Indonesia's blue carbon: a globally significant and vulnerable sink for seagrass and mangrove carbon

Abstract

The global significance of carbon storage in Indonesia’s coastal wetlands was assessed based on published and unpublished measurements of the organic carbon content of living seagrass and mangrove biomass and soil pools. For seagrasses, median above- and below-ground biomass was 0.29 and 1.13 Mg C ha−1 respectively; the median soil pool was 118.1 Mg C ha−1. Combining plant biomass and soil, median carbon storage in an Indonesian seagrass meadow is 119.5 Mg C ha−1. Extrapolated to the estimated total seagrass area of 30,000 km2, the national storage value is 368.5 Tg C. For mangroves, median above- and below-ground biomass was 159.1 and 16.7 Mg C ha−1, respectively; the median soil pool was 774.7 Mg C ha−1. The median carbon storage in an Indonesian mangrove forest is 950.5 Mg C ha−1. Extrapolated to the total estimated mangrove area of 31,894 km2, the national storage value is 3.0 Pg C, a likely underestimate if these habitats sequester carbon at soil depths >1 m and/or sequester inorganic carbon. Together, Indonesia’s seagrasses and mangroves conservatively account for 3.4 Pg C, roughly 17 % of the world’s blue carbon reservoir. Continued degradation and destruction of these wetlands has important consequences for CO2 emissions and dissolved carbon exchange with adjacent coastal waters. We estimate that roughly 29,040 Gg CO2 (eq.) is returned annually to the atmosphere–ocean pool. This amount is equivalent to about 3.2 % of Indonesia’s annual emissions associated with forest and peat land conversion. These results highlight the urgent need for blue carbon and REDD+ projects as a means to stem the decline in wetland area and to mitigate the release of a significant fraction of the world’s coastal carbon stores.

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1 23
Wetlands Ecology and Management
ISSN 0923-4861
Wetlands Ecol Manage
DOI 10.1007/s11273-015-9446-y
Indonesia’s blue carbon: a globally
significant and vulnerable sink for seagrass
and mangrove carbon
D.M.Alongi, D.Murdiyarso,
J.W.Fourqurean, J.B.Kauffman,
A.Hutahaean, S.Crooks, C.E.Lovelock,
J.Howard, D.Herr, et al.

1 23
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INVITED FEATURE ARTICLE
Indonesia’s blue carbon: a globally significant
and vulnerable sink for seagrass and mangrove carbon
D. M. Alongi .D. Murdiyarso .J. W. Fourqurean .J. B. Kauffman .
A. Hutahaean .S. Crooks .C. E. Lovelock .J. Howard .
D. Herr .M. Fortes .E. Pidgeon .T. Wagey
Received: 31 May 2015 / Accepted: 21 July 2015
ÓSpringer Science+Business Media Dordrecht 2015
Abstract The global significance of carbon storage
in Indonesia’s coastal wetlands was assessed based on
published and unpublished measurements of the
organic carbon content of living seagrass and man-
grove biomass and soil pools. For seagrasses, median
above- and below-ground biomass was 0.29 and
1.13 Mg C ha
-1
respectively; the median soil pool
was 118.1 Mg C ha
-1
. Combining plant biomass and
soil, median carbon storage in an Indonesian seagrass
meadow is 119.5 Mg C ha
-1
. Extrapolated to the
estimated total seagrass area of 30,000 km
2
, the
national storage value is 368.5 Tg C. For mangroves,
median above- and below-ground biomass was 159.1
and 16.7 Mg C ha
-1
, respectively; the median soil
pool was 774.7 Mg C ha
-1
. The median carbon
storage in an Indonesian mangrove forest is 950.5 Mg
Cha
-1
. Extrapolated to the total estimated mangrove
area of 31,894 km
2
, the national storage value is 3.0
Pg C, a likely underestimate if these habitats sequester
carbon at soil depths [1 m and/or sequester inorganic
carbon. Together, Indonesia’s seagrasses and man-
groves conservatively account for 3.4 Pg C, roughly
17 % of the world’s blue carbon reservoir. Continued
D. M. Alongi (&)
Australian Institute of Marine Science, PMB 3,
Townsville MC, QLD 4810, Australia
e-mail: d.alongi@aims.gov.au
D. Murdiyarso
Bogor Agricultural University, Kampus Darmaga,
Bogor 16810, Indonesia
J. W. Fourqurean
Department of Biological Sciences and Southeast
Environmental Research Center, Florida International
University, Miami, FL 33199, USA
J. B. Kauffman
Department of Fisheries and Wildlife, Oregon State
University, Corvallis, OR 97331, USA
A. Hutahaean T. Wagey
Agency for Research and Development of Marine Affairs
and Fisheries, Jakarta 12770, Indonesia
S. Crooks
Environmental Science Associates, 550 Kearny St Ste
800, San Francisco, CA 94108, USA
C. E. Lovelock
School of Biological Sciences, University of Queensland,
Saint Lucia, QLD 4072, Australia
J. Howard E. Pidgeon
Marine Climate Change Program, Conservation
International, 2011 Crystal Drive, Suite 500, Arlington,
VA 22202, USA
D. Herr
International Union for Conservation of Nature, Rue
Mauverney 28, 1196 Gland, Switzerland
M. Fortes
Marine Science Institute CS, University of the Philippines
Diliman, 1101 Quezon City, Philippines
123
Wetlands Ecol Manage
DOI 10.1007/s11273-015-9446-y
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degradation and destruction of these wetlands has
important consequences for CO
2
emissions and dis-
solved carbon exchange with adjacent coastal waters.
