307
Increased turbidity may function to either increase or decrease the CO 2 concentration of surface water, depending on the cause. If the source of turbidity is
phytoplankton, primary production is promoted and the surface CO 2 concentration
is reduced. If the source of turbidity is inorganic mineral particles (sand and mud),
photosynthesis is suppressed due to decreased light intensity in water and the concentration of CO 2 increases (Chen et al. 2012).
Furthermore, the residence time (exchange) of seawater is also an important factor determining CO 2 and organic matter concentrations (Gazeau et al. 2005). These
depend on the concentrations in and amount of incoming river water and offshore
seawater. Thus, changes in the residence time of seawater may function both in
increasing and decreasing CO 2 and organic matter concentrations.
11.3.3 Evidence from Field Studies
The air–seawater CO 2 exchange in the world’s SCEs was summarized by Borges
and Abril (2011), who noted only one case serving as a net sink for atmospheric
CO 2 , and by Laruelle et al. (2013) and Regnier et al. (2013), who concluding that
SCEs serve as a net emitter worldwide. In light of the growing literature after those
summaries were published, however, we used the Google Scholar and Scopus databases to identify new reported cases of SCEs serving as net sinks for CO 2 (Table 11.2)
to clarify the characteristics of these exceptional SCE cases.
First, a SCE serving as a net sink of atmospheric CO 2 is often located next to an
urbanized area or agricultural lands. These findings support our hypothesis that
human-impacted SCEs can act as a sink for atmospheric CO 2 . Second, such SCEs
are often affected by wastewater treatment, stratification, and hypoxia. These three
characteristics are consistent with the Japanese cases of Tokyo Bay (Fig. 11.6)
(Kubo et al. 2017) and Osaka Bay (Fig. 11.7) (Fujii et al. 2013). The effluent flowing into human-impacted SCEs has high nutrient and phytoplankton (chlorophyll a)
concentrations and high primary production due to loads derived from urban and
agricultural activities. In addition, previous studies revealed that net uptake of atmospheric CO 2 occurs when net ecosystem production increases (Maher and Eyre
2012; Tokoro et al. 2014, 2018).
As cases of vegetated ecosystems acting as net sinks of atmospheric CO 2 , seagrass meadows and one kelp bed were extracted. The uptake rate in the seagrass
meadows was 24.6 ± 44.1 mmol C/m
2
/day and that in the kelp bed was 59.4 mmol
C/m
2
/day (Ikawa and Oechel 2015), all of which were faster than the uptake rate of
SCEs without seagrass meadows (9.6 ± 6.7 mmol C/m
2
/day). There were also cases
of coral reefs in which the CO 2 concentration in water was undersaturated and the
system acted as a sink (Kayanne et al. 1995, 2005; Delille et al. 2009), although the
uptake rate was not described.
The global average of the net CO 2 emission rate from SCEs is about 40–50 mmol
C/m
2
/day (Laruelle et al. 2013). However, most of the data for these statistics were
acquired intermittently at fragmented spatial scales; there are very few cases for
11 CO 2 Uptake in the Shallow Coastal Ecosystems Affected…
Increased turbidity may function to either increase or decrease the CO 2 concentration of surface water, depending on the cause. If the source of turbidity is
phytoplankton, primary production is promoted and the surface CO 2 concentration
is reduced. If the source of turbidity is inorganic mineral particles (sand and mud),
photosynthesis is suppressed due to decreased light intensity in water and the concentration of CO 2 increases (Chen et al. 2012).
Furthermore, the residence time (exchange) of seawater is also an important factor determining CO 2 and organic matter concentrations (Gazeau et al. 2005). These
depend on the concentrations in and amount of incoming river water and offshore
seawater. Thus, changes in the residence time of seawater may function both in
increasing and decreasing CO 2 and organic matter concentrations.
11.3.3 Evidence from Field Studies
The air–seawater CO 2 exchange in the world’s SCEs was summarized by Borges
and Abril (2011), who noted only one case serving as a net sink for atmospheric
CO 2 , and by Laruelle et al. (2013) and Regnier et al. (2013), who concluding that
SCEs serve as a net emitter worldwide. In light of the growing literature after those
summaries were published, however, we used the Google Scholar and Scopus databases to identify new reported cases of SCEs serving as net sinks for CO 2 (Table 11.2)
to clarify the characteristics of these exceptional SCE cases.
First, a SCE serving as a net sink of atmospheric CO 2 is often located next to an
urbanized area or agricultural lands. These findings support our hypothesis that
human-impacted SCEs can act as a sink for atmospheric CO 2 . Second, such SCEs
are often affected by wastewater treatment, stratification, and hypoxia. These three
characteristics are consistent with the Japanese cases of Tokyo Bay (Fig. 11.6)
(Kubo et al. 2017) and Osaka Bay (Fig. 11.7) (Fujii et al. 2013). The effluent flowing into human-impacted SCEs has high nutrient and phytoplankton (chlorophyll a)
concentrations and high primary production due to loads derived from urban and
agricultural activities. In addition, previous studies revealed that net uptake of atmospheric CO 2 occurs when net ecosystem production increases (Maher and Eyre
2012; Tokoro et al. 2014, 2018).
As cases of vegetated ecosystems acting as net sinks of atmospheric CO 2 , seagrass meadows and one kelp bed were extracted. The uptake rate in the seagrass
meadows was 24.6 ± 44.1 mmol C/m
2
/day and that in the kelp bed was 59.4 mmol
C/m
2
/day (Ikawa and Oechel 2015), all of which were faster than the uptake rate of
SCEs without seagrass meadows (9.6 ± 6.7 mmol C/m
2
/day). There were also cases
of coral reefs in which the CO 2 concentration in water was undersaturated and the
system acted as a sink (Kayanne et al. 1995, 2005; Delille et al. 2009), although the
uptake rate was not described.
The global average of the net CO 2 emission rate from SCEs is about 40–50 mmol
C/m
2
/day (Laruelle et al. 2013). However, most of the data for these statistics were
acquired intermittently at fragmented spatial scales; there are very few cases for
11 CO 2 Uptake in the Shallow Coastal Ecosystems Affected…