37
seawater and are able to acquire carbon for photosynthesis from dissolved CO 2 and
bicarbonate (HCO 3
−
) in seawater (Beer et al. 2014). This characteristic of seagrasses
gives seagrass meadows a unique role in carbonate chemistry: mitigation of ocean
acidification by lowering the ambient partial pressure of CO 2 (pCO 2 ; Manzello et al.
2012; Unsworth et al. 2012).
Seagrasses typically occur on well-illuminated sandy or muddy bottoms where
wave energy is relatively low and the sedimentary environment stable. Excessive
sedimentation or erosion prohibits development of meadows (Cabaço et al. 2008).
Although some seagrass species occur as deep as 90 m (Duarte 1991), the seagrass
meadows that can contribute significantly to carbon capture and sequestration are
mostly confined to depths of 20 m or less, even under very transparent waters (e.g.,
Tanaka and Nakaoka 2007). Sediment materials that support seagrass meadows are
categorized into two groups: clastic and biogenic sediments. Clastic sediment materials are silicate and aluminosilicate minerals originally generated on land through
weathering of rocks and transported to coastal areas by rivers. Clastic sediment can
be classified according to particle size as gravels (>2 mm), sand (0.063–2 mm), silt
(0.004–0.063 mm), or clay (<0.004 mm). The particle size distribution of clastic
sediment is determined principally by geographic and oceanographic factors, such
as the distance from river mouths, water depth, and the hydrodynamic regime.
Seagrass meadows usually occur on sediments in which fine sand and/or silt fractions are dominant (Koch 2001). On the other hand, biogenic sediment includes
diatomaceous deposits made up of dead planktonic diatom frustules and biogenic
carbonate sands originally produced by carbonate-bearing organisms such as reef
corals, foraminifera, and calcareous algae. In tropical coastal regions, seagrass
meadows are often found on shallow carbonate sand in the back- reef lagoon of coral
reefs. As discussed below, the ecosystem function of seagrass meadows in terms of
the carbon cycle differs significantly between clastic and carbonate sediments.
Moreover, the mode and efficiency of OC sequestration in sediment are strongly
constrained by the particle size distribution of the sediment.
It should be emphasized that seagrass meadows and sediments interact closely
with each other, as the occurrence and dynamics of seagrass meadows are constrained by sedimentological conditions, while seagrass meadows also influence the
accretion and stability of sediment. Several factors are responsible for this function
of seagrass meadows. Firstly, seagrass meadows reduce water currents via friction
between leaf blades and seawater (Fonseca et al. 1982), accelerating the deposition
of fine-grained particles that would be flushed away in the absence of vegetation
(Fonseca and Fisher 1986; Gacia et al. 1999). At the same time, sediment resuspension is suppressed, and accumulation of organic detritus is facilitated (Ward et al.
1984). As a result of these effects, the sediment of dense and extensive seagrass
meadows is generally richer in fine particles such as silt and clay compared with that
of bare areas or sparse seagrass patches adjacent to meadows (Tanaka and Kayanne
2007). As discussed below, fine clastic sediment particles have a greater capacity to
hold OC per unit weight than do coarser particles. Thus, the capacity of seagrass
meadows to accumulate fine sediment is essential to their carbon sequestration role.
2 Carbon Sequestration in Sediment as an Ecosystem Function of Seagrass Meadows
seawater and are able to acquire carbon for photosynthesis from dissolved CO 2 and
bicarbonate (HCO 3
−
) in seawater (Beer et al. 2014). This characteristic of seagrasses
gives seagrass meadows a unique role in carbonate chemistry: mitigation of ocean
acidification by lowering the ambient partial pressure of CO 2 (pCO 2 ; Manzello et al.
2012; Unsworth et al. 2012).
Seagrasses typically occur on well-illuminated sandy or muddy bottoms where
wave energy is relatively low and the sedimentary environment stable. Excessive
sedimentation or erosion prohibits development of meadows (Cabaço et al. 2008).
Although some seagrass species occur as deep as 90 m (Duarte 1991), the seagrass
meadows that can contribute significantly to carbon capture and sequestration are
mostly confined to depths of 20 m or less, even under very transparent waters (e.g.,
Tanaka and Nakaoka 2007). Sediment materials that support seagrass meadows are
categorized into two groups: clastic and biogenic sediments. Clastic sediment materials are silicate and aluminosilicate minerals originally generated on land through
weathering of rocks and transported to coastal areas by rivers. Clastic sediment can
be classified according to particle size as gravels (>2 mm), sand (0.063–2 mm), silt
(0.004–0.063 mm), or clay (<0.004 mm). The particle size distribution of clastic
sediment is determined principally by geographic and oceanographic factors, such
as the distance from river mouths, water depth, and the hydrodynamic regime.
Seagrass meadows usually occur on sediments in which fine sand and/or silt fractions are dominant (Koch 2001). On the other hand, biogenic sediment includes
diatomaceous deposits made up of dead planktonic diatom frustules and biogenic
carbonate sands originally produced by carbonate-bearing organisms such as reef
corals, foraminifera, and calcareous algae. In tropical coastal regions, seagrass
meadows are often found on shallow carbonate sand in the back- reef lagoon of coral
reefs. As discussed below, the ecosystem function of seagrass meadows in terms of
the carbon cycle differs significantly between clastic and carbonate sediments.
Moreover, the mode and efficiency of OC sequestration in sediment are strongly
constrained by the particle size distribution of the sediment.
It should be emphasized that seagrass meadows and sediments interact closely
with each other, as the occurrence and dynamics of seagrass meadows are constrained by sedimentological conditions, while seagrass meadows also influence the
accretion and stability of sediment. Several factors are responsible for this function
of seagrass meadows. Firstly, seagrass meadows reduce water currents via friction
between leaf blades and seawater (Fonseca et al. 1982), accelerating the deposition
of fine-grained particles that would be flushed away in the absence of vegetation
(Fonseca and Fisher 1986; Gacia et al. 1999). At the same time, sediment resuspension is suppressed, and accumulation of organic detritus is facilitated (Ward et al.
1984). As a result of these effects, the sediment of dense and extensive seagrass
meadows is generally richer in fine particles such as silt and clay compared with that
of bare areas or sparse seagrass patches adjacent to meadows (Tanaka and Kayanne
2007). As discussed below, fine clastic sediment particles have a greater capacity to
hold OC per unit weight than do coarser particles. Thus, the capacity of seagrass
meadows to accumulate fine sediment is essential to their carbon sequestration role.
2 Carbon Sequestration in Sediment as an Ecosystem Function of Seagrass Meadows