61
mind, a tentative minimal estimation of long-term OC sequestration by global seagrass meadows may be 1.6 Tg C year
−1
, based on the average OC sequestration rate
excluding P. oceanica meadows and a global extent of seagrass meadows of
320,000 km
2
. If the average OC sequestration rate including P. oceanica meadows
is used, the global OC sequestration by seagrass meadows is estimated to be 9.4 Tg
C year
−1
.
It should be noted that these estimates represent the amount of OC expected to be
sequestered in sediment for hundreds to thousands of years, because millennial- scale
sediment cores were used to draw them. In the literature, several different estimates
of global carbon sequestration in seagrass meadows have been reported, and these
estimates are much larger than those described above: 27.4 Tg C year
−1
(based on
sediment delivery partitioning; Duarte et al. 2005; see also Table 2.3), 44 Tg C year
−1
(derived from community metabolic balancing; Duarte et al. 2005), and 48–112 Tg
C year
−1
(based on a compilation of various direct estimates of net primary production and allochthonous carbon accumulation; Kennedy et al. 2010). Such discrepancies among estimates are principally derived from differences in the time span
covered by the methods used (Miyajima et al. 2015). The shorter the time span used
for estimation of OC sequestration, the greater the resulting estimates are. Therefore,
when the OC sequestration rate of seagrass meadows is compared with the corresponding rates in other ecosystems or global carbon emissions flux, the time span
over which the rate is evaluated should be carefully chosen and clearly defined.
The overall potential of seagrass meadows and other coastal vegetated ecosystems for carbon sequestration may be considerably greater than the estimates noted
above, because these estimates focused solely on OC sequestered within the
ecosystems’ own sediments. In fact, seagrass meadows often export as large a
fraction of their net primary production to the pelagic ocean as that stored in their
own sediments (Duarte and Cebrián 1996). After OC is exported to the open ocean,
a portion of it may be sequestered within the ocean via several different mechanisms
(Fig. 2.8). Firstly, particulate OC (POC) exported from seagrass meadows as
detached seagrass leaves or resuspended organic sediment may settle onto pelagic
sediments and be stored for long term therein. For example, the export of detached
blades from the seagrass meadows of the Seto Inland Sea, Japan, to surrounding
pelagic sediment and further to the open Pacific Ocean has been estimated by
numerical hydrodynamic simulations (see Abo et al. (2018) in this volume). The
presence of seagrass-derived OC in the pelagic sediment of the Seto Inland Sea has
been observed directly using the eDNA approach (Hamaguchi et al., unpublished
data). Secondly, seagrasses excrete photosynthetic products as dissolved OC (DOC)
to the surrounding seawater (Wetzel and Penhale 1979; Ziegler and Benner 1999;
Barrón and Duarte 2009). Such DOC is presumably exported outside the meadows
to reside temporarily in the water column, where a portion of the DOC may be
converted to long-lived refractory DOC (RDOC) through complex microbial
reactions (Ogawa et al. 2001; see also Kuwae et al. (2018) in this volume). This
mechanism can be regarded as a type of carbon sequestration in the ocean, insofar
as the turnover time of the RDOC produced is sufficiently long. Thirdly, under some
conditions, dissolved inorganic carbon produced from remineralization of seagrass2 Carbon Sequestration in Sediment as an Ecosystem Function of Seagrass Meadows
mind, a tentative minimal estimation of long-term OC sequestration by global seagrass meadows may be 1.6 Tg C year
−1
, based on the average OC sequestration rate
excluding P. oceanica meadows and a global extent of seagrass meadows of
320,000 km
2
. If the average OC sequestration rate including P. oceanica meadows
is used, the global OC sequestration by seagrass meadows is estimated to be 9.4 Tg
C year
−1
.
It should be noted that these estimates represent the amount of OC expected to be
sequestered in sediment for hundreds to thousands of years, because millennial- scale
sediment cores were used to draw them. In the literature, several different estimates
of global carbon sequestration in seagrass meadows have been reported, and these
estimates are much larger than those described above: 27.4 Tg C year
−1
(based on
sediment delivery partitioning; Duarte et al. 2005; see also Table 2.3), 44 Tg C year
−1
(derived from community metabolic balancing; Duarte et al. 2005), and 48–112 Tg
C year
−1
(based on a compilation of various direct estimates of net primary production and allochthonous carbon accumulation; Kennedy et al. 2010). Such discrepancies among estimates are principally derived from differences in the time span
covered by the methods used (Miyajima et al. 2015). The shorter the time span used
for estimation of OC sequestration, the greater the resulting estimates are. Therefore,
when the OC sequestration rate of seagrass meadows is compared with the corresponding rates in other ecosystems or global carbon emissions flux, the time span
over which the rate is evaluated should be carefully chosen and clearly defined.
The overall potential of seagrass meadows and other coastal vegetated ecosystems for carbon sequestration may be considerably greater than the estimates noted
above, because these estimates focused solely on OC sequestered within the
ecosystems’ own sediments. In fact, seagrass meadows often export as large a
fraction of their net primary production to the pelagic ocean as that stored in their
own sediments (Duarte and Cebrián 1996). After OC is exported to the open ocean,
a portion of it may be sequestered within the ocean via several different mechanisms
(Fig. 2.8). Firstly, particulate OC (POC) exported from seagrass meadows as
detached seagrass leaves or resuspended organic sediment may settle onto pelagic
sediments and be stored for long term therein. For example, the export of detached
blades from the seagrass meadows of the Seto Inland Sea, Japan, to surrounding
pelagic sediment and further to the open Pacific Ocean has been estimated by
numerical hydrodynamic simulations (see Abo et al. (2018) in this volume). The
presence of seagrass-derived OC in the pelagic sediment of the Seto Inland Sea has
been observed directly using the eDNA approach (Hamaguchi et al., unpublished
data). Secondly, seagrasses excrete photosynthetic products as dissolved OC (DOC)
to the surrounding seawater (Wetzel and Penhale 1979; Ziegler and Benner 1999;
Barrón and Duarte 2009). Such DOC is presumably exported outside the meadows
to reside temporarily in the water column, where a portion of the DOC may be
converted to long-lived refractory DOC (RDOC) through complex microbial
reactions (Ogawa et al. 2001; see also Kuwae et al. (2018) in this volume). This
mechanism can be regarded as a type of carbon sequestration in the ocean, insofar
as the turnover time of the RDOC produced is sufficiently long. Thirdly, under some
conditions, dissolved inorganic carbon produced from remineralization of seagrass2 Carbon Sequestration in Sediment as an Ecosystem Function of Seagrass Meadows