We estimate that roughly 29,040 Gg CO
2
(eq.) is
returned annually to the atmosphere–ocean pool. This
amount is equivalent to about 3.2 % of Indonesia’s
annual emissions associated with forest and peat land
conversion. These results highlight the urgent need for
blue carbon and REDD?projects as a means to stem
the decline in wetland area and to mitigate the release
of a significant fraction of the world’s coastal carbon
stores.
Keywords Blue carbon Carbon sequestration 
Mangrove Seagrass Wetland Indonesia
Introduction
Indonesia, one of the world’s largest countries in area
and population, straddles nearly one-tenth (5120 km)
of the equator, encompassing 13,466 islands
1
of which
only about 4000 are inhabited. The coast of Indonesia
stretches for more than 95,180 km in length, giving it
one of the longest coastlines in the world, and houses
some of the world’s richest tropical marine ecosys-
tems, including coral reefs, mangroves and seagrass
meadows, within 3.6 million km
2
of territorial seas.
These ecosystems form a large part of the Coral
Triangle of the Indo-Pacific, termed ‘the center of
origin’ for many of the world’s tropical marine flora
and fauna (Veron et al. 2011).
Indonesia’s coral reefs and coastal wetlands are,
however, in trouble. Most of the nation’s coral reefs
are destroyed or badly degraded (Tun et al. 2008), and
nearly half of the archipelago’s mangroves have been
lost mostly to aquaculture and coastal development
during the past 50 years (Kusmana 2014). Of the
31,894 km
2
of existing mangrove wetland (Spalding
et al. 2010), 31 % are in good condition, 27 % are
moderately degraded and the remaining 42 % of
mangrove forests are heavily degraded (ASCNM
2009). The status of Indonesia’s seagrasses is poorly
known; some local seagrass beds are well described as
are the distribution of the major seagrass species, but
there is no national or regional inventory (Nadiarti
et al. 2012). At least one source (UNEP 2008) quotes a
decline in Indonesian seagrass cover of 30–40 % since
the 1960s and Green and Short (2003) estimate that
seagrasses cover 30,000 km
2
of Indonesian seas, but
there is little quantitative data. What can be said for
certain is that the trend is for a decline in seagrass area
(Ooi et al. 2011; Short et al. 2014).
Considering the large losses of mangrove and
seagrass habitats and the continuing trends throughout
Indonesia, it is important to know and conserve the size
of their below- and above-ground carbon pools because
the carbon from these destroyed and degraded ecosys-
tems is eventually lost to the atmosphere and coastal
ocean. Recent calculations (Donato et al. 2011;
Pendelton et al. 2012; Alongi and Mukhopadhyay
2014) indicate that the global destruction of mangrove
carbon stocks at the current deforestation rate of 1 %
results in an annual release of 90–970 Tg C years
-1
to
the atmosphere/ocean pools, and add an additional
10 % to global CO
2
release from deforestation of
tropical terrestrial forests. This estimate reflects the fact
that mangroves store more carbon (956 t C ha
-1
;Alongi
2014) than other ecosystems, such as rainforests (481 t
Cha
-1
) and salt marshes (593 t C ha
-1
); mangrove
carbon is stored mostly (75 %) below-ground as it is in
seagrass meadows ([90 %). Recent assessments of the
carbon sequestration capacity of seagrasses (Four-
qurean et al. 2012;Duarteetal.2013a,b;Lavery
et al. 2013; Siikama
¨ki et al. 2013; Macreadie et al.
