59
of determination between the logarithm of the eDNA copy number and seagrassderived OC (excluding the sample from the deepest layer that was apparently subject to eDNA contamination) is as high as 0.89. This example shows that eDNA is a
promising tool for quantitative evaluation of sediment OC derived from various
plant species. However, the quantitative relationship between eDNA and OC from
seagrasses appears to depend on the age of the samples and also on the environmental
settings of the meadows, which poses a challenge to the broader applicability of this
technique. This discrepancy may be partially related to differences in the specificity
to seagrasses of eDNA- and δ
13
C-based techniques. At present, more comprehensive
studies are required to understand which factors control the preservation of molecular biomarkers and eDNA in the diverse sediments of coastal vegetated
ecosystems.
2.10 Global Significance of Carbon Sequestration
in Seagrass Meadow Sediments
An obvious condition for a marine ecosystem to sequester carbon long term is that
the sediment mass accumulation rate in that ecosystem is greater than zero. The rate
of long-term carbon sequestration (μmol C m
−2
year
−1
) can then be defined as the
product of the sediment mass accumulation rate (g dry weight m
−2
year
−1
), the OC
concentration of surface sediment (μmol C (g dry weight)
−1
), and the OC burial
efficiency (dimensionless). However, estimation of the OC burial efficiency in
seagrass meadows is often problematic, because the OC concentration in anoxic
sediment does not necessarily decrease with increasing depth and may show a
subsurface maximum due to belowground seagrass production (see Sect. 2.4.1). For
this reason, the OC sequestration rate in seagrass meadows is usually estimated as
the product of the average OC concentration in the top 20–100 cm sediment core
and the mass accumulation rate. The mass accumulation rate is the product of the
sedimentation rate (mm year
−1
) and the dry bulk density of the sediment core (g dry
weight cm
−3
) times 1000.
Two methods have been conventionally used to determine the sedimentation rate
from intact sediment cores: radioactive carbon (
14
C) dating by accelerator mass
spectrometry and
210
Pb/
137
Cs dating by γ-spectroscopy. However, caution is required
to interpret the dating results obtained from each of these methods. Although the
14
C
method is, in principle, more straightforward for use in carbon sequestration studies,
it requires core samples covering a much longer time span (hundreds to thousands
of years) than does the
210
Pb/
137
Cs method (<100 years). In addition,
14
C dating is
affected by spatiotemporal variability in the influence of aged OC, such as old
terrestrial OC and deep-sea dissolved inorganic carbon (referred to as the reservoir
effect). On the other hand, the
210
Pb/
137
Cs method often provides higher sedimentation
rates than those of the
14
C method, presumably due to sediment compaction and
physical and biological vertical mixing of the top layers of sediment causing
overestimation of sedimentation rates in the
210
Pb/
137
Cs dating. Many available data
2 Carbon Sequestration in Sediment as an Ecosystem Function of Seagrass Meadows
of determination between the logarithm of the eDNA copy number and seagrassderived OC (excluding the sample from the deepest layer that was apparently subject to eDNA contamination) is as high as 0.89. This example shows that eDNA is a
promising tool for quantitative evaluation of sediment OC derived from various
plant species. However, the quantitative relationship between eDNA and OC from
seagrasses appears to depend on the age of the samples and also on the environmental
settings of the meadows, which poses a challenge to the broader applicability of this
technique. This discrepancy may be partially related to differences in the specificity
to seagrasses of eDNA- and δ
13
C-based techniques. At present, more comprehensive
studies are required to understand which factors control the preservation of molecular biomarkers and eDNA in the diverse sediments of coastal vegetated
ecosystems.
2.10 Global Significance of Carbon Sequestration
in Seagrass Meadow Sediments
An obvious condition for a marine ecosystem to sequester carbon long term is that
the sediment mass accumulation rate in that ecosystem is greater than zero. The rate
of long-term carbon sequestration (μmol C m
−2
year
−1
) can then be defined as the
product of the sediment mass accumulation rate (g dry weight m
−2
year
−1
), the OC
concentration of surface sediment (μmol C (g dry weight)
−1
), and the OC burial
efficiency (dimensionless). However, estimation of the OC burial efficiency in
seagrass meadows is often problematic, because the OC concentration in anoxic
sediment does not necessarily decrease with increasing depth and may show a
subsurface maximum due to belowground seagrass production (see Sect. 2.4.1). For
this reason, the OC sequestration rate in seagrass meadows is usually estimated as
the product of the average OC concentration in the top 20–100 cm sediment core
and the mass accumulation rate. The mass accumulation rate is the product of the
sedimentation rate (mm year
−1
) and the dry bulk density of the sediment core (g dry
weight cm
−3
) times 1000.
Two methods have been conventionally used to determine the sedimentation rate
from intact sediment cores: radioactive carbon (
14
C) dating by accelerator mass
spectrometry and
210
Pb/
137
Cs dating by γ-spectroscopy. However, caution is required
to interpret the dating results obtained from each of these methods. Although the
14
C
method is, in principle, more straightforward for use in carbon sequestration studies,
it requires core samples covering a much longer time span (hundreds to thousands
of years) than does the
210
Pb/
137
Cs method (<100 years). In addition,
14
C dating is
affected by spatiotemporal variability in the influence of aged OC, such as old
terrestrial OC and deep-sea dissolved inorganic carbon (referred to as the reservoir
effect). On the other hand, the
210
Pb/
137
Cs method often provides higher sedimentation
rates than those of the
14
C method, presumably due to sediment compaction and
physical and biological vertical mixing of the top layers of sediment causing
overestimation of sedimentation rates in the
210
Pb/
137
Cs dating. Many available data
2 Carbon Sequestration in Sediment as an Ecosystem Function of Seagrass Meadows