34
2.1 Introduction
“Blue carbon” is conventionally defined as the organic carbon (OC) produced and
stored by “coastal vegetated ecosystems”, such as mangroves, seagrass meadows,
and tidal salt marshes (Siikamäki et al. 2012). These ecosystems have developed as
a result of the adaptation and colonization of certain terrestrial vascular plants to
coastal marine environments (e.g., Les et al. 1997). The distribution of coastal
vegetated ecosystems is generally confined to well-illuminated land–sea interface
zones where clastic (alumino-)silicate and/or biogenic carbonate sediment
accumulates. According to recent estimates compiled by Siikamäki et al. (2012),
there are ~140,000 km
2
of mangroves, ~320,000 km
2
of seagrass meadows, and
~51,000 km
2
of salt marshes worldwide, although the estimates of seagrass and tidal
marsh areas are uncertain. Approximately 95% of mangroves and 70% of seagrass
meadows are believed to be in tropical and subtropical regions, whereas salt marsh
is typically found in temperate and boreal regions.
Geochemical and biogeochemical processes that transform an element from
highly mobile and/or reactive forms into inert, spatially confined reservoirs with
much longer residence times are collectively referred to as “sequestration” (Gorham
et al. 1979; Klee and Graedel 2004). Carbon biosequestration is a series of ecosystem
processes through which atmospheric carbon dioxide (CO 2 ) is captured first by
plants and incorporated into their biomass and then converted through multiple
reaction steps into refractory detrital OC, which is stored in soil and sediment
reservoirs for a long time. Coastal vegetated ecosystems are characterized by high
rates of carbon capture (primary productivity) by dominant vascular plants and
associated algae (Hori and Kuwae (2018) in this volume). An even more important
characteristic of these ecosystems is the disproportionally large belowground stock
of detrital OC compared with living biomass (Table 2.1). In fact, the quantity of OC
stored as detrital OC in soils and sediments is, on average, more than double
(mangroves; see Inoue (2018) in this volume) or one to two orders of magnitude
greater (seagrass meadows and salt marsh) than that of OC stored in living biomass
(Donato et al. 2011; Alongi 2012; Fourqurean et al. 2012; Siikamäki et al. 2012;
Alongi et al. 2016). For these reasons, these ecosystems have been regarded as the
most efficient natural systems for long-term carbon sequestration.
On average, at least half of the detrital OC stored in the soils and sediments of
these ecosystems is considered of autochthonous origin, derived mainly from the
belowground biomass of vegetation (Middelburg et al. 1997; Bouillon et al. 2008;
Kennedy et al. 2010; Miyajima et al. 2015). However, the correlation between the
standing stock of biomass OC and the detrital OC pool in soils and sediments is
generally weak or absent in mangroves and seagrass meadows (Kennedy et al. 2010;
Donato et al. 2011; Fourqurean et al. 2012; Alongi et al. 2016), indicating that the
size of the soil and sediment OC pool in these two ecosystem types is not determined
simply by the primary production of the vegetation but is also constrained
significantly by some other biological or geophysical factors. Identifying these
factors and understanding their roles are essential to evaluating the global carbon
sequestration capacity of coastal vegetated ecosystems and predicting its future
T. Miyajima and M. Hamaguchi
2.1 Introduction
“Blue carbon” is conventionally defined as the organic carbon (OC) produced and
stored by “coastal vegetated ecosystems”, such as mangroves, seagrass meadows,
and tidal salt marshes (Siikamäki et al. 2012). These ecosystems have developed as
a result of the adaptation and colonization of certain terrestrial vascular plants to
coastal marine environments (e.g., Les et al. 1997). The distribution of coastal
vegetated ecosystems is generally confined to well-illuminated land–sea interface
zones where clastic (alumino-)silicate and/or biogenic carbonate sediment
accumulates. According to recent estimates compiled by Siikamäki et al. (2012),
there are ~140,000 km
2
of mangroves, ~320,000 km
2
of seagrass meadows, and
~51,000 km
2
of salt marshes worldwide, although the estimates of seagrass and tidal
marsh areas are uncertain. Approximately 95% of mangroves and 70% of seagrass
meadows are believed to be in tropical and subtropical regions, whereas salt marsh
is typically found in temperate and boreal regions.
Geochemical and biogeochemical processes that transform an element from
highly mobile and/or reactive forms into inert, spatially confined reservoirs with
much longer residence times are collectively referred to as “sequestration” (Gorham
et al. 1979; Klee and Graedel 2004). Carbon biosequestration is a series of ecosystem
processes through which atmospheric carbon dioxide (CO 2 ) is captured first by
plants and incorporated into their biomass and then converted through multiple
reaction steps into refractory detrital OC, which is stored in soil and sediment
reservoirs for a long time. Coastal vegetated ecosystems are characterized by high
rates of carbon capture (primary productivity) by dominant vascular plants and
associated algae (Hori and Kuwae (2018) in this volume). An even more important
characteristic of these ecosystems is the disproportionally large belowground stock
of detrital OC compared with living biomass (Table 2.1). In fact, the quantity of OC
stored as detrital OC in soils and sediments is, on average, more than double
(mangroves; see Inoue (2018) in this volume) or one to two orders of magnitude
greater (seagrass meadows and salt marsh) than that of OC stored in living biomass
(Donato et al. 2011; Alongi 2012; Fourqurean et al. 2012; Siikamäki et al. 2012;
Alongi et al. 2016). For these reasons, these ecosystems have been regarded as the
most efficient natural systems for long-term carbon sequestration.
On average, at least half of the detrital OC stored in the soils and sediments of
these ecosystems is considered of autochthonous origin, derived mainly from the
belowground biomass of vegetation (Middelburg et al. 1997; Bouillon et al. 2008;
Kennedy et al. 2010; Miyajima et al. 2015). However, the correlation between the
standing stock of biomass OC and the detrital OC pool in soils and sediments is
generally weak or absent in mangroves and seagrass meadows (Kennedy et al. 2010;
Donato et al. 2011; Fourqurean et al. 2012; Alongi et al. 2016), indicating that the
size of the soil and sediment OC pool in these two ecosystem types is not determined
simply by the primary production of the vegetation but is also constrained
significantly by some other biological or geophysical factors. Identifying these
factors and understanding their roles are essential to evaluating the global carbon
sequestration capacity of coastal vegetated ecosystems and predicting its future
T. Miyajima and M. Hamaguchi