2014) indicate that, like mangroves, they are among the
most effective ecosystems for storing carbon, and that
losses of seagrass meadows could contribute an addi-
tional 11–90 Tg C years
-1
to the atmosphere (Four-
qurean et al. 2012; Pendleton et al. 2012).
The carbon stored in these vegetated ecosystems
(including salt marshes that are mainly temperate) has
been termed ‘blue carbon’ (Pendleton et al. 2012). A
growth of schemes to conserve and restore wetland
blue carbon (but not solely as part of the REDD?
2
1
GIS surveys beginning in 2011 have reduced the widely-
accepted island total from 17,080. This difference is due to the
increased accuracy of GIS as well as to not including inlets under
the definition of island in the UN Law of the Sea Convention
(https://www.unstats.un.org/unsd/geoinfo/UNGEGN/docs/10th-
uncsgn-doc/E_CONF.101_134_The%20Naming%20Procedures
%20%Of%20/Indonesia%20%20Islands.pdf).
2
REDD?stands for efforts to reduce emissions from defor-
estation and degradation, and the role of conservation, sustain-
able management of forests, and enhancement of forest carbon
stocks in developing countries (1).
Wetlands Ecol Manage
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umbrella) has emerged over the past few years. Their
objective is to enhance carbon sequestration both as a
means to ameliorate the rise in carbon emissions and to
protect and conserve these rapidly disappearing habi-
tats—partly to avoid the return of these considerable
carbon sinks to the atmosphere and coastal ocean
(Mcleod et al. 2011; Pendleton et al. 2012). Naturally,
Indonesia stands out as a high-priority nation for such
blue carbon projects, owing to the areal extent and size
of its mangrove forests and seagrass meadows, but to
date only one assessment of mangrove carbon stocks
(Murdiyarso et al. 2015) has been made.
This paper addresses this lack of information by
using published and unpublished measurements of the
organic carbon content (C
org
) of living seagrass and
mangrove biomass and soil pools to estimate the size
and significance of Indonesia’s blue carbon stocks
compared to the global wetland C stores. We also
estimate the magnitude of the continuing losses and
return of Indonesia’s wetland carbon to the atmo-
sphere/ocean pools compared with the current rates of
deforestation. We hope that these estimates will
highlight the global importance of Indonesia’s coastal
wetlands.
Data sources, assumptions and carbon estimates
Data on seagrass and mangrove C
org
content of living
biomass and C
org
content and dry bulk density (DBD)
of soils were compiled from a number of locations
(Fig. 1) to deliver preliminary estimates of carbon
stocks of these Indonesian ecosystems. These data are
from published sources obtained using literature
values and augmented by unpublished data, when
available. Different data sources invariably use dif-
ferent methods, but we treated all data equally and
standardized all nomenclature across the data. Units
are in Mg C ha
-1
except where noted.
Seagrasses
An additional criterion for data selection was that each
source contains empirical data from individual sites
for at least two of the three carbon pools: above-
ground biomass, below-ground biomass and the soil
pool. The total soil C
org
pool was standardized to a soil
depth of 1 m. Soil surface values were extrapolated to
1 m using the formulae derived by Fourqurean et al.
(2012) from all known seagrass soil C
org
data: for C
org
,
Fig. 1 Distribution map of Indonesia’s mangrove forests and seagrass meadows. The locations of the references cited in Tables 1and 2
are highlighted as ‘M’ for mangrove and ‘S’ for seagrass
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-0.005 ±0.003log
10
(C
org
?1) cm
-1
; for DBD,
8.6 ±4.0 (mg (dry weight) ml
-1
)cm
-1
, except in
one study (Alongi et al. 2008a) where the known
maximum depth of core penetration was less (Fig. 2).
Seagrass biomass (DW) was converted to C
org
using
the C
org
content measured for the dominant species in
each reference listed in Table 1.
