53
increase the availability of sediment mineral surfaces, which is expected to be
higher in larger patches, is a principal constraint on OC storage in these sediments.
The importance of the context of landscape in sediment OC storage for tropical and
subtropical seagrass habitats was also recently suggested (Gullström et al. 2017).
However, the OC concentration in some estuarine seagrass meadow sediments
clearly exceeded the value predicted by the typical OC–SSA relationship (Fig. 2.5a),
indicating inclusion of OC that is not adsorbed to a mineral surface. In fact, a
significant fraction of OC was found in density fractions <2.0 g cm
−3
in estuarine
seagrass meadow samples (e.g., Fig. 2.4b), and the δ
13
C indicated that the OC in
these lighter fractions originated mainly from terrestrial C 3 plants. This result
implies that terrestrial refractory OC transported by rivers can be effectively trapped
and stored in estuarine seagrass meadows in excess of the OC concentration
expected from the normal OC–SSA relationship. Sediments from seagrass-free
estuarine tidal flats and sandy macroalgal beds in the Seto Inland Sea were generally
depleted of OC and showed lower OC loading compared with seagrass meadow
sediments. Sediments from bare areas adjacent to seagrass meadows usually showed
similar OC loading but lower OC concentrations compared with the nearest seagrass
meadow sediments. OC in these seagrass-free sediments was divided almost equally
between the mineral-associated (>2.0 g cm
−3
) and mineral-free (<2.0 g cm
−3
)
density fractions (e.g., Fig. 2.4c).
The implications of these results can be summarized as follows: (i) Z. marina
meadows in the Seto Inland Sea have the capacity to increase the OC loading of
sediments from depleted values to near saturation; (ii) seagrass meadow sediments
vary widely in OC concentration but often have similar saturated OC loading values;
(iii) the rate of OC accumulation in seagrass meadow sediments is constrained
mainly by the supply of mineral surfaces available for sorptive preservation of OC,
rather than by the supply of OC itself, unless refractory OC is supplied from
allochthonous (terrestrial) sources; and (iv) the capacity of seagrass meadows to
accumulate available mineral surfaces and therefore to store OC in sediments,
depends on spatial factors such as the patch size of individual meadows, which is
presumably related to enhanced sediment stability. These characteristics are
generally consistent with the hypothesis proposed in Sect. 2.6.
2.8 Factors That Influence the Carbon Storage Capacity
of Seagrass Meadows
The conclusions of the previous section regarding Z. marina meadows in temperate
Japan are thought to be generally applicable to perennial seagrass meadows growing
on clastic sediments. However, the sources of OC supplied to seagrass meadows are
highly variable depending on the oceanographic setting, and the source-specific
characteristics of OC can influence its long-term stability when stored in sediment.
Various types of OC supplied to seagrass meadows are conceptually categorized
into allochthonous and autochthonous OC, and allochthonous OC can be further
divided into two groups based on an association with minerals (Table 2.5).
2 Carbon Sequestration in Sediment as an Ecosystem Function of Seagrass Meadows
increase the availability of sediment mineral surfaces, which is expected to be
higher in larger patches, is a principal constraint on OC storage in these sediments.
The importance of the context of landscape in sediment OC storage for tropical and
subtropical seagrass habitats was also recently suggested (Gullström et al. 2017).
However, the OC concentration in some estuarine seagrass meadow sediments
clearly exceeded the value predicted by the typical OC–SSA relationship (Fig. 2.5a),
indicating inclusion of OC that is not adsorbed to a mineral surface. In fact, a
significant fraction of OC was found in density fractions <2.0 g cm
−3
in estuarine
seagrass meadow samples (e.g., Fig. 2.4b), and the δ
13
C indicated that the OC in
these lighter fractions originated mainly from terrestrial C 3 plants. This result
implies that terrestrial refractory OC transported by rivers can be effectively trapped
and stored in estuarine seagrass meadows in excess of the OC concentration
expected from the normal OC–SSA relationship. Sediments from seagrass-free
estuarine tidal flats and sandy macroalgal beds in the Seto Inland Sea were generally
depleted of OC and showed lower OC loading compared with seagrass meadow
sediments. Sediments from bare areas adjacent to seagrass meadows usually showed
similar OC loading but lower OC concentrations compared with the nearest seagrass
meadow sediments. OC in these seagrass-free sediments was divided almost equally
between the mineral-associated (>2.0 g cm
−3
) and mineral-free (<2.0 g cm
−3
)
density fractions (e.g., Fig. 2.4c).
The implications of these results can be summarized as follows: (i) Z. marina
meadows in the Seto Inland Sea have the capacity to increase the OC loading of
sediments from depleted values to near saturation; (ii) seagrass meadow sediments
vary widely in OC concentration but often have similar saturated OC loading values;
(iii) the rate of OC accumulation in seagrass meadow sediments is constrained
mainly by the supply of mineral surfaces available for sorptive preservation of OC,
rather than by the supply of OC itself, unless refractory OC is supplied from
allochthonous (terrestrial) sources; and (iv) the capacity of seagrass meadows to
accumulate available mineral surfaces and therefore to store OC in sediments,
depends on spatial factors such as the patch size of individual meadows, which is
presumably related to enhanced sediment stability. These characteristics are
generally consistent with the hypothesis proposed in Sect. 2.6.
2.8 Factors That Influence the Carbon Storage Capacity
of Seagrass Meadows
The conclusions of the previous section regarding Z. marina meadows in temperate
Japan are thought to be generally applicable to perennial seagrass meadows growing
on clastic sediments. However, the sources of OC supplied to seagrass meadows are
highly variable depending on the oceanographic setting, and the source-specific
characteristics of OC can influence its long-term stability when stored in sediment.
Various types of OC supplied to seagrass meadows are conceptually categorized
into allochthonous and autochthonous OC, and allochthonous OC can be further
divided into two groups based on an association with minerals (Table 2.5).
2 Carbon Sequestration in Sediment as an Ecosystem Function of Seagrass Meadows