,18= 11.63,
P= 0.003, r2= 0.39) (Fig. 7a), OCsed and
aboveground biomass (F1,18 = 16.38, P < 0.001, r2= 0.48)
(Fig. 7b), OCsed and belowground biomass (F1,18= 10.95,
P= 0.004, r2= 0.38) (Fig. 7c), OCsed and
OCdead (F1,18= 4.55, P= 0.047, r2= 0.20) (Fig. 7d),
and OCsed and δ13Csed (F1,18= 11.51,
P= 0.003, r2= 0.39) (Fig. 7e). We also found significant
correlations between δ13Csed and belowground biomass
(F1,18= 4.68, P= 0.044, r2= 0.21) (Fig. 7f), and between
δ13Csed and OCdead (F1,18= 13.18, P= 0.002, r2= 0.42) (Fig. 7g). We also found significant positive
correlations between aboveground and belowground biomass (F1,18= 94.10,
P < 0.001, r2= 0.84). At the estuarine site, we found
significant correlations between OCsed and aboveground biomass
(F1,6= 8.18, P= 0.029, r2= 0.58) (Fig. 7b) and between
OCsed and OCdead (F1,6= 6.94, P= 0.039,
r2= 0.54) (Fig. 7d) but not between OCsed and biomass
(F1,6= 3.08, P > 0.05, r2= 0.34) (Fig. 7a),
OCsed and belowground biomass (Fig. 7c) (F1,6= 2.94, P > 0.05,
r2= 0.33), or OCsed and δ13Csed (F1,6= 0.040, P > 0.05, r2 < 0.01)
(Fig. 7e). The slope of the regression line of OCsed against aboveground biomass did not differ significantly
between the sites (ANCOVA, F= 1.09, d.f. = 1, P > 0.05)
(Fig. 7b), and that of OCsed against OCdead also did
not differ significantly between the sites (F= 0.36, d.f. = 1, P > 0.05)
(Fig. 7d). We also found significant positive correlations
between aboveground and belowground biomass (F1,6= 78.40, P < 0.001, r2= 0.93).
(a) OCbio (sum of aboveground and belowground
biomass) (g C m-2); (b) contribution of belowground biomass
to OCbio (%); (c) OCdead (sum of
aboveground and belowground detritus (g C m-2); and
(d) contribution of belowground detritus to
OCdead (%). Boxes show the 25 and 75 % quantiles;
horizontal bands inside the box are median values; whiskers show maximum and
minimum values; and open circles show outliers. (a) and
(b) show the data of vegetated sampling points and (c) and
(d) show the data of vegetated and bare sampling points.
δ13Csed at each site and the δ13C
values of potential sources of OC of δ13Csed (means ± SE).
Relationships at the back-reef (blue) and estuarine (orange) sites
between OCsed and (a) biomass (g m-2),
(b) aboveground biomass (g m-2), (c) belowground
biomass (g m-2), and (d) OCdead (g C m-2),
and between (e) OCsed and δ13Csed, (f) δ13Csed and
belowground biomass, and (g) δ13Csed and
OCdead.
Proposed mechanisms of OC accumulation at our study sites. At the
back-reef site dominated by Thalassia hemprichii, direct supply of recalcitrant belowground
seagrass detritus is a major pathway of OCsed accumulation. At the
estuarine site dominated by Enhalus acoroides, trapping of suspended autochthonous and
allochthonous OC is the major pathway of OC accumulation. A difference in
the turnover rate of belowground biomass likely caused the major mechanism
of OC accumulation to differ between the sites.
Discussion
Components of OC stock in seagrass meadows
Our results showed that the sedimentary OC mass (OCsed) was the
main component of the total organic carbon mass (OCtotal; i.e.,
all stock components: live and dead aboveground and belowground biomass and
sediments) at our study sites. The averaged OCbio was
significantly higher in this study than that in the previous study by
Fourqurean et al. (2012) (W= 1691, P= 0.006), whereas the averaged
OCsed was significantly lower in this study than in the previous
study at both vegetated and no-vegetation points (vegetation, W= 6952,
P < 0.001; no-vegetation, W= 225, P= 0.036) (Table 3).
