5
have assimilated via photosynthesis and incorporated into organic matter. In a terrestrial ecosystem, green carbon is taken up directly as CO 2 from the atmosphere
into a plant to make organic carbon. Hence the amount of organic carbon in the
terrestrial realm can be equated to the amount of green carbon itself.
However, unlike terrestrial plants, generally marine plants cannot fix CO 2 until
CO 2 from the atmosphere has become dissolved in seawater. The process of dissolution is complex. In a nutshell, physicochemical processes control the rates of CO 2
absorption from the atmosphere by seawater and its release from seawater to the
atmosphere. The factors that control CO 2 absorption and release by seawater are the
difference between the partial pressure of atmospheric CO 2 and the partial pressure
of CO 2 dissolved in the seawater. When the partial pressure in the atmosphere is
greater than (less than) the partial pressure in the seawater, there is a net influx
(efflux) of CO 2 from the atmosphere (ocean) to the ocean (atmosphere). Chapter 6
provides details (Tokoro et al. 2018).
Photosynthesis by marine plants lowers the dissolved CO 2 concentration in the
ocean. In addition, increases (decreases) in the water temperature cause the CO 2
solubility to decrease (increase). Upwelling and vertical mixing can cause large
amounts of dissolved CO 2 from lower depths to be introduced into the surface mixed
layer and thereby affect the vertical distribution of CO 2 . The wind speed at the sea
surface also influences the exchange of CO 2 with the atmosphere. Most of the CO 2
dissolved in seawater dissociates into bicarbonate and carbonate ions. At the pH of
typical seawater, the bicarbonate ion concentration greatly exceeds the carbonate
ion concentration at equilibrium. As dissolved CO 2 in seawater increases, the
amount of dissolved inorganic carbon (DIC) that occurs in the form of bicarbonate
ions increases, and hydrogen ions are released. These hydrogen ions are the cause
of ocean acidification; they lower the pH of the seawater. Marine plants grow by
taking up some of the DIC in seawater and converting it into organic matter via
photosynthesis. At the present time, the CO 2 partial pressure is lower in the sea than
in the atmosphere. Hence, there is a net influx of CO 2 from the atmosphere into the
ocean (see Chaps. 6 and 11; Tokoro et al. 2018, Kuwae et al. 2018). The net influx
of CO 2 into the ocean is therefore not always equal to the rate of CO 2 sequestration
by marine plants. Seagrass species are well known to take up dissolved CO 2 in preference to bicarbonate ions (Larkum et al. 2006).
CO 2 taken up by marine plants is converted to organic carbon and becomes part
of the plant biomass. Chapter 4 provides details (Yoshida et al. 2018). Afterward,
depending on the rates of photosynthesis, respiration, growth, and mortality, some
of the organic carbon is converted back into inorganic carbon and returned to the
sea. Chapter 9 provides details (Abo et al. 2018). Alternatively, the plants may be
consumed by herbivores, in which case the organic carbon becomes part of the biomass of the herbivores. The animals convert some of the organic carbon to DIC via
respiration, but the carbon that is not respired remains in the form of organic matter.
As predators consume prey in the food chain, some organic carbon consumed by
predators is converted to DIC via respiration, but the rest is retained as organic carbon. The overall process by which CO 2 from the atmosphere enters the ocean and is
1 Blue Carbon: Characteristics of the Ocean’s Sequestration and Storage Ability…
have assimilated via photosynthesis and incorporated into organic matter. In a terrestrial ecosystem, green carbon is taken up directly as CO 2 from the atmosphere
into a plant to make organic carbon. Hence the amount of organic carbon in the
terrestrial realm can be equated to the amount of green carbon itself.
However, unlike terrestrial plants, generally marine plants cannot fix CO 2 until
CO 2 from the atmosphere has become dissolved in seawater. The process of dissolution is complex. In a nutshell, physicochemical processes control the rates of CO 2
absorption from the atmosphere by seawater and its release from seawater to the
atmosphere. The factors that control CO 2 absorption and release by seawater are the
difference between the partial pressure of atmospheric CO 2 and the partial pressure
of CO 2 dissolved in the seawater. When the partial pressure in the atmosphere is
greater than (less than) the partial pressure in the seawater, there is a net influx
(efflux) of CO 2 from the atmosphere (ocean) to the ocean (atmosphere). Chapter 6
provides details (Tokoro et al. 2018).
Photosynthesis by marine plants lowers the dissolved CO 2 concentration in the
ocean. In addition, increases (decreases) in the water temperature cause the CO 2
solubility to decrease (increase). Upwelling and vertical mixing can cause large
amounts of dissolved CO 2 from lower depths to be introduced into the surface mixed
layer and thereby affect the vertical distribution of CO 2 . The wind speed at the sea
surface also influences the exchange of CO 2 with the atmosphere. Most of the CO 2
dissolved in seawater dissociates into bicarbonate and carbonate ions. At the pH of
typical seawater, the bicarbonate ion concentration greatly exceeds the carbonate
ion concentration at equilibrium. As dissolved CO 2 in seawater increases, the
amount of dissolved inorganic carbon (DIC) that occurs in the form of bicarbonate
ions increases, and hydrogen ions are released. These hydrogen ions are the cause
of ocean acidification; they lower the pH of the seawater. Marine plants grow by
taking up some of the DIC in seawater and converting it into organic matter via
photosynthesis. At the present time, the CO 2 partial pressure is lower in the sea than
in the atmosphere. Hence, there is a net influx of CO 2 from the atmosphere into the
ocean (see Chaps. 6 and 11; Tokoro et al. 2018, Kuwae et al. 2018). The net influx
of CO 2 into the ocean is therefore not always equal to the rate of CO 2 sequestration
by marine plants. Seagrass species are well known to take up dissolved CO 2 in preference to bicarbonate ions (Larkum et al. 2006).
CO 2 taken up by marine plants is converted to organic carbon and becomes part
of the plant biomass. Chapter 4 provides details (Yoshida et al. 2018). Afterward,
depending on the rates of photosynthesis, respiration, growth, and mortality, some
of the organic carbon is converted back into inorganic carbon and returned to the
sea. Chapter 9 provides details (Abo et al. 2018). Alternatively, the plants may be
consumed by herbivores, in which case the organic carbon becomes part of the biomass of the herbivores. The animals convert some of the organic carbon to DIC via
respiration, but the carbon that is not respired remains in the form of organic matter.
As predators consume prey in the food chain, some organic carbon consumed by
predators is converted to DIC via respiration, but the rest is retained as organic carbon. The overall process by which CO 2 from the atmosphere enters the ocean and is
1 Blue Carbon: Characteristics of the Ocean’s Sequestration and Storage Ability…