50
saturation levels of ~60 μmol C m
−2
to undersaturation levels of ~10 μmol C m
−2
in
shallow exposed sediments (Mayer 1994; Aller and Blair 2006).
Once seagrass meadows are formed on shallow coastal sediments, accumulation
of fine-grained sediment is enhanced due to the energy-dissipating and sedimentstabilizing functions of the meadows. This facilitates long-term sequestration of OC
through sorptive preservation. The supply rate of OC to sediment also increases due
to autochthonous OC production by seagrasses and associated algae as well as
increased input of allochthonous organic seston such as pelagic plankton and riverborne terrestrial detritus trapped by seagrasses. On the other hand, accumulation of
fine-grained sediment reduces the mobility of O 2 in the pore water, resulting in a
thinner surface oxic layer. The increased OC supply stimulates microbial
consumption of O 2 in the sediment. These effects, acting synergistically with the
increase in the sediment accumulation rate, drastically shorten the OET compared
with unvegetated sediment. A portion of the O 2 produced by seagrasses through
photosynthesis is translocated to the rhizomes and excreted from the roots to the
surrounding sediment (Iizumi et al. 1980; Hemminga 1998). Simultaneously,
soluble LOC is excreted from the roots, stimulating microbial activity (Moriarty
et al. 1986; Bouillon and Boschker 2006) and resulting in the development of a
microbial community specific to the seagrass rhizosphere (Jensen et al. 2007). Such
an active microbial community could produce copious secondary metabolites
including extracellular polymers, providing precursors for mineral-associated LOC
(Cyle et al. 2016) and elevating the mineral surface OC loading to saturation levels.
These characteristic functions of seagrass meadows act cooperatively to enhance
the OC sequestration rate in the sediment.
From a historical perspective, the role of seagrass meadows (and probably also
salt marshes and mangroves) described above may be viewed as an emergent
ecosystem function of the sediment source–sink system (Fig. 2.6). According to this
hypothesis, recruitment and proliferation of seagrasses can render previously
OC-depleted coastal sediments into hot spots of OC accumulation and sequestration.
If such habitat changes occur worldwide, it would significantly alter the global
inventories and cycling of carbon, resulting in secular reduction of atmospheric
CO 2 . Quantitative evidence supporting this hypothesis is introduced in the following
sections.
2.7 Organic Carbon Storage in Seagrass Meadow Sediments
Kennedy et al. (2010) investigated the variability in sediment OC concentrations in
32 individual seagrass beds (predominantly Z. marina and P. oceanica) and adjacent
non-vegetated areas from temperate coastal environments. They reported that the
average OC concentration in seagrass sediments was approximately twice as high as
that in corresponding barren sediments, and this difference was statistically
significant. Our data also showed similar patterns not only in OC, but also in total
nitrogen and organic phosphorus concentrations, between subtropical
T. Miyajima and M. Hamaguchi
saturation levels of ~60 μmol C m
−2
to undersaturation levels of ~10 μmol C m
−2
in
shallow exposed sediments (Mayer 1994; Aller and Blair 2006).
Once seagrass meadows are formed on shallow coastal sediments, accumulation
of fine-grained sediment is enhanced due to the energy-dissipating and sedimentstabilizing functions of the meadows. This facilitates long-term sequestration of OC
through sorptive preservation. The supply rate of OC to sediment also increases due
to autochthonous OC production by seagrasses and associated algae as well as
increased input of allochthonous organic seston such as pelagic plankton and riverborne terrestrial detritus trapped by seagrasses. On the other hand, accumulation of
fine-grained sediment reduces the mobility of O 2 in the pore water, resulting in a
thinner surface oxic layer. The increased OC supply stimulates microbial
consumption of O 2 in the sediment. These effects, acting synergistically with the
increase in the sediment accumulation rate, drastically shorten the OET compared
with unvegetated sediment. A portion of the O 2 produced by seagrasses through
photosynthesis is translocated to the rhizomes and excreted from the roots to the
surrounding sediment (Iizumi et al. 1980; Hemminga 1998). Simultaneously,
soluble LOC is excreted from the roots, stimulating microbial activity (Moriarty
et al. 1986; Bouillon and Boschker 2006) and resulting in the development of a
microbial community specific to the seagrass rhizosphere (Jensen et al. 2007). Such
an active microbial community could produce copious secondary metabolites
including extracellular polymers, providing precursors for mineral-associated LOC
(Cyle et al. 2016) and elevating the mineral surface OC loading to saturation levels.
These characteristic functions of seagrass meadows act cooperatively to enhance
the OC sequestration rate in the sediment.
From a historical perspective, the role of seagrass meadows (and probably also
salt marshes and mangroves) described above may be viewed as an emergent
ecosystem function of the sediment source–sink system (Fig. 2.6). According to this
hypothesis, recruitment and proliferation of seagrasses can render previously
OC-depleted coastal sediments into hot spots of OC accumulation and sequestration.
If such habitat changes occur worldwide, it would significantly alter the global
inventories and cycling of carbon, resulting in secular reduction of atmospheric
CO 2 . Quantitative evidence supporting this hypothesis is introduced in the following
sections.
2.7 Organic Carbon Storage in Seagrass Meadow Sediments
Kennedy et al. (2010) investigated the variability in sediment OC concentrations in
32 individual seagrass beds (predominantly Z. marina and P. oceanica) and adjacent
non-vegetated areas from temperate coastal environments. They reported that the
average OC concentration in seagrass sediments was approximately twice as high as
that in corresponding barren sediments, and this difference was statistically
significant. Our data also showed similar patterns not only in OC, but also in total
nitrogen and organic phosphorus concentrations, between subtropical
T. Miyajima and M. Hamaguchi