Hence, the contribution of OCbio to OCtotal at our
sites was higher than the global average. The high OCbio was due
to the well-developed belowground biomass, which accounted for
90.8 ± 3.9 % of DW-based biomass at our sites. This value is also
among the highest among globally compiled data (Duarte and Chiscano, 1999).
Possible reasons for the exceptional development of belowground biomass
include (1) morphological plasticity for resistance to high wave energy
(Fonseca and Bell, 1998), which is supported by the low mud content at our
sites compared to that reported by previous studies (Koch, 2001; Serrano et
al., 2016a), and (2) nutrient limitation, which can lead to more allocation
of biomass to belowground parts to enable the plant to acquire nutrients in
deeper sediment layers (Lee et al., 2007). The low OCsed may be
attributable to (1) high wave energy in association with increased OC
lability due to the low specific surface area of sediments (Miyajima et al.,
2017) and (2) the low gross primary production/respiration (P / R)
ratio in this geographical region (Duarte et al., 2010).
Values of seagrass biomass organic carbon and sedimentary
organic carbon mass in globally compiled data (Fourqurean
et al., 2012) and this study (mean ± SD,
n).
Vegetated
No-vegetation
Seagrass biomass OC
Sedimentary OC
Seagrass biomass OC
Sedimentary OC
(g C m-2)
(g C L-1)
(g C m-2)
(g C L-1)
mean ± SD (n)
mean ± SD (n)
mean ± SD (n)
mean ± SD (n)
Fourqurean et al. (2012)
251.4 ± 395.6 (251)
12.32 ± 8.04 (410)
–
8.08 ± 5.90 (43)
This study
283.0 ± 200.8 (21)
5.03 ± 1.32 (21)
–
2.93 ± 0.73 (7)
Belowground detritus (i.e., sheath and rhizome, and root) was the major
component of OCdead, accounting for 90.8 ± 12.0 % of
OCdead at our sites. This result is consistent with a previous
report on Cymodocea nodosa (Cebrian et al., 2000) and suggests that
belowground detritus is more easily stored in the sediment than aboveground
detritus. A mechanism supporting this hypothesis might be either (1) a higher
belowground biomass and an associated higher supply of seagrass detritus or
(2) higher recalcitrance of belowground detritus. Here, a higher supply is
more likely because at our sites the belowground biomass is among the highest
reported values for each species (Duarte and Chiscano, 1999), although the
reported aboveground/belowground production ratio of T. hemprichii
and E. acoroides varies among studies (Duarte et al., 1998; Duarte
and Chiscano, 1999; Erftemeijer et al., 1993). Higher recalcitrance is also
possible; Holmer and Olsen (2002) reported that during a 43-day decomposition
experiment, E. acoroides rhizomes did not lose weight, whereas
buried leaves lost 80.3 ± 4.2 % of their weight. Also, Fourqurean
and Schrlau (2003) showed that only 5 ± 2 % of Thalassia testudinum leaves, but 49 ± 6 % of
T. testudinum rhizomes, remained after 348 days of decomposition.
Mechanism of the OC supply to sediment
OCsed was significantly and positively correlated with
aboveground biomass at both sites (Fig. 7b) and with belowground biomass at
the back-reef site (Fig. 7c). This result is contrary to the finding of most
previous studies that there is no relationship between biomass and %OC or
OC mass (Kennedy et al., 2004, 2010; Howard et al., 2018; Rozaimi et al.,
2017; but cf. Samper-Villarreal et al., 2016). This contrary result may be
due to our data collection strategy of (1) sympatric sampling of all stock
components (live and dead aboveground and belowground biomass and sediments)
in intact cores, and (2) selection of sampling points aiming at controlling
for variables other than seagrass biomass (i.e., mud content, wave height,
and the amount of allochthonous OC inputs were relatively homogenous among
points), although we could not exclude the possibility that our sites may
have specific sedimentary OC storage characteristics different from those of
other Indo-Pacific seagrass meadows. Several mechanisms can plausibly explain
the positive relationship between seagrass biomass and OCsed,
including (1) trapping of suspended OC (both allochthonous and autochthonous
OC) by seagrass leaves, and (2) the direct supply of belowground
seagrass-derived autochthonous OC. If it is assumed that the suspended OC
settling on the sediment surface is spatially homogeneous in nature (quality)
and that the contribution of trapped OC is larger than that of directly
supplied OC, then δ13Csed should be constant
regardless of the aboveground biomass and its associated trapping capacity.
