247
In the intertidal area, the CO 2 budget ranged from −50.4 to 4.7 kg CO 2 /day, thus
this area was a CO 2 sink in every season except winter. Both CO 2 absorption and
emissions were largest in summer. In the subtidal area, the CO 2 budget ranged from
−17.0 to 4.8 kg CO 2 /day, thus this area was a CO 2 source in every season except
winter. The total CO 2 budget for the bird sanctuary site ranged from −50.4 to
−9.1 kg CO 2 /day, constituting a carbon sink throughout the year. Both the intertidal
and subtidal areas made important contributions to the CO 2 budget because while
the air–water CO 2 flux in the subtidal area was smaller than the air–sediment CO 2
flux in the intertidal area, the subtidal area was larger than the intertidal area.
8.5 Summary
In this chapter, we showed that the air–water, air–sediment, water–sediment, and
air–salt marsh CO 2 fluxes and their temporal variations are affected by different
vegetation (Table 8.1). This is the case for our sites and for tidal flats and salt
marshes throughout the world.
At our sites, the air–sediment and air–salt marsh CO 2 fluxes showed net absorption (Figs. 8.7 and 8.12). However, the air–water CO 2 flux showed net emission
throughout the year, except in winter at the bird sanctuary site (Table 8.1). The
water–sediment CO 2 flux showed net emission during the warm months of May and
July (Table 8.4).
The air–sediment CO 2 fluxes at our sites were almost the similar value, especially GPP per Chl.a (Figs. 8.8 and 8.13a). The absorption of CO 2 by sediments is
thought to be due to microphytobenthos, which has high primary production as
summarized by Cahoon (1999) and Underwood and Kromkamp (1999). GPP and
CO 2 emission from sediment in the subtidal area, where light levels are low, is an
order of magnitude smaller than from sediment in the intertidal area (Figs. 8.13 and
8.15).
The air–water CO 2 flux at our sites may have been influenced by freshwater discharge in the water column. At the bird sanctuary site, where salinity is 13–30
(Yamochi et al. 2017), the air–water CO 2 flux tended to be smaller than the airsediment CO 2 flux, but in the river-mouth site, where salinity is 0–23 (Otani et al.
2017), the opposite was true. Moreover, it has been shown that atmospheric CO 2 is
absorbed into seawater in the eastern part of Osaka Bay (−8.3 mmol C/m
2
/day; Fujii
et al. 2013), which is affected by discharge from the Yodo River.
Air–salt marsh CO 2 flux measured by the eddy correlation method tended to be
larger than air–sediment and water–sediment CO 2 fluxes at all sites (Table 8.1). One
factor that may be significant is that CO 2 is absorbed during the growth period of
Phragmites australis and Spartina alterniflora. It has also been reported that Zostera
marina directly absorbs CO 2 (Watanabe and Kuwae 2015). Vegetation appears to be
a major driver of air–ecosystem CO 2 flux and other carbon cycles in blue carbon
ecosystems and tidal flats.
8 CO 2 Flux in Tidal Flats and Salt Marshes
In the intertidal area, the CO 2 budget ranged from −50.4 to 4.7 kg CO 2 /day, thus
this area was a CO 2 sink in every season except winter. Both CO 2 absorption and
emissions were largest in summer. In the subtidal area, the CO 2 budget ranged from
−17.0 to 4.8 kg CO 2 /day, thus this area was a CO 2 source in every season except
winter. The total CO 2 budget for the bird sanctuary site ranged from −50.4 to
−9.1 kg CO 2 /day, constituting a carbon sink throughout the year. Both the intertidal
and subtidal areas made important contributions to the CO 2 budget because while
the air–water CO 2 flux in the subtidal area was smaller than the air–sediment CO 2
flux in the intertidal area, the subtidal area was larger than the intertidal area.
8.5 Summary
In this chapter, we showed that the air–water, air–sediment, water–sediment, and
air–salt marsh CO 2 fluxes and their temporal variations are affected by different
vegetation (Table 8.1). This is the case for our sites and for tidal flats and salt
marshes throughout the world.
At our sites, the air–sediment and air–salt marsh CO 2 fluxes showed net absorption (Figs. 8.7 and 8.12). However, the air–water CO 2 flux showed net emission
throughout the year, except in winter at the bird sanctuary site (Table 8.1). The
water–sediment CO 2 flux showed net emission during the warm months of May and
July (Table 8.4).
The air–sediment CO 2 fluxes at our sites were almost the similar value, especially GPP per Chl.a (Figs. 8.8 and 8.13a). The absorption of CO 2 by sediments is
thought to be due to microphytobenthos, which has high primary production as
summarized by Cahoon (1999) and Underwood and Kromkamp (1999). GPP and
CO 2 emission from sediment in the subtidal area, where light levels are low, is an
order of magnitude smaller than from sediment in the intertidal area (Figs. 8.13 and
8.15).
The air–water CO 2 flux at our sites may have been influenced by freshwater discharge in the water column. At the bird sanctuary site, where salinity is 13–30
(Yamochi et al. 2017), the air–water CO 2 flux tended to be smaller than the airsediment CO 2 flux, but in the river-mouth site, where salinity is 0–23 (Otani et al.
2017), the opposite was true. Moreover, it has been shown that atmospheric CO 2 is
absorbed into seawater in the eastern part of Osaka Bay (−8.3 mmol C/m
2
/day; Fujii
et al. 2013), which is affected by discharge from the Yodo River.
Air–salt marsh CO 2 flux measured by the eddy correlation method tended to be
larger than air–sediment and water–sediment CO 2 fluxes at all sites (Table 8.1). One
factor that may be significant is that CO 2 is absorbed during the growth period of
Phragmites australis and Spartina alterniflora. It has also been reported that Zostera
marina directly absorbs CO 2 (Watanabe and Kuwae 2015). Vegetation appears to be
a major driver of air–ecosystem CO 2 flux and other carbon cycles in blue carbon
ecosystems and tidal flats.
8 CO 2 Flux in Tidal Flats and Salt Marshes