244
comparatively small in winter because biological activity is suppressed at low temperatures and adds little CO 2 to seawater, such that the CO 2 concentration in seawater becomes lower than the atmospheric concentration. The CO 2 balance was
negative in winter because atmospheric CO 2 is more readily absorbed owing to its
increased solubility in colder water.
Correlation coefficients between the air–water flux, the air–sediment flux, photon density flux, and different measured water qualities are listed in Table 8.3. The
air–water flux is weakly related to the air–sediment flux because their CO 2 exchange
processes differ. Atmospheric CO 2 gas is exchanged directly with sediment by biological processes during the emersion periods. On the other hand, air–water CO 2
exchange is governed by physical processes that depend on the gradient of CO 2
concentration between air and water, along with changes in CO 2 concentrations in
water from biological activity as well as chemical effects related to solubility and
aqueous carbonate equilibrium. Therefore, air–water CO 2 flux is related to the difference in CO 2 concentrations between air and seawater (ΔpCO 2 ). The table also
shows that there is a negative correlation between air–water CO 2 flux and photon
density flux, showing an effect arising from photosynthesis by phytoplankton.
8.4.2.3 Water–Sediment CO 2 Flux in Subtidal Area
Water–sediment CO 2 fluxes in the subtidal area during daytime are listed in
Table 8.4. In spring, the averaged water–sediment CO 2 flux was 1.11 mg CO 2 /m
2
/
min, signifying net release of CO 2 from sediment to seawater. In summer, the average flux was −0.65 mg CO 2 /m
2
/min, signifying net absorption by sediment. The
water–sediment CO 2 flux was smaller than the air–sediment CO 2 flux and the same
as the air–water CO 2 flux. The water–sediment CO 2 flux had seasonal trends similar
to those of the air–sediment CO 2 flux.
We found that both absorption and emission of CO 2 from subtidal sediment were
strongly correlated with sediment temperature in a relationship that could be
approximated by Eq. 8.6 because they were for the intertidal zone at this site
(Fig. 8.15). However, the values of CO 2 absorption and emission at the water–
sediment interface in the subtidal area were about one-tenth of those in the intertidal
area. The Q 10 value was 3.0 for CO 2 absorption and 3.3 for CO 2 emission. These
values are larger than those for the intertidal area, suggesting that CO 2 exchange at
the water–sediment interface in the subtidal area was strongly influenced by sediment temperature, unlike the case in the intertidal area.
8.4.2.4 CO 2 Budget at the Bird Sanctuary Site
We used the air–sediment and air–water CO 2 flux data to estimate the daily CO 2
budget at the bird sanctuary site. We calculated the daily CO 2 budget in the intertidal
and subtidal areas considering the temporal changes of their CO 2 fluxes and the
changes in their respective surface areas caused by the tides. The results are shown
schematically in Fig. 8.16.
S. Otani and T. Endo
comparatively small in winter because biological activity is suppressed at low temperatures and adds little CO 2 to seawater, such that the CO 2 concentration in seawater becomes lower than the atmospheric concentration. The CO 2 balance was
negative in winter because atmospheric CO 2 is more readily absorbed owing to its
increased solubility in colder water.
Correlation coefficients between the air–water flux, the air–sediment flux, photon density flux, and different measured water qualities are listed in Table 8.3. The
air–water flux is weakly related to the air–sediment flux because their CO 2 exchange
processes differ. Atmospheric CO 2 gas is exchanged directly with sediment by biological processes during the emersion periods. On the other hand, air–water CO 2
exchange is governed by physical processes that depend on the gradient of CO 2
concentration between air and water, along with changes in CO 2 concentrations in
water from biological activity as well as chemical effects related to solubility and
aqueous carbonate equilibrium. Therefore, air–water CO 2 flux is related to the difference in CO 2 concentrations between air and seawater (ΔpCO 2 ). The table also
shows that there is a negative correlation between air–water CO 2 flux and photon
density flux, showing an effect arising from photosynthesis by phytoplankton.
8.4.2.3 Water–Sediment CO 2 Flux in Subtidal Area
Water–sediment CO 2 fluxes in the subtidal area during daytime are listed in
Table 8.4. In spring, the averaged water–sediment CO 2 flux was 1.11 mg CO 2 /m
2
/
min, signifying net release of CO 2 from sediment to seawater. In summer, the average flux was −0.65 mg CO 2 /m
2
/min, signifying net absorption by sediment. The
water–sediment CO 2 flux was smaller than the air–sediment CO 2 flux and the same
as the air–water CO 2 flux. The water–sediment CO 2 flux had seasonal trends similar
to those of the air–sediment CO 2 flux.
We found that both absorption and emission of CO 2 from subtidal sediment were
strongly correlated with sediment temperature in a relationship that could be
approximated by Eq. 8.6 because they were for the intertidal zone at this site
(Fig. 8.15). However, the values of CO 2 absorption and emission at the water–
sediment interface in the subtidal area were about one-tenth of those in the intertidal
area. The Q 10 value was 3.0 for CO 2 absorption and 3.3 for CO 2 emission. These
values are larger than those for the intertidal area, suggesting that CO 2 exchange at
the water–sediment interface in the subtidal area was strongly influenced by sediment temperature, unlike the case in the intertidal area.
8.4.2.4 CO 2 Budget at the Bird Sanctuary Site
We used the air–sediment and air–water CO 2 flux data to estimate the daily CO 2
budget at the bird sanctuary site. We calculated the daily CO 2 budget in the intertidal
and subtidal areas considering the temporal changes of their CO 2 fluxes and the
changes in their respective surface areas caused by the tides. The results are shown
schematically in Fig. 8.16.
S. Otani and T. Endo