All seagrass meadows sampled in Indonesia were
composed of sandy soils, or in some cases, were
growing on coral rubble (Table 1), and were domi-
nated by Enhalus acoroides,Thalassia hemprichii,
Halodule uninervis, Cymodocea rotundata,orCy-
modocea serrulata. The data from all sites (Fig. 3)
showed asymmetric distribution with very few values
greater than 0.6 Mg C ha
-1
for above-ground carbon
TOC (% soil dry weight)
0123456
Depth (cm)
0
10
20
30
40
50
SB1
SB2
SB3
Fig. 2 Vertical profiles of mean (±1 SE) organic carbon
concentrations (as % soil dry weight) with increasing soil depth
in three seagrass meadows in Sulawesi, Indonesia. Site SB1 was
a meadow dominated by Enhalus acoroides and located a few
meters from the mouth of a mangrove-lined creek, with very fine
sandy soil (40 % CaCO
3
). Site SB2 was a meadow dominated
by E. acoroides,Halodule uninervis, and Cymodocea rotundata
and located further seawards, composed of very fine sandy soil
(70 % CaCO
3
). Site SB3 was a meadow located adjacent to a
coral reef, composed of fine carbonate (83 % CaCO
3
) sand, and
dominated by the above species as well as C. serrulata and
Thalassia hemprichii. Alongi et al. (2008a) provides a full
description of these sites and soil C methods. Briefly, triplicate
sediment samples were taken using a hand-held stainless steel
corer. Dried sub-samples were processes for determination of
total C (TC) on a Perkin-Elmer 2400 CHNS/O Series II
Analyzer and for total organic carbon (TOC) on a Shimadzu
TOC Analyzer with solid sampler. Total inorganic carbon was
assumed CaCO
3
, as determined by difference between the TC
and TOC concentrations
Seagrass
AGCG (Mg C ha -1)
Number of observations
0
5
10
15
20
25
BGCB (Mg C ha -1)
Number of observations
0
5
10
15
20
25
30
Soil C (Mg C ha -1)
0.0 0.2 0.4 0.6 0.8 1.0 1.2
2
01 345
50 100 150 200 250 300
Number of observations
0
2
4
6
8
10
Fig. 3 Frequency distribution of reported observations of
above-ground carbon biomass (AGCB, top histogram), below-
ground carbon-biomass (BGCB, middle histogram) and soil
carbon (bottom histogram) from individual sample sites in
Indonesian seagrass meadows (mean values for locations are in
Table 1)
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biomass (AGCB) and 3 Mg C ha
-1
for below-ground
carbon biomass (BGCB). AGCB ranged from 0.01 to
1.15 Mg C ha
-1
with a median of 0.29 and a mean (±1
SE) of 0.32 ±0.03 Mg C ha
-1
(Table 1). BGCB was
greater than ABCG at all locations and ranged from
0.02 to 4.84 Mg C ha
-1
with a median of 1.13 and a
mean (±1 SE) of 1.23 ±0.11 Mg C ha
-1
(Table 1).
The largest carbon pool was the soil, ranging from
31.3 to 293.3 Mg C ha
-1
with a median of 118.1 and a
mean (±1 SE) of 129.9 ±9.6 Mg C ha
-1
(Table 1).
Seagrass AGCB and BGCB correlated positively
(Pearson’s r =0.501; P \0.00000167) but biomass
did not correlate with soil carbon. The sum of these
three pools is the median carbon storage for an
Indonesian seagrass meadow, which is 119.5 Mg C
ha
-1
. Extrapolating this value to the total area of
seagrasses (30,000 km
2
) gives a total seagrass carbon
storage value in Indonesia of 368.5 ±19.5 Tg C or
0.3685 ±0.002 Pg C.
Mangroves
An additional criterion for the acceptance of mangrove
data sources was documentation in appropriate detail
of above- and below biomass and of soil C stocks to a
soil depth of 1 m or down to bedrock; other biomass
data exist for Indonesia, but these sources had
inadequate methodological details, lack of replicate
sample plots, and/or insufficient soil data, so they were
excluded from our analysis. We realize that soil
carbon stocks of mangroves are frequently [1 m (see
Donato et al. 2011), but in order to make comparisons
we limited to this depth. Similarly, references con-
taining only soil C data but no or insufficient above-
and below-ground biomass data were also excluded.
