302
dissolved organic carbon (DOC) export (Sugimatsu et al. 2015; Abo et al. 2018).
Although the components of RDOC and the reasons for the refractory properties are
still not fully understood, Arrieta et al. (2015) proposed that RDOC has a molecular
or physical structure (unspecified) that is difficult for bacteria to use, or its concentration is too low to be available for bacteria. Because these three pumps tend to not
flow in reverse (i.e., the opposite process is weak), they function to suppress both
the decomposition of organic matter and the emission of CO 2 to the atmosphere
over a long period.
11.2.4 Carbon Storage in Water and Organisms
Of the three requirements for a net long-term CO 2 sink (Fig. 11.4), the storage of
RDOC in water is the least understood (Jiao et al. 2014). The organic carbon pool is
not limited to sediments and organisms but can be in the water column as well if
only the DOC is refractory. DOC accounts for 28% (246 Tg C/year) of the total
green carbon flowing from rivers to oceans on the global scale (Cai 2011). Thus,
how much DOC decomposes in microbial and photochemical reactions (Moran
et al. 2000), how much DOC remains as refractory dissolved green carbon (Kubo
et al. 2015), and how much refractory dissolved blue carbon is newly formed at the
site can be major factors determining the amount of CO 2 uptake and emission.
Among them, new formation of refractory dissolved blue carbon is particularly
unclear. Phytoplankton, bacteria, macrophytes (seagrasses and seaweeds), and corals are organisms responsible for the formation of refractory dissolved blue carbon
(Ogawa et al. 2001; Wada et al. 2008; Kragh and Søndergaard 2009; Lønborg et al.
2009; Tanaka et al. 2011a, b). From a technical perspective, however, it is extremely
difficult to quantify refractory dissolved blue carbon separately from refractory dissolved green carbon because the concentration of DOC is low and salt in seawater
acts as an inhibitor in the chemical analysis.
The sequence of (1) uptake of CO 2 by macrophytes and phytoplankton, (2) production of their body (POC) and mucus (DOC), and (3) transportation and sinking
of POC and DOC in the deep ocean is also an important mechanism for carbon
storage. Even if POC and DOC get decomposed in the deep ocean and become CO 2 ,
the transport to the surface and return to the atmosphere occur over geological time
scales. According to recent reports, the global estimate of POC and DOC derived
from seagrasses transported from SCEs to the deep ocean is about 24 Tg C/year
(Duarte and Krause-Jensen 2017) and that derived from kelps is around 36–279 Tg
C/year (Krause-Jensen and Duarte 2016). However, the variability in these amounts
and the factors controlling their transport are still unknown, leading to high uncertainty in the estimates.
T. Kuwae et al.
dissolved organic carbon (DOC) export (Sugimatsu et al. 2015; Abo et al. 2018).
Although the components of RDOC and the reasons for the refractory properties are
still not fully understood, Arrieta et al. (2015) proposed that RDOC has a molecular
or physical structure (unspecified) that is difficult for bacteria to use, or its concentration is too low to be available for bacteria. Because these three pumps tend to not
flow in reverse (i.e., the opposite process is weak), they function to suppress both
the decomposition of organic matter and the emission of CO 2 to the atmosphere
over a long period.
11.2.4 Carbon Storage in Water and Organisms
Of the three requirements for a net long-term CO 2 sink (Fig. 11.4), the storage of
RDOC in water is the least understood (Jiao et al. 2014). The organic carbon pool is
not limited to sediments and organisms but can be in the water column as well if
only the DOC is refractory. DOC accounts for 28% (246 Tg C/year) of the total
green carbon flowing from rivers to oceans on the global scale (Cai 2011). Thus,
how much DOC decomposes in microbial and photochemical reactions (Moran
et al. 2000), how much DOC remains as refractory dissolved green carbon (Kubo
et al. 2015), and how much refractory dissolved blue carbon is newly formed at the
site can be major factors determining the amount of CO 2 uptake and emission.
Among them, new formation of refractory dissolved blue carbon is particularly
unclear. Phytoplankton, bacteria, macrophytes (seagrasses and seaweeds), and corals are organisms responsible for the formation of refractory dissolved blue carbon
(Ogawa et al. 2001; Wada et al. 2008; Kragh and Søndergaard 2009; Lønborg et al.
2009; Tanaka et al. 2011a, b). From a technical perspective, however, it is extremely
difficult to quantify refractory dissolved blue carbon separately from refractory dissolved green carbon because the concentration of DOC is low and salt in seawater
acts as an inhibitor in the chemical analysis.
The sequence of (1) uptake of CO 2 by macrophytes and phytoplankton, (2) production of their body (POC) and mucus (DOC), and (3) transportation and sinking
of POC and DOC in the deep ocean is also an important mechanism for carbon
storage. Even if POC and DOC get decomposed in the deep ocean and become CO 2 ,
the transport to the surface and return to the atmosphere occur over geological time
scales. According to recent reports, the global estimate of POC and DOC derived
from seagrasses transported from SCEs to the deep ocean is about 24 Tg C/year
(Duarte and Krause-Jensen 2017) and that derived from kelps is around 36–279 Tg
C/year (Krause-Jensen and Duarte 2016). However, the variability in these amounts
and the factors controlling their transport are still unknown, leading to high uncertainty in the estimates.
T. Kuwae et al.