However, OCsed was significantly and positively correlated with
δ13Csed (Fig. 7e), and the average δ13C of
OCsed was significantly higher than δ13C values of
allochthonous OC (algae and corals, suspended POM, and terrestrial POM) at
the back-reef site (Fig. 6). Furthermore, OCsed was positively
correlated with OCdead at both sites (Fig. 7d), and the main
component of OCdead was belowground detritus (Fig. 5d). Taken
together, these results suggest that directly supplied seagrass-derived OC
was mainly from the belowground detritus. The positive correlations between
δ13Csed and belowground biomass (Fig. 7f) and between
δ13Csed and OCdead (Fig. 7g) at the
back-reef site also support this mechanism. From these lines of evidence, we
conclude that the direct supply of recalcitrant belowground seagrass detritus
is a major mechanism of OCsed accumulation at the back-reef site
(Fig. 8). Although we inferred that a direct autochthonous OC supply from
belowground biomass is the major mechanism of OCsed accumulation,
suspended allochthonous OC may also have been supplied from the water column
at the back-reef site, as has been reported elsewhere (Kennedy et al.,
2010), because the average δ13Csed at the back-reef
site was significantly lower than δ13C of seagrass (Fig. 6).
The seagrass-derived OC increase according to the development of the seagrass
meadows at the back-reef site (Fig. 7a and e) suggests that seagrass
meadows are autotrophic and the time since seagrass colonization is longer.
This inference is consistent with a previous report that net primary
production (NPP) at the back-reef site is higher where the seagrass cover is
high (cover 91.7 %; NPP 68.14 mmol C m-2 d-1) than where the
seagrass cover is low (cover 55.1 %; NPP 34.20 mmol C m-2 d-1)
(Nakamura and Nakamori, 2009). It is also possible that seagrass mortality
increases with time since colonization, leading to an increase in dead plant
structures (Cebrian et al., 2000).
At the estuarine site, OCsed increased with increasing aboveground
seagrass biomass (Fig. 7b), but it did not increase with increasing
belowground seagrass biomass (Fig. 7c), indicating that trapping of suspended
OC by seagrass leaves surpassed the direct supply of belowground
seagrass-derived OC (Fig. 8). However, OCdead was significantly
and positively correlated with OCsed (Fig. 7d), indicating that
direct supply also contributed to OCsed accumulation at the site.
A plausible mechanism explaining the hypothesized dominance of suspended OC
trapping is a lower belowground turnover rate (i.e., the production / biomass
ratio) at the estuarine site than at the back-reef site. Because
OCsed was not significantly different between the sites and
directly supplied seagrass-derived OC was the major component of
OCsed at the back-reef site and only a minor component at the
estuarine site, the capacity of the estuarine site to directly supply
belowground seagrass-derived OC to the sediment was lower than that of the
back-reef site (Fig. 8). Moreover, given that the directly supplied amount is
determined by two factors, the belowground biomass and its turnover rate, and
that the belowground biomass was not significantly different between the
sites (Fig. 5a), we anticipate that a difference in the belowground turnover
rate was responsible for the difference in the direct supply contribution
between the sites. Another possible explanation for the inferred difference
is that the absolute input of allochthonous OC was higher at the estuarine
site than at the back-reef site. The slope of the regression between
aboveground biomass and OCsed was not significantly different
between the sites (Fig. 7b), which suggests that the trapping ability for
autochthonous and allochthonous OC was not different between the sites.
However, the fact that OCsed at vegetated points was not significantly
different between the sites (Fig. 4b and d) together with the
apparently minor direct belowground supply at the estuarine site implies that
the contribution of OC from the water column to OCsed was larger
at the estuarine site. Moreover, the fact that average δ13Csed was significantly lower at the estuarine site than at
the back-reef site (Fig. 6) would support a major role of allochthonous OC
from the water column in OCsed accumulation. The effect of
particle trapping by seagrasses is reported to be enhanced particularly in
particle-poor waters (Duarte et al., 1999). Thus, trapping is likely to be an
important mechanism especially at sites with particle-poor water such as
coral reef sites.