These criteria resulted in the acceptance of only two
sources (Table 2). Nevertheless, both of these pub-
lished sources used identical methods (Howard et al.
2014) and contain data from 37 different mangrove
forests in Sumatra, Sulawesi, Java, West Papua and
Kalimantan and encompass all known forest types
(estuarine, riverine, fringing, carbonate settings).
The data from all forest plots (Fig. 4) showed
asymmetric distribution especially for above-ground
carbon biomass (AGCB) with very few values greater
than 350 Mg C ha
-1
. AGCB ranged from 8.2 to
478.4 Mg C ha
-1
with a median of 159.1 and a mean
Mangroves
AGCB (Mg C ha -1)
Number of observations
0
2
4
6
8
10
Soil C (Mg C ha -1)
Number of observations
0
1
2
3
4
5
6
7
BGCB (Mg C ha -1)
0 100 200 300 400 500
0 200 400 600 800 1000 1200 1400 1600
0 102030405060
Number of observations
0
2
4
6
8
10
Fig. 4 Frequency distribution of reported observations of
above-ground carbon biomass (AGCB, top histogram), below-
ground carbon-biomass (BGCB, middle histogram) and soil
carbon (bottom histogram) from individual sample sites in
Indonesian mangrove forests (mean values for locations are in
Table 2)
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Table 1 Mean (±1 SE) above-(AGCB) and below-ground carbon (BGCB) biomass and soil carbon storage in various seagrass beds
throughout Indonesia. Soil stocks are to a depth of 1 m except where noted
Location Dominant species
(by biomass)
AGCB
(Mg C ha
-1
)
BGCB
(Mg C ha
-1
)
Soil C
(Mg C ha
-1
)
Reference
Salayer, Flores
Sea
E. acoroides,
T. hemprichii
0.73 ±0.41 1.07 ±0.63 NA (medium carbonate
sand to coral rubble)
Nienhuis et al. (1989)
Taka Bone Rate,
Flores Sea
Thalassodendron
ciliatum,
H. uninervis,T.
hemprichii
0.71 ±0.42 1.01 ±0.67 NA (medium carbonate
sand to coral rubble)
Komodo E. acoroides,
T. hemprichii
0.63 ±0.33 1.29 ±0.76 NA (fine carbonate sand to
coral rubble)
Sumbawa T. hemprichii,
Syringodium
isoetifolium,
C. serrulata
0.57 ±0.40 1.92 ±2.71 NA (fine carbonate sand to
coral rubble)
NW Java E. acoroides,
C. rotundata H.
uninervis,
C. serrulata,
T. hemprichii
0.66 ±0.32 1.75 ±0.82 62.4 ±12.4
a,b
Kiswara (1992)
S Sulawesi T. hemprichii,
E. acoroides
0.31 ±0.16 1.79 ±0.80 31.3 ±2.1
a
Erftemeijer and
Middelburg (1993),
Erftemeijer (1994),
and Erftemeijer and
Herman (1994)
T. hemprichii,
Halodule univervis
E. acoroides
0.34 ±0.16 1.81 ±1.09 NA (fine to coarse
carbonate sand)
Mixed 0.38 ±0.24 1.30 ±0.05 NA (fine to coarse
carbonate sand)
C. rotundata,
T. hemprichii,
E. acoroides,
H. univervis
0.55 ±0.25 2.84 ±1.53 NA (fine to coarse
carbonate sand)
E. acoroides 0.26 ±0.20 0.55 ±0.36 148.4 ±22.2
a
T. hemprichii,
C. serrulata
0.15 ±0.07 0.59 ±0.28 NA (terrigenous sandy
mud)
Moluccas H. univervis (sparse) 0.03 ±0.01 0.21 ±0.05 NA Delongh et al. (1995)
H. univervis 0.07 ±0.01 0.52 ±0.10 NA
SW Sulawesi T. hemprichii 0.73 ±0.19 1.18 ±0.52 NA Stapel et al. (2001)
SW Sulawesi E. acoroides 0.33 ±0.17 1.53 ±0.22 239.2 ±44.9
c
(1.08–1.30)
c
Priosambodo (2006),
Alongi et al. (2008a),
and Alongi (unpubl)
C. rotundata,
C. serrulata,
E. acoroides,
H. univervis
0.38 ±0.21 1.82 ±0.39 102.9 ±29.3
c
(0.91–1.38)
d
C. rotundata,
C. serrulata,
E. acoroides
0.61 ±0.18 2.41 ±0.50 87.9 ±28.4
c
(1.20–1.93)
d
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(±1 SE) of 191.2 ±20.2 Mg C ha
-1
(Table 2).
