167
6.4 Air–Water CO 2 Flux in Shallow Coastal Waters
An increasing number of measurements of CO 2 fluxes between the atmosphere and
shallow coastal waters have recently been made in seagrass meadows (Kone et al.
2009; Maher and Eyre 2012; Tokoro et al. 2014; Tokoro and Kuwae 2018), macroalgal beds (Ikawa and Oechel 2014), mangroves (Borges et al. 2003; Akhand et al.
2016, 2018), tidal flats (Otani and Endo 2018), saltmarshes (Otani and Endo 2018),
and coral reefs (Watanabe et al. 2013; Watanabe and Nakamura 2018). Additional
measurements will be necessary to reveal the factors that regulate CO 2 fluxes and to
predict future changes in those fluxes. In this section, we discuss results of some
studies of air–water CO 2 fluxes in shallow coastal waters as practical examples of
the measurement methods.
6.4.1 Air–Water CO 2 Fluxes in Seagrass Meadows: Combined
Measurements Using Multiple Methods
Here, we show empirical studies of air–water CO 2 fluxes and the factors that regulate them in seagrass meadows based on the use of multiple methods (Tokoro et al.
2014, Tokoro and Kuwae 2015, 2018). These studies were conducted in Furen
Lagoon, which is located on the east coast of Hokkaido, Japan (Fig. 6.7). Furen
Lagoon has an area of 57.4 km
2
, and its water is brackish. Most of the lagoon is
shallower than 1 m, and eelgrass meadows (dominant species: Zostera marina)
cover 70% of the bottom. Organic carbon dynamics in the lagoon have been reported
elsewhere (Watanabe and Kuwae 2015b). Three methods (bulk formula method,
floating chamber method, and eddy covariance method) were used to measure the
air–water CO 2 fluxes.
The results showed that the air–water CO 2 fluxes varied seasonally (Figs. 6.8,
6.9, and 6.10). The system was a sink of atmospheric CO 2 during the summer and a
small source during the winter. The annual average CO 2 flux was negative, i.e., the
eelgrass meadows functioned as a net CO 2 sink over the course of 1 year (Fig. 6.10).
Diurnal changes of air–water CO 2 fluxes were clearly identified by both the bulk
formula and floating chamber methods (Fig. 6.8), whereas the seasonal variations
were recorded by the eddy covariance method and bulk formula method (Figs. 6.9
and 6.10). The results of the eddy covariance method included some time intervals
when data were lacking because the post processing filtered out some low quality
data. The post-processing procedure for the eddy covariance method is still in the
development stage (Tokoro and Kuwae 2018).
Results of conventional studies have indicated that estuarine systems are net
sources of CO 2 to the atmosphere because of the input of DIC and organic carbon
from terrestrial systems (e.g., Cai 2011; Laruelle et al. 2010; Regnier et al. 2013).
However, air–water CO 2 fluxes estimated with multiple methods in this case showed
that seagrass meadows were atmospheric CO 2 sinks. As mentioned in Sect. 6.2, the
6 Air–Water CO 2 Flux in Shallow Coastal Waters: Theory, Methods…
6.4 Air–Water CO 2 Flux in Shallow Coastal Waters
An increasing number of measurements of CO 2 fluxes between the atmosphere and
shallow coastal waters have recently been made in seagrass meadows (Kone et al.
2009; Maher and Eyre 2012; Tokoro et al. 2014; Tokoro and Kuwae 2018), macroalgal beds (Ikawa and Oechel 2014), mangroves (Borges et al. 2003; Akhand et al.
2016, 2018), tidal flats (Otani and Endo 2018), saltmarshes (Otani and Endo 2018),
and coral reefs (Watanabe et al. 2013; Watanabe and Nakamura 2018). Additional
measurements will be necessary to reveal the factors that regulate CO 2 fluxes and to
predict future changes in those fluxes. In this section, we discuss results of some
studies of air–water CO 2 fluxes in shallow coastal waters as practical examples of
the measurement methods.
6.4.1 Air–Water CO 2 Fluxes in Seagrass Meadows: Combined
Measurements Using Multiple Methods
Here, we show empirical studies of air–water CO 2 fluxes and the factors that regulate them in seagrass meadows based on the use of multiple methods (Tokoro et al.
2014, Tokoro and Kuwae 2015, 2018). These studies were conducted in Furen
Lagoon, which is located on the east coast of Hokkaido, Japan (Fig. 6.7). Furen
Lagoon has an area of 57.4 km
2
, and its water is brackish. Most of the lagoon is
shallower than 1 m, and eelgrass meadows (dominant species: Zostera marina)
cover 70% of the bottom. Organic carbon dynamics in the lagoon have been reported
elsewhere (Watanabe and Kuwae 2015b). Three methods (bulk formula method,
floating chamber method, and eddy covariance method) were used to measure the
air–water CO 2 fluxes.
The results showed that the air–water CO 2 fluxes varied seasonally (Figs. 6.8,
6.9, and 6.10). The system was a sink of atmospheric CO 2 during the summer and a
small source during the winter. The annual average CO 2 flux was negative, i.e., the
eelgrass meadows functioned as a net CO 2 sink over the course of 1 year (Fig. 6.10).
Diurnal changes of air–water CO 2 fluxes were clearly identified by both the bulk
formula and floating chamber methods (Fig. 6.8), whereas the seasonal variations
were recorded by the eddy covariance method and bulk formula method (Figs. 6.9
and 6.10). The results of the eddy covariance method included some time intervals
when data were lacking because the post processing filtered out some low quality
data. The post-processing procedure for the eddy covariance method is still in the
development stage (Tokoro and Kuwae 2018).
Results of conventional studies have indicated that estuarine systems are net
sources of CO 2 to the atmosphere because of the input of DIC and organic carbon
from terrestrial systems (e.g., Cai 2011; Laruelle et al. 2010; Regnier et al. 2013).
However, air–water CO 2 fluxes estimated with multiple methods in this case showed
that seagrass meadows were atmospheric CO 2 sinks. As mentioned in Sect. 6.2, the
6 Air–Water CO 2 Flux in Shallow Coastal Waters: Theory, Methods…