281
contained in carbonates mainly originating from corals, foraminifera, and calcareous algae, which typically have values less than 0.2% to 0.4% (Miyajima et al.
1998).
Table 10.2 also summarizes the OC content of seagrass bed and mangrove forest
sediments. Please also refer to Chaps. 2 and 3 for discussions of OC in seagrass beds
(Miyajima and Hamaguchi 2018) and mangrove sediments (Inoue 2018), respectively. OC in seagrass sediments shows large variability, ranging from 0.1% to 11%
(Kennedy et al. 2010). Most tropical and subtropical seagrass beds have a relatively
low OC content of 0.6% or 0.7%; the median of the values surveyed here is 0.67%,
which is about twice that of coral reef sediments (0.35%). Coral reef sands vegetated
by seagrasses have OC contents 2–5 times those of unvegetated reef sands, possibly
from the accumulation of detrital OC in seagrass beds (Miyajima et al. 2015). OC in
mangrove sediments also shows large variability, ranging from 0.6% to 36%
(Bouillon et al. 2003; Breithaupt et al. 2012). The median of values surveyed here is
6.3%, which is close to the median mangrove sediment OC content of 7.0% reported
by Breithaupt et al. (2012) and about 18 times the OC content of reef sediments.
The differences between OC concentrations in coral reef, seagrass meadow, and
mangrove sediments can be explained in terms of the supply and preservation of
organic carbon in each ecosystem. The sedimentation rates compiled from previous
studies are not very different across these ecosystems (Table 10.2): 0.26 g cm
−2
year
−1
for coral reefs, 0.12 g cm
−2
year
−1
for seagrass beds (although only limited
data are reported), and 0.28 g cm
−2
year
−1
for mangrove forests. The OC burial rates,
however, are much higher in mangroves and in seagrass beds than in coral reefs; the
median values are 12.8 gC m
−2
year
−1
for coral reefs, 83.0 gC m
−2
year
−1
for seagrass beds, and 139 gC m
−2
year
−1
for mangrove forests. This difference results not
only from the supply of organic matter, but also from the differences in decomposition and preservation of organic matter in the sediments. In coral reef sediments, the
top several centimeters of sediments are usually supplied with oxygen (Werner et al.
2006; Yamamoto et al. 2015) mainly through pore-water advection due to highly
permeable sediments with relatively large grain size (Werner et al. 2006). This oxygen supply can facilitate carbon mineralization in the surface sediments (Miyajima
and Hamaguchi 2018). This mechanism can help maintain low OC content in reef
sediments when the supply of organic matter is not too high.
10.2.5 Carbon Export from Coral Reefs
Coral reefs can export organic matter from primary production, either to their internal sediments or to the open ocean. As explained in Sect. 10.2.2, coral reefs have
high GPP as well as respiration, but many reef flats are slightly autotrophic and
show positive net primary production (NPP) (for example Gattuso et al. 1993,
1996b; Kayanne et al. 1995, 2005; Ohde and van Woesik 1999; Hata et al. 2002).
Positive NPP means that OC can be either stored within the ecosystem or exported
to adjacent systems such as the open ocean. Note that the pCO 2 decrease due to NPP
10 Carbon Dynamics in Coral Reefs
contained in carbonates mainly originating from corals, foraminifera, and calcareous algae, which typically have values less than 0.2% to 0.4% (Miyajima et al.
1998).
Table 10.2 also summarizes the OC content of seagrass bed and mangrove forest
sediments. Please also refer to Chaps. 2 and 3 for discussions of OC in seagrass beds
(Miyajima and Hamaguchi 2018) and mangrove sediments (Inoue 2018), respectively. OC in seagrass sediments shows large variability, ranging from 0.1% to 11%
(Kennedy et al. 2010). Most tropical and subtropical seagrass beds have a relatively
low OC content of 0.6% or 0.7%; the median of the values surveyed here is 0.67%,
which is about twice that of coral reef sediments (0.35%). Coral reef sands vegetated
by seagrasses have OC contents 2–5 times those of unvegetated reef sands, possibly
from the accumulation of detrital OC in seagrass beds (Miyajima et al. 2015). OC in
mangrove sediments also shows large variability, ranging from 0.6% to 36%
(Bouillon et al. 2003; Breithaupt et al. 2012). The median of values surveyed here is
6.3%, which is close to the median mangrove sediment OC content of 7.0% reported
by Breithaupt et al. (2012) and about 18 times the OC content of reef sediments.
The differences between OC concentrations in coral reef, seagrass meadow, and
mangrove sediments can be explained in terms of the supply and preservation of
organic carbon in each ecosystem. The sedimentation rates compiled from previous
studies are not very different across these ecosystems (Table 10.2): 0.26 g cm
−2
year
−1
for coral reefs, 0.12 g cm
−2
year
−1
for seagrass beds (although only limited
data are reported), and 0.28 g cm
−2
year
−1
for mangrove forests. The OC burial rates,
however, are much higher in mangroves and in seagrass beds than in coral reefs; the
median values are 12.8 gC m
−2
year
−1
for coral reefs, 83.0 gC m
−2
year
−1
for seagrass beds, and 139 gC m
−2
year
−1
for mangrove forests. This difference results not
only from the supply of organic matter, but also from the differences in decomposition and preservation of organic matter in the sediments. In coral reef sediments, the
top several centimeters of sediments are usually supplied with oxygen (Werner et al.
2006; Yamamoto et al. 2015) mainly through pore-water advection due to highly
permeable sediments with relatively large grain size (Werner et al. 2006). This oxygen supply can facilitate carbon mineralization in the surface sediments (Miyajima
and Hamaguchi 2018). This mechanism can help maintain low OC content in reef
sediments when the supply of organic matter is not too high.
10.2.5 Carbon Export from Coral Reefs
Coral reefs can export organic matter from primary production, either to their internal sediments or to the open ocean. As explained in Sect. 10.2.2, coral reefs have
high GPP as well as respiration, but many reef flats are slightly autotrophic and
show positive net primary production (NPP) (for example Gattuso et al. 1993,
1996b; Kayanne et al. 1995, 2005; Ohde and van Woesik 1999; Hata et al. 2002).
Positive NPP means that OC can be either stored within the ecosystem or exported
to adjacent systems such as the open ocean. Note that the pCO 2 decrease due to NPP
10 Carbon Dynamics in Coral Reefs