Below-ground carbon biomass (BGCB) was less than
AGCB at all locations, ranging from 2.5 to 58.6 Mg C
ha
-1
with a median of 16.7 and a mean (±1 SE) of
21.1 ±2.7 Mg C ha
-1
(Table 2). The largest reservoir
was the soil pool, ranging from 18.8 to 1569.3 Mg C
ha
-1
with a median of 774.7 and a mean (±1 SE) of
761.3 ±73.6 Mg C ha
-1
(Table 2). In one Sumatran
forest growing atop a relict coral reef, the largest C
pool was the mangrove forest canopy (Alongi et al.
2008b). Mangrove AGCB and BGCB correlated
positively (Pearson’s r =0.436; P \0.007) but nei-
ther biomass pool correlated with soil carbon.
Summing the median AGCB, BGCB and soil pools
(limited to 1 m depth), we derive a median carbon
storage value for an Indonesian mangrove forest of
950.5 ±29.9 Mg C ha
-1
. Extrapolating this value to
the total area of mangroves (31,894 km
2
) results in a
total mangrove carbon storage in Indonesia of
3.0 ±0.1 Pg C.
Discussion
The sum of seagrass and mangrove carbon storage in
Indonesia is 3.4 Pg C and can be compared to the
global estimate of carbon storage for all coastal
wetlands of 20 Pg C (10 PgC estimated by Chmura
et al. (2003) for tidal marshes and mangroves, plus an
equivalent C
org
storage (Fourqurean et al. 2012) for
seagrasses). If accurate, we can estimate that Indone-
sia’s estuarine and marine wetlands store roughly
17 % of the world’s blue carbon. Unfortunately, due to
the lack of sufficient data on mass sediment accumu-
lation of mangrove and seagrass soils in Indonesia, it is
not possible to derive an annual carbon burial rate.
Table 1 continued
Location Dominant species
(by biomass)
AGCB
(Mg C ha
-1
)
BGCB
(Mg C ha
-1
)
Soil C
(Mg C ha
-1
)
Reference
S Sulawesi T. hemprichii
(closed canopy)
0.41 ±0.01 2.19 ±0.06 NA Vonk et al. (2010)
T. hemprichii (open
canopy)
0.16 ±0.01 1.77 ±0.06 NA
E Kalimantan H. univervis
(coastal)
0.01 ±0.01 0.02 ±0.01 243.3 ±30.2
a
Van Katwijk et al.
(2011)
H. univervis
(intermediate)
0.05 ±0.03 0.19 ±0.11 121.6 ±14.8
a
H. univervis
(offshore)
0.14 ±0.09 0.23 ±0.16 80.0 ±10.2
a
E Kalimantan H. univervis 0.04 ±0.00 0.18 ±0.02 NA Christianen et al. (2012,
2013)
H. univervis
(grazed)
0.06 ±0.01 0.18 ±0.02 NA
H. univervis
(ungrazed)
0.10 ±0.01 0.18 ±0.01 NA
S Sulawesi E. acoroides,
T. hemprichii,
C. rotundata
0.27 ±0.03 0.86 ±0.03 214.4 ±48.7
a
Supriadi et al. (2014)
Median 0.29 1.13 118.1
Mean 0.32 ±0.03
(n =82)
1.23 ±0.11
(n =82)
129.9 ±9.6 (n =32)
a
An average sediment dry bulk density (g ml
-1
) of 0.92 was used for terrigenous deposits (Fourqurean et al. 2012) and 1.39 for
carbonates (Campbell et al. 2015)
b
Estimated using a median surface soil C
org
value of 0.595 % (by sediment dry weight) for these deposits (Booij et al. 2001; Pratono
et al. 2009)
c
Soil depth only to 50 cm
d
Range of increasing soil dry bulk densities (g ml
-1
) with soil depth (Alongi, unpublished data)
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Such data is urgently needed. Sequestration rates can
be derived for plant biomass (assuming such stocks are
not burned or clear-felled), but these are wholly
inadequate because soils account for 82 and 99 % of
the total carbon reserves for the archipelago’s man-
groves and seagrasses, respectively (Tables 1,2). One
empirical estimate indicates carbon sequestration
(estimated from sediment accretion using surface
elevation tables) in mangroves in Bali of
2.2 ±0.6 Mg C ha
-1
years
-1
(Sidik 2014) which is
very close to the global mean burial rate of 1.7 Mg C
ha
-1
years
-1
. Using the Bali estimate, this means that
approximately 7 Tg of carbon is buried in mangrove
soils throughout Indonesia annually.
