52
seagrass bed sediments varied widely, from 0.1% to 11% of dry weight depending
on the sampling site, and did not correlate with seagrass biomass. The average OC
concentration in seagrass bed sediments was 1.8%, although the frequency
distribution of the observations was clearly skewed, with most data confined in a
range of 0–2.0%. For this reason, the authors recommend using the median OC
value of 1.2% rather than the average as the representative OC concentration for
seagrass bed sediments worldwide. Similar results based on a more comprehensive
dataset (219 core sites) were published by Fourqurean et al. (2012), who found
average and median OC concentrations in seagrass bed sediments of 2.0 and 1.4%
of dry weight, respectively. They also estimated the OC stock in the upper 1 m of
sediment, which was in the range 9–830 Mg C ha
−1
with a median value of 140 Mg
C ha
−1
. According to their online supplementary dataset, the OC concentration in
surface sediments does not correlate with the aboveground biomass of seagrasses,
even when the correlation is limited to seagrass meadows with similar species
compositions. Alongi et al. (2016) also found that the average OC concentration in
82 sediment samples from Indonesian tropical seagrass meadows was 1.3% of dry
weight (ranging from 0.2% to 3.0%) and that, on average, the detrital OC stock in
the upper 1 m of sediment was equivalent to ~400 times the average OC stock in
aboveground seagrass biomass and ~100 times that in belowground biomass.
However, according to their data, there was no correlation between aboveground
seagrass biomass and the OC stock in the sediment.
These findings suggest that despite the fact that the presence of seagrass meadows actually enhances sediment OC storage at a local scale, the degree of enhancement depends not only on biological factors such as biomass and species composition,
but on some non-biological factor or factors that significantly influence sediment
OC accumulation at regional and global scales. It follows that estimating the
aboveground biomass or coverage of seagrasses using remote-sensing techniques,
which has been undertaken in many ongoing survey projects to constrain “blue
carbon” stocks worldwide, is not necessarily an appropriate way to quantify total
blue carbon in seagrass meadows.
In the case of Z. marina meadows in the Seto Inland Sea (central Japan), the OC
concentration in the top 30 cm of sediment varies widely from 0.1% to 1.6% of dry
weight, with a strong positive correlation with the SSA of mineral sediments
(Miyajima et al. 2017). This OC concentration range is consistent with those of the
previous reports noted above. The average OC loading (OC/SSA ratio) was
61.5 ± 5.5 μmol C m
−2
(Fig. 2.5a), in accordance with known saturated OC loading
values for continental shelf sediments (63.3 ± 2.5 μmol C m
−2
; Keil et al. 1994a).
Most of the OC in these sediments was found in >2.0 g cm
−3
density fractions
(Fig. 2.4a as an example), indicating a close association between OC and sediment
minerals. The carbon isotope ratio (δ
13
C) implied that approximately half of the OC
in these heavy fractions was derived from seagrasses. Furthermore, the OC
concentration and SSA were correlated with the patch size of the seagrass meadows,
with larger patches having larger mineral surface areas and storing more OC per
unit weight of sediment (Miyajima et al. 2017). These findings suggest that the
capacity of seagrass meadows to capture fine-grained sediment and thereby to
T. Miyajima and M. Hamaguchi
seagrass bed sediments varied widely, from 0.1% to 11% of dry weight depending
on the sampling site, and did not correlate with seagrass biomass. The average OC
concentration in seagrass bed sediments was 1.8%, although the frequency
distribution of the observations was clearly skewed, with most data confined in a
range of 0–2.0%. For this reason, the authors recommend using the median OC
value of 1.2% rather than the average as the representative OC concentration for
seagrass bed sediments worldwide. Similar results based on a more comprehensive
dataset (219 core sites) were published by Fourqurean et al. (2012), who found
average and median OC concentrations in seagrass bed sediments of 2.0 and 1.4%
of dry weight, respectively. They also estimated the OC stock in the upper 1 m of
sediment, which was in the range 9–830 Mg C ha
−1
with a median value of 140 Mg
C ha
−1
. According to their online supplementary dataset, the OC concentration in
surface sediments does not correlate with the aboveground biomass of seagrasses,
even when the correlation is limited to seagrass meadows with similar species
compositions. Alongi et al. (2016) also found that the average OC concentration in
82 sediment samples from Indonesian tropical seagrass meadows was 1.3% of dry
weight (ranging from 0.2% to 3.0%) and that, on average, the detrital OC stock in
the upper 1 m of sediment was equivalent to ~400 times the average OC stock in
aboveground seagrass biomass and ~100 times that in belowground biomass.
However, according to their data, there was no correlation between aboveground
seagrass biomass and the OC stock in the sediment.
These findings suggest that despite the fact that the presence of seagrass meadows actually enhances sediment OC storage at a local scale, the degree of enhancement depends not only on biological factors such as biomass and species composition,
but on some non-biological factor or factors that significantly influence sediment
OC accumulation at regional and global scales. It follows that estimating the
aboveground biomass or coverage of seagrasses using remote-sensing techniques,
which has been undertaken in many ongoing survey projects to constrain “blue
carbon” stocks worldwide, is not necessarily an appropriate way to quantify total
blue carbon in seagrass meadows.
In the case of Z. marina meadows in the Seto Inland Sea (central Japan), the OC
concentration in the top 30 cm of sediment varies widely from 0.1% to 1.6% of dry
weight, with a strong positive correlation with the SSA of mineral sediments
(Miyajima et al. 2017). This OC concentration range is consistent with those of the
previous reports noted above. The average OC loading (OC/SSA ratio) was
61.5 ± 5.5 μmol C m
−2
(Fig. 2.5a), in accordance with known saturated OC loading
values for continental shelf sediments (63.3 ± 2.5 μmol C m
−2
; Keil et al. 1994a).
Most of the OC in these sediments was found in >2.0 g cm
−3
density fractions
(Fig. 2.4a as an example), indicating a close association between OC and sediment
minerals. The carbon isotope ratio (δ
13
C) implied that approximately half of the OC
in these heavy fractions was derived from seagrasses. Furthermore, the OC
concentration and SSA were correlated with the patch size of the seagrass meadows,
with larger patches having larger mineral surface areas and storing more OC per
unit weight of sediment (Miyajima et al. 2017). These findings suggest that the
capacity of seagrass meadows to capture fine-grained sediment and thereby to
T. Miyajima and M. Hamaguchi