229
The air–sediment CO 2 flux has been measured in two field studies using a closed
dark chamber system during emersion periods. One study, in the tidal flat of the
Westerschelde estuary, Netherlands, measured a maximum flux of 381 mmol C/m
2
/
day (Middelburg et al. 1996) and the other, in the salt marsh of the Ribble estuary,
England, measured a flux of 229 mmol C/m
2
/day (Ford et al. 2012). These areas
were considered to be CO 2 emission sites dominated by decomposition of organic
matter. Primary production in these tidal flats was also estimated for the submerged
sediments, using in situ data for temporal O 2 concentration changes. Migné et al.
(2002, 2016) and Spilmont et al. (2005) measured CO 2 flux by monitoring changes
of CO 2 concentration in situ in benthic light and dark chambers, and found that CO 2
was absorbed by microphytobenthos in the light chamber. Gross primary production appeared to be larger than respiration and decomposition in these intertidal
sediments, and the researchers suggested that CO 2 absorption by microphytobenthos on the sediment surface was important in the carbon cycle of the tidal flat
ecosystem.
In many cases the air–water CO 2 flux has been measured by the bulk method, and
the water surface has been shown to be a place of CO 2 emission in tidal flats and salt
marshes. The air–water CO 2 flux in estuaries indicates overall emission of CO 2 ,
given the influx of dissolved inorganic carbon and organic carbon from the basin
and the excess of mineralization and respiration over primary production (Raymond
and Cole 2001; Borges et al. 2004a, 2005; Jiang et al. 2008; Laruelle et al. 2010; Cai
2011; Chen et al. 2013; Regnier et al. 2013). For example, in a summary of published CO 2 flux emissions for about 165 estuaries and 87 continental shelves, Chen
et al. (2013) found that upper estuaries with salinities less than 2 are strong sources
of atmospheric CO 2 (39 ± 56 mol C/m
2
/year); middle estuaries with salinities
between 2 and 25 are moderate sources (17.5 ± 34 mol C/m
2
/year); and lower estuaries with salinities more than 25 are weak sources (8.4 ± 14 mol C/m
2
/year). Flecha
et al. (2015) found that the air–water CO 2 flux in the Guadalquivir estuary, Spain,
fluctuates due to differences in salinity and emitters in the upstream part of the estuary. And Frankignoulle et al. (1998) and Borges et al. (2006) reported that CO 2
emissions from European estuaries (averaging 0.05 mmol C/m
2
/year) are significant
in the regional CO 2 budget.
8.3 Materials and Methods
8.3.1 Study Sites
The two study sites were near the city of Osaka in a eutrophicated area. The sites
were characterized in terms of their CO 2 flux in the atmosphere, water surface, sediments of supratidal area, intertidal area, and subtidal area in tidal benthic ecosystems. Further details are presented (Endo and Otani 2018).
8 CO 2 Flux in Tidal Flats and Salt Marshes
The air–sediment CO 2 flux has been measured in two field studies using a closed
dark chamber system during emersion periods. One study, in the tidal flat of the
Westerschelde estuary, Netherlands, measured a maximum flux of 381 mmol C/m
2
/
day (Middelburg et al. 1996) and the other, in the salt marsh of the Ribble estuary,
England, measured a flux of 229 mmol C/m
2
/day (Ford et al. 2012). These areas
were considered to be CO 2 emission sites dominated by decomposition of organic
matter. Primary production in these tidal flats was also estimated for the submerged
sediments, using in situ data for temporal O 2 concentration changes. Migné et al.
(2002, 2016) and Spilmont et al. (2005) measured CO 2 flux by monitoring changes
of CO 2 concentration in situ in benthic light and dark chambers, and found that CO 2
was absorbed by microphytobenthos in the light chamber. Gross primary production appeared to be larger than respiration and decomposition in these intertidal
sediments, and the researchers suggested that CO 2 absorption by microphytobenthos on the sediment surface was important in the carbon cycle of the tidal flat
ecosystem.
In many cases the air–water CO 2 flux has been measured by the bulk method, and
the water surface has been shown to be a place of CO 2 emission in tidal flats and salt
marshes. The air–water CO 2 flux in estuaries indicates overall emission of CO 2 ,
given the influx of dissolved inorganic carbon and organic carbon from the basin
and the excess of mineralization and respiration over primary production (Raymond
and Cole 2001; Borges et al. 2004a, 2005; Jiang et al. 2008; Laruelle et al. 2010; Cai
2011; Chen et al. 2013; Regnier et al. 2013). For example, in a summary of published CO 2 flux emissions for about 165 estuaries and 87 continental shelves, Chen
et al. (2013) found that upper estuaries with salinities less than 2 are strong sources
of atmospheric CO 2 (39 ± 56 mol C/m
2
/year); middle estuaries with salinities
between 2 and 25 are moderate sources (17.5 ± 34 mol C/m
2
/year); and lower estuaries with salinities more than 25 are weak sources (8.4 ± 14 mol C/m
2
/year). Flecha
et al. (2015) found that the air–water CO 2 flux in the Guadalquivir estuary, Spain,
fluctuates due to differences in salinity and emitters in the upstream part of the estuary. And Frankignoulle et al. (1998) and Borges et al. (2006) reported that CO 2
emissions from European estuaries (averaging 0.05 mmol C/m
2
/year) are significant
in the regional CO 2 budget.
8.3 Materials and Methods
8.3.1 Study Sites
The two study sites were near the city of Osaka in a eutrophicated area. The sites
were characterized in terms of their CO 2 flux in the atmosphere, water surface, sediments of supratidal area, intertidal area, and subtidal area in tidal benthic ecosystems. Further details are presented (Endo and Otani 2018).
8 CO 2 Flux in Tidal Flats and Salt Marshes