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8.1 Introduction
Estuarine Tidal flats and marshes are influenced by freshwater from rivers and seawater from offshore and are also sites where tides cause large environmental fluctuations. Because they serve as places of decomposition of organic matter inflowing
from the watershed, these ecosystems have typically been regarded as CO 2 sources
(Laruelle et al. 2013; Regnier et al. 2013). However, salt marshes are drawing attention for their role in blue carbon storage (Nellemann et al. 2009). Researchers are
investigating the plants that directly and indirectly absorb atmospheric CO 2 as well
as the animals and soils that store carbon.
Besides hosting vigorous biogenic production and consumption, tidal flats and
marshes display wide variations in CO 2 absorption and emission in response to
carbon influx driven by tides. Primary producers in these settings include large vegetation and benthic microalgae (microphytobenthos), and all participate in the
cycling of material through the food web. The macroalgae and seagrass are consumed by fish (Yoshida et al. 2015), mangrove leaves are a food source of crabs
(Robertson 1986; Skov and Hartnoll 2002), and microphytobenthos are major food
sources for macrobenthos.
The strong influence of tides means that tidal flats and salt marshes fluctuate
between being CO 2 sources and sinks over the course of a day. Therefore, it is necessary to evaluate CO 2 and carbon fluxes on a detailed temporal and seasonal scale.
However, compared with data from land areas and the open ocean, CO 2 gas flux data
from shallow coastal waters are scarce (Laruelle et al. 2013).
In this study, we sought to review the published CO 2 gas flux data from tidal flats
and salt marshes and to quantitatively evaluate CO 2 flux at two field sites near
Osaka, Japan, as model cases. Our main focus was CO 2 fluxes across the air–water
and air–sediment interfaces. Positive values of CO 2 flux and heat flux indicate
sources to the atmosphere and negative values indicate sinks from the atmosphere.
CO 2 fluxes in tidal flat and salt marsh ecosystems are shown schematically in
Fig. 8.1.
8.2 Literature Review on CO 2 Flux in Tidal Flats
and Marshes
Direct ways to measure air–water CO 2 flux include the eddy correlation method and
the chamber method, but the indirect bulk method is widely used because of the
ease in obtaining the necessary parameters such as wind speed and temperature
(Tokoro et al. 2018). Table 8.1 lists representative studies of CO 2 flux using these
three methods in intertidal and subtidal areas.
More studies of air–ecosystem CO 2 flux using the eddy correlation method have
been conducted in salt marshes than in tidal flats, and these focused on absorption of
CO 2 flux throughout the year. In particular, much research has targeted the common
S. Otani and T. Endo
8.1 Introduction
Estuarine Tidal flats and marshes are influenced by freshwater from rivers and seawater from offshore and are also sites where tides cause large environmental fluctuations. Because they serve as places of decomposition of organic matter inflowing
from the watershed, these ecosystems have typically been regarded as CO 2 sources
(Laruelle et al. 2013; Regnier et al. 2013). However, salt marshes are drawing attention for their role in blue carbon storage (Nellemann et al. 2009). Researchers are
investigating the plants that directly and indirectly absorb atmospheric CO 2 as well
as the animals and soils that store carbon.
Besides hosting vigorous biogenic production and consumption, tidal flats and
marshes display wide variations in CO 2 absorption and emission in response to
carbon influx driven by tides. Primary producers in these settings include large vegetation and benthic microalgae (microphytobenthos), and all participate in the
cycling of material through the food web. The macroalgae and seagrass are consumed by fish (Yoshida et al. 2015), mangrove leaves are a food source of crabs
(Robertson 1986; Skov and Hartnoll 2002), and microphytobenthos are major food
sources for macrobenthos.
The strong influence of tides means that tidal flats and salt marshes fluctuate
between being CO 2 sources and sinks over the course of a day. Therefore, it is necessary to evaluate CO 2 and carbon fluxes on a detailed temporal and seasonal scale.
However, compared with data from land areas and the open ocean, CO 2 gas flux data
from shallow coastal waters are scarce (Laruelle et al. 2013).
In this study, we sought to review the published CO 2 gas flux data from tidal flats
and salt marshes and to quantitatively evaluate CO 2 flux at two field sites near
Osaka, Japan, as model cases. Our main focus was CO 2 fluxes across the air–water
and air–sediment interfaces. Positive values of CO 2 flux and heat flux indicate
sources to the atmosphere and negative values indicate sinks from the atmosphere.
CO 2 fluxes in tidal flat and salt marsh ecosystems are shown schematically in
Fig. 8.1.
8.2 Literature Review on CO 2 Flux in Tidal Flats
and Marshes
Direct ways to measure air–water CO 2 flux include the eddy correlation method and
the chamber method, but the indirect bulk method is widely used because of the
ease in obtaining the necessary parameters such as wind speed and temperature
(Tokoro et al. 2018). Table 8.1 lists representative studies of CO 2 flux using these
three methods in intertidal and subtidal areas.
More studies of air–ecosystem CO 2 flux using the eddy correlation method have
been conducted in salt marshes than in tidal flats, and these focused on absorption of
CO 2 flux throughout the year. In particular, much research has targeted the common
S. Otani and T. Endo