299
“storing carbon but emitting CO 2 ” occurs because water intervenes between the
sediment (the main pool of carbon) and the atmosphere. Similarly, this dilemma can
also occur in inland waters such as lakes and rivers (Cole et al. 2007). In turn, in a
forest or grassland ecosystem not interrupted by water, the same amount of carbon
taken up from the atmosphere is stored in organisms and/or in the soil, assuming a
closed system. Because of this, the dilemma has not been specifically discussed in
terrestrial ecosystems. In reality, however, terrestrial ecosystems are also open systems, as large amounts of green carbon flow out of these ecosystems through rivers
and into the sea (Fig. 11.1). Therefore, in forest and grassland ecosystems, the
amount of net CO 2 uptake is larger than the amount of organic carbon stored.
11.2.2 Carbon Storage in SCEs
Among the various processes that influence carbon storage in SCEs, the major ones
supporting the high accumulation of organic carbon in the sediment include (1)
large supplies of autochthonous organic matter (i.e., blue carbon formed within
SCEs) and/or allochthonous organic matter (green carbon flowing in from terrestrial
sources and/or blue carbon flowing from outside the SCE); (2) a large supply of
mineral particles, which are the main component of the sediment (Sholkovitz 1976);
and (3) aggregation of the mineral particles and organic matter to promote sedimentation (Kennedy et al. 2010; Zonneveld et al. 2010).
The seabed of SCEs where carbon is deposited is also dynamic. Due to external
forcing by waves and currents, the sediment surface layer is disturbed, its thickness
varies, and the seabed topography changes. For example, erosion at the sea bottom
implies outflow of sedimentary mineral particles and carbon from the sediment surface. In turn, when the waves and currents near the seabed are calm, the sediment
and carbon accumulation rates increase. Furthermore, in such calm physical conditions, fine sediment particles are more easily deposited and organic matter adsorbs
to the fine particles, often resulting in the formation of muddy sediments where
much carbon is stored.
Decomposition of organic matter becomes slower after deposition on the sea
bottom. This is related to the anoxic environment of the sediment except for its
vast surface layer (e.g., about the top several millimeters in a muddy sediment).
Because terrestrial soil is aerobically decomposed by exposure to oxygen from the
air, its decomposition proceeds on a scale of decades, whereas in the anaerobic
environment of the seabed, organic matter is decomposed and mineralized over
thousands of years (Chambers et al. 2001). This suppression of the decomposition
rate promotes accumulation of organic matter at the seabed (Miyajima and
Hamaguchi 2018).
Vegetated SCEs such as mangroves, salt marshes, and seagrass meadows have
among the fastest rates of carbon storage to their sediments, with average values
ranging from 138 to 226 g C/m
2
/year (range: 18–1713 g C/m
2
/year); the rates are
at least 1000 times greater than that in the open ocean (0.018 g C/m
2
/year)
11 CO 2 Uptake in the Shallow Coastal Ecosystems Affected…
“storing carbon but emitting CO 2 ” occurs because water intervenes between the
sediment (the main pool of carbon) and the atmosphere. Similarly, this dilemma can
also occur in inland waters such as lakes and rivers (Cole et al. 2007). In turn, in a
forest or grassland ecosystem not interrupted by water, the same amount of carbon
taken up from the atmosphere is stored in organisms and/or in the soil, assuming a
closed system. Because of this, the dilemma has not been specifically discussed in
terrestrial ecosystems. In reality, however, terrestrial ecosystems are also open systems, as large amounts of green carbon flow out of these ecosystems through rivers
and into the sea (Fig. 11.1). Therefore, in forest and grassland ecosystems, the
amount of net CO 2 uptake is larger than the amount of organic carbon stored.
11.2.2 Carbon Storage in SCEs
Among the various processes that influence carbon storage in SCEs, the major ones
supporting the high accumulation of organic carbon in the sediment include (1)
large supplies of autochthonous organic matter (i.e., blue carbon formed within
SCEs) and/or allochthonous organic matter (green carbon flowing in from terrestrial
sources and/or blue carbon flowing from outside the SCE); (2) a large supply of
mineral particles, which are the main component of the sediment (Sholkovitz 1976);
and (3) aggregation of the mineral particles and organic matter to promote sedimentation (Kennedy et al. 2010; Zonneveld et al. 2010).
The seabed of SCEs where carbon is deposited is also dynamic. Due to external
forcing by waves and currents, the sediment surface layer is disturbed, its thickness
varies, and the seabed topography changes. For example, erosion at the sea bottom
implies outflow of sedimentary mineral particles and carbon from the sediment surface. In turn, when the waves and currents near the seabed are calm, the sediment
and carbon accumulation rates increase. Furthermore, in such calm physical conditions, fine sediment particles are more easily deposited and organic matter adsorbs
to the fine particles, often resulting in the formation of muddy sediments where
much carbon is stored.
Decomposition of organic matter becomes slower after deposition on the sea
bottom. This is related to the anoxic environment of the sediment except for its
vast surface layer (e.g., about the top several millimeters in a muddy sediment).
Because terrestrial soil is aerobically decomposed by exposure to oxygen from the
air, its decomposition proceeds on a scale of decades, whereas in the anaerobic
environment of the seabed, organic matter is decomposed and mineralized over
thousands of years (Chambers et al. 2001). This suppression of the decomposition
rate promotes accumulation of organic matter at the seabed (Miyajima and
Hamaguchi 2018).
Vegetated SCEs such as mangroves, salt marshes, and seagrass meadows have
among the fastest rates of carbon storage to their sediments, with average values
ranging from 138 to 226 g C/m
2
/year (range: 18–1713 g C/m
2
/year); the rates are
at least 1000 times greater than that in the open ocean (0.018 g C/m
2
/year)
11 CO 2 Uptake in the Shallow Coastal Ecosystems Affected…