300
(Nellemann et al. 2009; McLeod et al. 2011). This difference cannot be explained
solely by the difference in the production rate of blue carbon (SCE: 1044–2784 g
C/m
2
/year, open ocean: 120 g C/m
2
/year; Gattuso et al. 1998). Rather, the existence
of vegetation slows water currents and promotes the trapping and sedimentation of
suspended particulate organic matter, causing an increase in the carbon accumulation rate (Hendriks et al. 2007; Kennedy et al. 2010).
Other factors also influence the carbon storage rate in SCEs, as explained in
Chap. 2 (Miyajima and Hamaguchi 2018). These include chemical factors such as
the quality (e.g., whether it is easy or difficult to decompose) of organic matter
supplied and degradation enzyme activity; geophysical factors such as temperature, water depth, and the grain size and surface area of the sediment (Miyajima
et al. 2017); and biological factors such as bioturbation (Zonneveld et al. 2010;
Koho et al. 2013).
11.2.3 Requirements for a SCE Becoming a Net Sink
of Atmospheric CO 2
As we explained, the carbon storage rate in aquatic ecosystems is not equal to the
net atmospheric carbon uptake rate because these are open systems in which water
intervenes between the sediment and atmosphere. In addition, the inorganic–organic
conversion in the water column is complex. As a result, the amount of material
exchanged at the air–water interface and that exchanged at the water–sediment
interface generally do not balance.
Gas exchange between the atmosphere and the ocean occurs at the air–water
interface. If the concentration of CO 2 in seawater is lower than that in air, then atmospheric CO 2 will be absorbed into the sea (Wanninkhof 1992). Currently, the atmospheric CO 2 concentration fluctuates from about 350 to 450 ppm; in turn, the CO 2
concentration in SCE surface waters ranges from about 20 ppm to more than
3000 ppm. Thus, the actual gas exchange rate and direction of the flux (i.e., whether
the ecosystem is a sink or a source of CO 2 ) are dependent on the CO 2 concentration
in the surface water.
The CO 2 concentration in the surface water becomes undersaturated and atmospheric CO 2 is taken up (1) if the CO 2 concentration in the influent water from outside the target area is lower than that of the atmosphere, or (2) if the concentration
decreases below the atmospheric concentration due to the occurrence of processes
lowering the CO 2 concentration in the surface water. Rivers are major CO 2 influents
from outside SCEs, and their CO 2 concentrations are high. The partial pressure of
CO 2 in more than 95% of global inland waters is higher than that in the air, with a
median value of about 3100 μatm (Raymond et al. 2013). Thus, in order for the
surface water of SCEs to be undersaturated, it is necessary to have a process that
lowers the CO 2 concentration in surface water. As explained in Chap. 6, such processes include decreasing temperature, increasing total alkalinity (Ca
2+
and NO 3
−
T. Kuwae et al.
(Nellemann et al. 2009; McLeod et al. 2011). This difference cannot be explained
solely by the difference in the production rate of blue carbon (SCE: 1044–2784 g
C/m
2
/year, open ocean: 120 g C/m
2
/year; Gattuso et al. 1998). Rather, the existence
of vegetation slows water currents and promotes the trapping and sedimentation of
suspended particulate organic matter, causing an increase in the carbon accumulation rate (Hendriks et al. 2007; Kennedy et al. 2010).
Other factors also influence the carbon storage rate in SCEs, as explained in
Chap. 2 (Miyajima and Hamaguchi 2018). These include chemical factors such as
the quality (e.g., whether it is easy or difficult to decompose) of organic matter
supplied and degradation enzyme activity; geophysical factors such as temperature, water depth, and the grain size and surface area of the sediment (Miyajima
et al. 2017); and biological factors such as bioturbation (Zonneveld et al. 2010;
Koho et al. 2013).
11.2.3 Requirements for a SCE Becoming a Net Sink
of Atmospheric CO 2
As we explained, the carbon storage rate in aquatic ecosystems is not equal to the
net atmospheric carbon uptake rate because these are open systems in which water
intervenes between the sediment and atmosphere. In addition, the inorganic–organic
conversion in the water column is complex. As a result, the amount of material
exchanged at the air–water interface and that exchanged at the water–sediment
interface generally do not balance.
Gas exchange between the atmosphere and the ocean occurs at the air–water
interface. If the concentration of CO 2 in seawater is lower than that in air, then atmospheric CO 2 will be absorbed into the sea (Wanninkhof 1992). Currently, the atmospheric CO 2 concentration fluctuates from about 350 to 450 ppm; in turn, the CO 2
concentration in SCE surface waters ranges from about 20 ppm to more than
3000 ppm. Thus, the actual gas exchange rate and direction of the flux (i.e., whether
the ecosystem is a sink or a source of CO 2 ) are dependent on the CO 2 concentration
in the surface water.
The CO 2 concentration in the surface water becomes undersaturated and atmospheric CO 2 is taken up (1) if the CO 2 concentration in the influent water from outside the target area is lower than that of the atmosphere, or (2) if the concentration
decreases below the atmospheric concentration due to the occurrence of processes
lowering the CO 2 concentration in the surface water. Rivers are major CO 2 influents
from outside SCEs, and their CO 2 concentrations are high. The partial pressure of
CO 2 in more than 95% of global inland waters is higher than that in the air, with a
median value of about 3100 μatm (Raymond et al. 2013). Thus, in order for the
surface water of SCEs to be undersaturated, it is necessary to have a process that
lowers the CO 2 concentration in surface water. As explained in Chap. 6, such processes include decreasing temperature, increasing total alkalinity (Ca
2+
and NO 3
−
T. Kuwae et al.