166
because there is no need for fast-response sensors. However, the eddy diffusion
coefficient, which is the metric of turbulence for this method, is difficult to estimate
accurately.
Because there are advantages and disadvantages to each method, there is no perfect method for determining the true values of air–water CO 2 fluxes over any spatial
and temporal scales. It is thus important to apply multiple methods to estimate air–
water CO 2 fluxes in shallow coastal waters. When a discrepancy between fluxes
measured by these different methods is reported, especially in aquatic environments, elucidation of the causes of the discrepancy will be an important subject of
future research.
6.3.3 Direct Measurement Methods of Air–Ecosystem CO 2
Flux
Although we focus on the air–water CO 2 flux in this chapter, we also briefly introduce methods for measuring air–non-water (ecosystem) CO 2 fluxes because of their
relevance to some blue carbon ecosystems such as mangroves and saltmarshes. CO 2
fluxes between the air and mangrove/saltmarsh ecosystems, which are exposed to
the atmosphere, obviously cannot be estimated by the bulk formula method. Direct
measurement methods have therefore been applied to determine the fluxes (Akhand
et al. 2018; Otani and Endo 2018).
The benthic chamber method is a common method for measuring air–ecosystem
CO 2 fluxes. The floating chamber method is analogous to this method, and the two
methods are based on a common principle. The methodology of the benthic chamber is more established than the floating chamber methodology, and benthic chambers are commercially available (e.g., Li-8100A, LI-COR, Nebraska, USA).
The eddy covariance method and other micro-meteorological methods have also
been applied to air–terrestrial ecosystem CO 2 fluxes (e.g., FLUXNET, Baldocchi
2008). The terrestrial application was somewhat earlier than the aquatic application.
The methodology is the same for estimating air–terrestrial ecosystem and air–water
fluxes. Because terrestrial CO 2 fluxes are generally more than an order of magnitude
larger than air–water fluxes, the post-processing procedures for terrestrial CO 2
fluxes tend to be easier than those for air–water fluxes.
Like the methods for air–water fluxes, the methods for air–ecosystem fluxes
cover different spatial and temporal scales and should be applied in combination.
The eddy covariance method can be used to measure the net flux from all components of a mangrove/saltmarsh ecosystem. Although the air–vegetation CO 2 flux
and air–soil CO 2 flux should be quite different, the eddy covariance method cannot
quantify the contributions of soils and vegetation separately. In this case, the benthic
chamber method should also be used to measure the air–soil flux, and the contribution of vegetation can be estimated from the difference between the fluxes estimated
by the eddy covariance and benthic chamber methods.
T. Tokoro et al.
because there is no need for fast-response sensors. However, the eddy diffusion
coefficient, which is the metric of turbulence for this method, is difficult to estimate
accurately.
Because there are advantages and disadvantages to each method, there is no perfect method for determining the true values of air–water CO 2 fluxes over any spatial
and temporal scales. It is thus important to apply multiple methods to estimate air–
water CO 2 fluxes in shallow coastal waters. When a discrepancy between fluxes
measured by these different methods is reported, especially in aquatic environments, elucidation of the causes of the discrepancy will be an important subject of
future research.
6.3.3 Direct Measurement Methods of Air–Ecosystem CO 2
Flux
Although we focus on the air–water CO 2 flux in this chapter, we also briefly introduce methods for measuring air–non-water (ecosystem) CO 2 fluxes because of their
relevance to some blue carbon ecosystems such as mangroves and saltmarshes. CO 2
fluxes between the air and mangrove/saltmarsh ecosystems, which are exposed to
the atmosphere, obviously cannot be estimated by the bulk formula method. Direct
measurement methods have therefore been applied to determine the fluxes (Akhand
et al. 2018; Otani and Endo 2018).
The benthic chamber method is a common method for measuring air–ecosystem
CO 2 fluxes. The floating chamber method is analogous to this method, and the two
methods are based on a common principle. The methodology of the benthic chamber is more established than the floating chamber methodology, and benthic chambers are commercially available (e.g., Li-8100A, LI-COR, Nebraska, USA).
The eddy covariance method and other micro-meteorological methods have also
been applied to air–terrestrial ecosystem CO 2 fluxes (e.g., FLUXNET, Baldocchi
2008). The terrestrial application was somewhat earlier than the aquatic application.
The methodology is the same for estimating air–terrestrial ecosystem and air–water
fluxes. Because terrestrial CO 2 fluxes are generally more than an order of magnitude
larger than air–water fluxes, the post-processing procedures for terrestrial CO 2
fluxes tend to be easier than those for air–water fluxes.
Like the methods for air–water fluxes, the methods for air–ecosystem fluxes
cover different spatial and temporal scales and should be applied in combination.
The eddy covariance method can be used to measure the net flux from all components of a mangrove/saltmarsh ecosystem. Although the air–vegetation CO 2 flux
and air–soil CO 2 flux should be quite different, the eddy covariance method cannot
quantify the contributions of soils and vegetation separately. In this case, the benthic
chamber method should also be used to measure the air–soil flux, and the contribution of vegetation can be estimated from the difference between the fluxes estimated
by the eddy covariance and benthic chamber methods.
T. Tokoro et al.