Refinement of the carbon storage estimates in the
future may result in an even higher percentage of the
world’s total, especially for Indonesia’s seagrasses, for
several reasons. First, the areal coverage of Indone-
sia’s seagrass meadows is most likely greater than
30,000 km
2
because many of the shallow coastal
waters of the country’s 13,466 islands remain unsur-
veyed (Ooi et al. 2011). Green and Short (2003)
themselves maintained that their figure was a likely
underestimate. Second, only the senior author has
made any attempt to sample Indonesian seagrass
deposits to maximum depth of penetration; most of the
available studies (Table 1) sampled only surface soils.
Third, in other areas of Southeast Asia experiencing
increased river runoff due to deforestation (Terrados
et al. 1998; Gacia et al. 2003), many seagrass
meadows in Indonesia likely inhabit finer sediment
deposits than those locations cited in Table 1. Thus,
the average soil carbon pool in an Indonesian seagrass
meadow is probably much greater than estimated here,
considering also that recent seagrass carbon studies
(Fourqurean et al. 2012) have been able to penetrate
seagrass meadows across the globe to a soil depth of
1 meter. The mangrove soil data are also
Table 2 Mean (±1 SE) above-(AGCB) and below-ground carbon (BGCB) biomass and soil carbon storage in various mangrove
forests throughout Indonesia
Location Dominant species AGCB
(Mg C ha
-1
)
BGCB
(Mg C ha
-1
)
Soil C
(Mg C ha
-1
)
Reference
S. Sumatra Avicennia marina 24.3 ±4.6 3.1 ±2.8 174. 8 ±27.2
(50 cm)
Alongi et al. (2008b)
Rhizophora stylosa 18.2 ±4.1 4.4 ±2.6 1169.3 ±227.6
R. stylosa,R. apiculata 117.1 ±6.8 18.2 ±9.9 18.8 ±4.4
(12 cm)
W. Sulawesi Sonneratia caseolaris,
S. lanceolata,
Lumnitzera racemosa
30.0 ±3.4 8.2 ±3.8 777.4 ±110.4
(50 cm)
E Sumatra A. marina, R. apiculata,
R. stylosa, R. alba, R.
mucronata, Bruguiera
spp., Xylocarpus
granatum
311.8 ±77.2 27.9 ±4.4 979.5 ±152.4 Donato et al. (2011) and
Murdiyarso et al.
(2015)
N Java 18.8 ±6.4 2.5 ±3.3 571.6 ±200.3
W Kalimantan 159.1 ±69.5 14.2 ±6.0 620.9 ±61.2
S. Kalimantan 159.5 ±62.9 21.3 ±12.3 1059.2 ±188.9
N Sulawesi 111.9 ±30.3 14.9 ±10.8 811.7 ±630.1
W Papua 213.8 ±129.8 36.7 ±18.8 660.5 ±282.1
W Papua 338.4 ±61.7 43.6 ±19.4 1014.8 ±189.0
W Papua 282.9 ±158.5 27.2 ±13.7 965.1 ±169.2
Bali Avicennia spp.,
Rhizophora spp.,
Sonneratia spp.,
Bruguiera
gymnorrhiza,Ceriops
tagal, X. granatum
171 ±43 40 ±5 290 ±40 Sidik (2014)
Median 159.1 16.7 774.7
Mean 191.2 ±20.3
(n =37)
21.1 ±2.7
(n =37)
761.3 ±73.6
(n =37)
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underestimates of the true soil stocks as a number of
studies (Donato et al. 2011; Murdiyarso et al. 2015)
have found significant carbon stocks deeper than 1
meter. Further, it has been demonstrated that when
mangroves are disturbed carbon losses from depth [1
m have been measured (Ong 1993; Kauffman et al.
2014).
