12
equivalent to the amount stored in terrestrial forests and the deep ocean on the same
timeframe. The mechanism responsible for the high carbon sequestration and storage of blue carbon ecosystems accounts for this equivalence.
A characteristic that is not common to the three ecosystems is the habitat where
the vegetation is formed. Saltmarshes and mangrove forests are formed in intertidal
zones that undergo repeated submergence and emergence because of the tides. In
extremely shallow waters, stems and leaves may emerge above the water but never
dry out. These conditions are found especially at the mouths of brackish water estuaries, where freshwater and seawater mix with each other. Carbon dioxide from the
atmosphere is then directly fixed via photosynthesis when the leaves are exposed to
the atmosphere. For that reason, although these two ecosystems are clearly defined
as blue carbon in the UNEP report, some researchers may regard saltmarshes and
mangrove forests as being intermediate between green carbon and blue carbon systems. In other words, of the three ecosystems called blue carbon ecosystems, only
seagrass beds consist of plants that are strictly marine and can sequester and store
carbon in the subtidal zone. However, it has become clear that CO 2 in the atmosphere can also be directly absorbed by eelgrass. Chapter 6 provides details (Tokoro
et al. 2018). The depths at which seagrass beds are found depend on which seagrass
species forms the bed and can be as great as about 60 m (Coles et al. 2009). Seagrass
beds are found over a wide range of salinities (Lirman and Cropper 2003, Koch
et al. 2007). These characteristics of seagrasses suggest that they can be much more
widely distributed than saltmarshes and mangrove forests, the habitats of which are
restricted to intertidal areas along the coast. In the future, it will be very desirable to
find a way to expand the areal distribution of seagrass beds (See Chaps. 12 and 13
for details; Nobutoki et al. 2018, Kuwae and Hori 2018) to achieve the coastal management goal of maintaining and improving the role of shallow coastal ecosystems
in sequestering and storing blue carbon. Achieving that goal is one of the main
reasons that seagrass beds are frequently cited as representative examples of blue
carbon ecosystem in this chapter.
Table 1.1 compares some of the characteristics of seagrass beds, mangrove forests, and saltmarshes and the amounts of blue carbon they store versus the whole
ocean. The values for seagrass beds, however, are derived from data for the genus
Posidonia in the Mediterranean Sea and coastal waters of Australia as well as for
some other fast-growing tropical seagrass species. It is therefore possible that the
rates for seagrass beds are overestimates. The authors of the UNEP report have
pointed out this potential bias, and caution is therefore necessary when citing this
value. In particular, it is believed that belowground production tends to be larger for
the genus Posidonia than for other seagrass genera, and thus the amount of organic
carbon stored in the sediment tends to be greater in seagrass beds dominated by
Posidonia. For example, in Japan the rate of organic carbon storage by eelgrass
(Zostera marina) is almost equal to the rate of storage by the genus Posidonia in
Australia, but it is only about one-fifth the rate of storage by the genus Posidonia in
the Mediterranean (see Chap. 2 for details; Miyajima and Hamaguchi 2018).
Among the coastal macrophyte communities, mangrove forests and saltmarshes
have the same storage rate per unit area, and those rates are slightly higher than the
M. Hori et al.
equivalent to the amount stored in terrestrial forests and the deep ocean on the same
timeframe. The mechanism responsible for the high carbon sequestration and storage of blue carbon ecosystems accounts for this equivalence.
A characteristic that is not common to the three ecosystems is the habitat where
the vegetation is formed. Saltmarshes and mangrove forests are formed in intertidal
zones that undergo repeated submergence and emergence because of the tides. In
extremely shallow waters, stems and leaves may emerge above the water but never
dry out. These conditions are found especially at the mouths of brackish water estuaries, where freshwater and seawater mix with each other. Carbon dioxide from the
atmosphere is then directly fixed via photosynthesis when the leaves are exposed to
the atmosphere. For that reason, although these two ecosystems are clearly defined
as blue carbon in the UNEP report, some researchers may regard saltmarshes and
mangrove forests as being intermediate between green carbon and blue carbon systems. In other words, of the three ecosystems called blue carbon ecosystems, only
seagrass beds consist of plants that are strictly marine and can sequester and store
carbon in the subtidal zone. However, it has become clear that CO 2 in the atmosphere can also be directly absorbed by eelgrass. Chapter 6 provides details (Tokoro
et al. 2018). The depths at which seagrass beds are found depend on which seagrass
species forms the bed and can be as great as about 60 m (Coles et al. 2009). Seagrass
beds are found over a wide range of salinities (Lirman and Cropper 2003, Koch
et al. 2007). These characteristics of seagrasses suggest that they can be much more
widely distributed than saltmarshes and mangrove forests, the habitats of which are
restricted to intertidal areas along the coast. In the future, it will be very desirable to
find a way to expand the areal distribution of seagrass beds (See Chaps. 12 and 13
for details; Nobutoki et al. 2018, Kuwae and Hori 2018) to achieve the coastal management goal of maintaining and improving the role of shallow coastal ecosystems
in sequestering and storing blue carbon. Achieving that goal is one of the main
reasons that seagrass beds are frequently cited as representative examples of blue
carbon ecosystem in this chapter.
Table 1.1 compares some of the characteristics of seagrass beds, mangrove forests, and saltmarshes and the amounts of blue carbon they store versus the whole
ocean. The values for seagrass beds, however, are derived from data for the genus
Posidonia in the Mediterranean Sea and coastal waters of Australia as well as for
some other fast-growing tropical seagrass species. It is therefore possible that the
rates for seagrass beds are overestimates. The authors of the UNEP report have
pointed out this potential bias, and caution is therefore necessary when citing this
value. In particular, it is believed that belowground production tends to be larger for
the genus Posidonia than for other seagrass genera, and thus the amount of organic
carbon stored in the sediment tends to be greater in seagrass beds dominated by
Posidonia. For example, in Japan the rate of organic carbon storage by eelgrass
(Zostera marina) is almost equal to the rate of storage by the genus Posidonia in
Australia, but it is only about one-fifth the rate of storage by the genus Posidonia in
the Mediterranean (see Chap. 2 for details; Miyajima and Hamaguchi 2018).
Among the coastal macrophyte communities, mangrove forests and saltmarshes
have the same storage rate per unit area, and those rates are slightly higher than the
M. Hori et al.