6
incorporated as organic carbon in the organisms that make up the marine food chain
is called “sequestration”.
There is another important process in addition to sequestration. Some of the
organic carbon sequestered in the sea is refractory. This refractory organic carbon is
difficult for organisms to respire because of its chemical nature, and even the organic
carbon that can be easily decomposed is often stored because it is incorporated into
sediments, where low oxygen concentrations slow rates of decomposition. Some
organic carbon may also be transported to the deep sea, where it is isolated from the
CO 2 flux between atmosphere and ocean. Such organic matter may be preserved in
the sediment or in the deep ocean on a time scale of decades, centuries, or even millennia (Chap. 2; Miyajima and Hamaguchi 2018, Chap. 9; Abo et al. 2018, and
Chap. 11; Kuwae et al. 2018). The processes by which organic carbon is isolated
from the carbon cycle between the ocean and atmosphere and converted to a state
where it is stored for a long time is called “storage” in this book.
There are six major carbon pools in marine ecosystems: (1) sedimentary organic
carbon (SOC), (2) particulate inorganic carbon in sediment (carbonates), (3) organic
carbon in living marine organisms, (4) shell and skeletal inorganic carbon, (5) dissolved organic carbon in seawater, and (6) DIC in seawater. All of these six pools
contribute to the sequestration and storage of atmospheric CO 2 , but the residence
time of carbon in these pools (the time during which the carbon is stored in the
ocean before it is returned to the atmospheric as CO 2 ) varies.
It should be noted that an increase of a carbon pool does not necessarily lead to
a reduction of atmospheric CO 2 concentrations. For example, when inorganic carbonates are produced (e.g., calcification associated with the growth of shellfish and
corals), CO 2 is also formed as a byproduct of the chemical reaction (see Chaps. 6
and 10) (Tokoro et al. 2018; Watanabe and Nakamura 2018). Therefore, as carbonates are formed, the partial pressure of dissolved CO 2 increases, and this increase
may lead to an efflux of CO 2 into the atmosphere.
The carbon residence time is long in both SOC (because of the slow decomposition of organic matter) and RDOC in seawater (see Chaps. 2 and 11) (Miyajima and
Hamaguchi 2018; Kuwae et al. 2018). Although the DIC in seawater is the largest
among the six pools, the residence time of CO 2 in the DIC is short because the bicarbonate ion in the DIC is readily converted into CO 2 as a result of carbonate chemistry equilibrium (pH decrease). Moreover, the organic carbon in living organisms
also undergoes large spatiotemporal fluctuations and is rather unstable compared to
SOC and RDOC.
In the UNEP report, blue carbon is described as the CO 2 absorbed by living
marine organisms. However, blue carbon is more accurately characterized as “carbon that is sequestered or stored in the ocean by marine organisms”. An ecosystem
refers to an interactive complex of biological communities and the abiotic environment affecting them within a particular place. The physicochemical factors are more
complex in the water column of the ocean than in terrestrial ecosystems. To evaluate
the role of blue carbon in mitigating effects of climate change, it is therefore important to understand not only the amount of organic carbon sequestered and stored by
marine organisms but also the carbon cycle in the ecosystem.
M. Hori et al.
incorporated as organic carbon in the organisms that make up the marine food chain
is called “sequestration”.
There is another important process in addition to sequestration. Some of the
organic carbon sequestered in the sea is refractory. This refractory organic carbon is
difficult for organisms to respire because of its chemical nature, and even the organic
carbon that can be easily decomposed is often stored because it is incorporated into
sediments, where low oxygen concentrations slow rates of decomposition. Some
organic carbon may also be transported to the deep sea, where it is isolated from the
CO 2 flux between atmosphere and ocean. Such organic matter may be preserved in
the sediment or in the deep ocean on a time scale of decades, centuries, or even millennia (Chap. 2; Miyajima and Hamaguchi 2018, Chap. 9; Abo et al. 2018, and
Chap. 11; Kuwae et al. 2018). The processes by which organic carbon is isolated
from the carbon cycle between the ocean and atmosphere and converted to a state
where it is stored for a long time is called “storage” in this book.
There are six major carbon pools in marine ecosystems: (1) sedimentary organic
carbon (SOC), (2) particulate inorganic carbon in sediment (carbonates), (3) organic
carbon in living marine organisms, (4) shell and skeletal inorganic carbon, (5) dissolved organic carbon in seawater, and (6) DIC in seawater. All of these six pools
contribute to the sequestration and storage of atmospheric CO 2 , but the residence
time of carbon in these pools (the time during which the carbon is stored in the
ocean before it is returned to the atmospheric as CO 2 ) varies.
It should be noted that an increase of a carbon pool does not necessarily lead to
a reduction of atmospheric CO 2 concentrations. For example, when inorganic carbonates are produced (e.g., calcification associated with the growth of shellfish and
corals), CO 2 is also formed as a byproduct of the chemical reaction (see Chaps. 6
and 10) (Tokoro et al. 2018; Watanabe and Nakamura 2018). Therefore, as carbonates are formed, the partial pressure of dissolved CO 2 increases, and this increase
may lead to an efflux of CO 2 into the atmosphere.
The carbon residence time is long in both SOC (because of the slow decomposition of organic matter) and RDOC in seawater (see Chaps. 2 and 11) (Miyajima and
Hamaguchi 2018; Kuwae et al. 2018). Although the DIC in seawater is the largest
among the six pools, the residence time of CO 2 in the DIC is short because the bicarbonate ion in the DIC is readily converted into CO 2 as a result of carbonate chemistry equilibrium (pH decrease). Moreover, the organic carbon in living organisms
also undergoes large spatiotemporal fluctuations and is rather unstable compared to
SOC and RDOC.
In the UNEP report, blue carbon is described as the CO 2 absorbed by living
marine organisms. However, blue carbon is more accurately characterized as “carbon that is sequestered or stored in the ocean by marine organisms”. An ecosystem
refers to an interactive complex of biological communities and the abiotic environment affecting them within a particular place. The physicochemical factors are more
complex in the water column of the ocean than in terrestrial ecosystems. To evaluate
the role of blue carbon in mitigating effects of climate change, it is therefore important to understand not only the amount of organic carbon sequestered and stored by
marine organisms but also the carbon cycle in the ecosystem.
M. Hori et al.