The data reported here for Indonesia’s mangroves is
likely more spatially representative than that for the
seagrasses, but nevertheless, many islands remain
unsurveyed (Fig. 1). Alongi et al. (2008b)discovered
that many coral reefs throughout the archipelago have
been smothered by catchment soils due to enhanced
land erosion and subsequent transport, leading to
colonization and development of mangroves. Although
no quantitative data exist, sedimentation has increased
since the 1970s to the extent that fringing reefs are
rapidly disappearing and being replaced by mangroves
and seagrasses (Budiman et al. 1986;Yuliantoetal.
2004;Alongietal.2008b; Sekiguchi and Aksornkoae
2008). Therefore, despite ongoing deforestation, there
are some areas where mangrove and seagrass ecosys-
tems area are expanding at the expense of the
archipelago’s coral reefs, providing that increased
sediment loading does not decrease the amount of light
reaching the bottom to fuel seagrass photosynthesis.
The net result of enhanced land erosion may well be
enhanced coastal carbon storage.
That Indonesia’s coastal wetlands store approxi-
mately one-fifth of the world’s blue carbon is not
totally unexpected if one considers that the archipe-
lago’s mangroves and seagrasses account for about
21 % of the global total coverage of mangroves and at
least 10 % of seagrasses (Green and Short 2003;
Spalding et al. 2010). Further, the carbon stocks of
mangroves located at lower latitudes are higher than
those of higher latitudes (Alongi 2009). Indonesian
seagrasses may or may not store more carbon in soils
as we only report on the top 50 cm; data currently in
hand would suggest that Indonesian seagrasses may
store less C
org
in above- (mean =0.32 Mg C ha
-1
)
and below-ground biomass (mean =1.23 Mg C ha
-1
)
than the global averages of 0.76 and 1.76 Mg ha
-1
recorded by Fourqurean et al. (2012). The ‘average’
mangrove forest in Indonesia stores roughly as much
C
org
(mean =950.5 Mg C ha
-1
) as they do globally
(mean =956 Mg C ha
-1
), but slightly more propor-
tionally below-ground (83 vs 75 %) in soil and roots
(Alongi 2014). But these differences with the global
averages may be more representative of the small
sample size in Indonesia compared with the global
dataset rather than of true differences in the distribu-
tion of carbon.
While Indonesia likely has at least one-fifth of the
world’s blue carbon stores, continuing and alarming
destruction and degradation of the nation’s mangrove
forests and seagrass meadows does have important
consequences. This is not limited only to coastal
stability and sustainability, but also in terms of
greenhouse gas emissions. CO
2
emissions have been
both estimated (Alongi et al. 1998; Pendleton et al.
2012;Siikama
¨ki et al. 2012) and observed after
degradation of mangrove forests (Lovelock et al.
2011; Sidik and Lovelock 2013;Kauffmanetal.
2014). Given the continuing mangrove deforestation
rate of 1 % of area annually, and assuming that 88 % of
the entire carbon store in above- and below ground
biomass as well as in soil to a depth of 1 meter is
oxidized to CO
2
during the destruction process (Kauff-
man et al. 2014), the amount of CO
2
returned annually
to the atmosphere/ocean pools is roughly 26,400 Gg
CO
2
or 29,040 Gg CO
2
if we make the identical
assumptions for seagrasses (see Kennedy et al. (2014)
for level of uncertainty and validity of these assump-
tions). This estimate is equivalent to about 3.2 % of the
annual CO
2
emissions associated with forest and net
forest and peatland conversion (906,874 Gg CO
2
years
-1
) in Indonesia (FAO 2012), which is roughly
proportional to the total area of coastal wetlands to
forest area (World Bank 2012). Our emissions estimate
is only a small proportion of the 0.15–1.02 Pg CO
2
that
may be released by land-use change of all of the world’s
coastal wetlands (Pendleton et al. 2012).
Nevertheless, REDD?and other climate mitiga-
tion projects that conserve the present carbon stores
as well as those activities that restore mangrove and
seagrass habitats are urgently required. This is not
simply to maintain and restore Indonesia’s carbon
stocks and the other important ecosystem services
that arise from these ecosystems, but to prevent a
significant fraction of the world’s marine carbon
reserves from being lost to the atmosphere and
ocean carbon pools.
Acknowledgments This is contribution no. 736 from the
Southeast Environmental Research Center at Florida
International University and is a publication of the Blue
Carbon Initiative, Washington DC.
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