163
recent studies have used higher values (approximately 400 ppm) because atmospheric CO 2 concentrations have been increasing at 10–15 ppm per decade as a
result of anthropogenic releases of CO 2 .
6.3.2 Direct Measurement Methods for Air–Water CO 2 Fluxes
There are advantages to using direct methods for measuring CO 2 fluxes in shallow
coastal waters because the accuracies of indirect methods such as the bulk-formula
method are compromised by the difficulty of accurately estimating transfer velocities in shallow coastal waters (see Sect. 6.2.3). However, most of the direct measurement methods require more expensive instruments and greater technical expertise
than the bulk formula method. The cost and spatiotemporal coverage of the fluxes
vary among the direct measurement methods.
The tracer method uses chemical tracers to provide estimates of air–water gas
fluxes that are easily quantifiable and recalcitrant to decomposition. A commonly
used tracer is
14
C. Prior to atmospheric bomb testing during the 1950s,
14
C was a
very small fraction of naturally occurring carbon. In recent years, however,
14
C has
been widely used as a tracer of atmospheric CO 2 and has been used to estimate
regional and global CO 2 fluxes in the ocean (e.g., Sweeney et al. 2007; Wanninkhof
1992). Sulfur hexafluoride (SF 6 ) has also been used as a gas tracer in the ocean (e.g.,
Watson et al. 1991). SF 6 is artificially injected into the water, and the CO 2 flux is
calculated from the rate of change of the SF 6 concentration. Correction for the dilution effect of the water mass is made by using another concurrently injected gas
(e.g.,
3
He) that has a different transfer velocity. The relationship between the transfer velocity and wind speed (Eq. 6.4) has been determined mainly from gas tracer
studies. A disadvantage of the tracer method is that CO 2 gas fluxes can be measured
only as long-term and area-wide average values. The tracer method is therefore not
suitable for detecting diurnal fluctuations of CO 2 fluxes. Application of the tracer
method to shallow coastal waters is also limited by the fact that shallow coastal
waters consist of heterogeneous water bodies.
The floating chamber method estimates CO 2 fluxes from continuous measurements of CO 2 concentrations made inside a chamber floating on the water surface
(Fig. 6.5). This method is designed to directly measure CO 2 fluxes in shallow coastal
waters (e.g., Frankignoulle 1988; Tokoro et al. 2007). However, there is no wind
inside the chamber, and although the wind blowing against the chamber may cause
the chamber to move relative to the water and thereby generate artificial turbulence,
the measured flux may be biased from the true flux. Unlike other direct methods,
this method is poor at measuring long-term fluxes over wide areas. Despite acknowledging these problems, some authors have concluded that the floating chamber
method is suitable for shallow coastal waters (Tokoro et al. 2007, 2014).
The eddy covariance method is one meteorological method for measuring the
transfers of gas and heat by atmospheric turbulence (Fig. 6.6). This method has been
used mainly to measure CO 2 fluxes between the air and terrestrial ecosystems
6 Air–Water CO 2 Flux in Shallow Coastal Waters: Theory, Methods…
recent studies have used higher values (approximately 400 ppm) because atmospheric CO 2 concentrations have been increasing at 10–15 ppm per decade as a
result of anthropogenic releases of CO 2 .
6.3.2 Direct Measurement Methods for Air–Water CO 2 Fluxes
There are advantages to using direct methods for measuring CO 2 fluxes in shallow
coastal waters because the accuracies of indirect methods such as the bulk-formula
method are compromised by the difficulty of accurately estimating transfer velocities in shallow coastal waters (see Sect. 6.2.3). However, most of the direct measurement methods require more expensive instruments and greater technical expertise
than the bulk formula method. The cost and spatiotemporal coverage of the fluxes
vary among the direct measurement methods.
The tracer method uses chemical tracers to provide estimates of air–water gas
fluxes that are easily quantifiable and recalcitrant to decomposition. A commonly
used tracer is
14
C. Prior to atmospheric bomb testing during the 1950s,
14
C was a
very small fraction of naturally occurring carbon. In recent years, however,
14
C has
been widely used as a tracer of atmospheric CO 2 and has been used to estimate
regional and global CO 2 fluxes in the ocean (e.g., Sweeney et al. 2007; Wanninkhof
1992). Sulfur hexafluoride (SF 6 ) has also been used as a gas tracer in the ocean (e.g.,
Watson et al. 1991). SF 6 is artificially injected into the water, and the CO 2 flux is
calculated from the rate of change of the SF 6 concentration. Correction for the dilution effect of the water mass is made by using another concurrently injected gas
(e.g.,
3
He) that has a different transfer velocity. The relationship between the transfer velocity and wind speed (Eq. 6.4) has been determined mainly from gas tracer
studies. A disadvantage of the tracer method is that CO 2 gas fluxes can be measured
only as long-term and area-wide average values. The tracer method is therefore not
suitable for detecting diurnal fluctuations of CO 2 fluxes. Application of the tracer
method to shallow coastal waters is also limited by the fact that shallow coastal
waters consist of heterogeneous water bodies.
The floating chamber method estimates CO 2 fluxes from continuous measurements of CO 2 concentrations made inside a chamber floating on the water surface
(Fig. 6.5). This method is designed to directly measure CO 2 fluxes in shallow coastal
waters (e.g., Frankignoulle 1988; Tokoro et al. 2007). However, there is no wind
inside the chamber, and although the wind blowing against the chamber may cause
the chamber to move relative to the water and thereby generate artificial turbulence,
the measured flux may be biased from the true flux. Unlike other direct methods,
this method is poor at measuring long-term fluxes over wide areas. Despite acknowledging these problems, some authors have concluded that the floating chamber
method is suitable for shallow coastal waters (Tokoro et al. 2007, 2014).
The eddy covariance method is one meteorological method for measuring the
transfers of gas and heat by atmospheric turbulence (Fig. 6.6). This method has been
used mainly to measure CO 2 fluxes between the air and terrestrial ecosystems
6 Air–Water CO 2 Flux in Shallow Coastal Waters: Theory, Methods…