155
uncertainty in the estimates for shallow coastal waters reflects the fact that
environments in coastal waters are far more diverse than those in the open ocean and
continental shelves. Although results of several studies of air–water CO 2 fluxes in
shallow coastal waters have recently been reported, the estimates are far from constrained, and the mechanisms are now under discussion (Akhand et al. 2018; Kuwae
et al. 2016; Otani and Endo 2018; Watanabe and Nakamura 2018). Because the
absorption and release of atmospheric CO 2 in shallow coastal waters undergo
extreme temporal and spatial fluctuations, a database of more extensive and intensive measurements in shallow coastal waters is needed to guide effective climate
change mitigation using blue carbon ecosystems.
In this chapter, we first explain the theoretical background of air–water CO 2 flux
measurements and methods for making those measurements. We then introduce
examples of in situ measurements made in shallow coastal waters and analyses
thereof, and we show that these waters can be sinks of atmospheric CO 2 .
6.2 Theoretical Background of Air–Water CO 2 Fluxes
The methods of measuring air–water CO 2 fluxes can be assigned to one or the other
of the following two approaches. The first approach is the direct measurement of the
CO 2 flux, such as by a tracer method (e.g., Sweeney et al. 2007), the floating chamber method (e.g., Frankignoulle 1988), or the eddy covariance method (e.g., Lee
et al. 2004). The other is indirect estimation using a theoretical model (Danckwerts
1951). The bulk formula method is the most general theoretical model. Although
indirect estimation may be inaccurate in shallow coastal waters, indirect estimation
has been used more than direct measurement methods because it is easy and less
expensive. Furthermore, indirect estimation can reveal the environmental factors
that affect the gas flux. We therefore first explain the theoretical background relevant to air–water gas fluxes. We then assess the bulk formula method as a representative indirect estimation method.
6.2.1 Thin–Film Model
There are several theoretical models of air–water CO 2 fluxes. Here, we describe a
simple model called the “thin-film model”. According to this model, just below the
water surface there is a thin film, within which there is a CO 2 concentration gradient. That concentration gradient is used to determine the CO 2 flux (Fig. 6.2).
In this chapter, the terms “concentration” and “partial pressure” are frequently
used; we briefly define them here. The former is the amount of a substance per unit
volume, and the latter is the pressure of a gas. In the case of water, the concentration
of a gas divided by its solubility equals the partial pressure. The air–water CO 2 flux
can be determined from the difference between the partial pressures of CO 2 (pCO 2 )
6 Air–Water CO 2 Flux in Shallow Coastal Waters: Theory, Methods…
uncertainty in the estimates for shallow coastal waters reflects the fact that
environments in coastal waters are far more diverse than those in the open ocean and
continental shelves. Although results of several studies of air–water CO 2 fluxes in
shallow coastal waters have recently been reported, the estimates are far from constrained, and the mechanisms are now under discussion (Akhand et al. 2018; Kuwae
et al. 2016; Otani and Endo 2018; Watanabe and Nakamura 2018). Because the
absorption and release of atmospheric CO 2 in shallow coastal waters undergo
extreme temporal and spatial fluctuations, a database of more extensive and intensive measurements in shallow coastal waters is needed to guide effective climate
change mitigation using blue carbon ecosystems.
In this chapter, we first explain the theoretical background of air–water CO 2 flux
measurements and methods for making those measurements. We then introduce
examples of in situ measurements made in shallow coastal waters and analyses
thereof, and we show that these waters can be sinks of atmospheric CO 2 .
6.2 Theoretical Background of Air–Water CO 2 Fluxes
The methods of measuring air–water CO 2 fluxes can be assigned to one or the other
of the following two approaches. The first approach is the direct measurement of the
CO 2 flux, such as by a tracer method (e.g., Sweeney et al. 2007), the floating chamber method (e.g., Frankignoulle 1988), or the eddy covariance method (e.g., Lee
et al. 2004). The other is indirect estimation using a theoretical model (Danckwerts
1951). The bulk formula method is the most general theoretical model. Although
indirect estimation may be inaccurate in shallow coastal waters, indirect estimation
has been used more than direct measurement methods because it is easy and less
expensive. Furthermore, indirect estimation can reveal the environmental factors
that affect the gas flux. We therefore first explain the theoretical background relevant to air–water gas fluxes. We then assess the bulk formula method as a representative indirect estimation method.
6.2.1 Thin–Film Model
There are several theoretical models of air–water CO 2 fluxes. Here, we describe a
simple model called the “thin-film model”. According to this model, just below the
water surface there is a thin film, within which there is a CO 2 concentration gradient. That concentration gradient is used to determine the CO 2 flux (Fig. 6.2).
In this chapter, the terms “concentration” and “partial pressure” are frequently
used; we briefly define them here. The former is the amount of a substance per unit
volume, and the latter is the pressure of a gas. In the case of water, the concentration
of a gas divided by its solubility equals the partial pressure. The air–water CO 2 flux
can be determined from the difference between the partial pressures of CO 2 (pCO 2 )
6 Air–Water CO 2 Flux in Shallow Coastal Waters: Theory, Methods…