38
Secondly, seagrass meadows have a filtering effect through which suspended
organic particles conveyed by the water current are trapped in the bundle of leaf
blades and accumulate in the sediment (Hendriks et al. 2008). This factor greatly
increases the supply rate of allochthonous OC per area of sediment.
Thirdly, seagrasses extend horizontal rhizomes outward, forming a network of rhizomes and fine roots that stabilizes the sediment surface under the meadow (Fonseca
1989; Gacia and Duarte 2001). Sediment stabilization by the belowground structure is
one of the most notable characteristics shared by all coastal vegetated ecosystems,
although the strength of this effect differs largely among species. The most extreme
known example may be the dense fine-root structure of the peat- forming mangrove
Rhizophora spp. (Fujimoto et al. 1999; Ono et al. 2015). No seagrass species produces
such a dense root/rhizome system that genuine peat is generated in the underlying
sediment. However, a few species, such as Mediterranean Posidonia oceanica and
subtropical Thalassia spp., can accumulate and stabilize fine sediment so effectively
using their robust root systems that mound-like microtopography is often generated
under their dense meadows (Wanless 1981; Lo Iacono et al. 2008).
2.3 Global Pattern of Sediment Organic Carbon
Accumulation
Before proceeding to the specific characteristics and functions of seagrass meadow
sediments, we briefly review carbon sequestration in marine sediments from a
global perspective in the following sections.
Most of the carbon on the Earth’s surface exists in the geosphere as crustal carbonates and sedimentary OC, with only a small fraction (<1%) circulating in the
biosphere (Table 2.2). The surface “reactive layer” of marine sediment represents
the boundary or interface zone connecting the carbon reservoirs in the biosphere
(biogeochemical carbon cycle) to those in the geosphere (geological carbon cycle).
Conversely, carbon may be returned from the geosphere to the biosphere, mainly
through volcanic eruption, tectonism, and fossil fuel mining. Due to the increasing
use of fossil fuels, the flux of carbon from the geosphere to the biosphere, particularly to the atmosphere (>8 Pg year
−1
), overwhelms the return flux of permanent
sequestration in marine sediment at present (~0.2 Pg year
−1
; Ciais et al. 2013). It has
been estimated that the annual atmospheric loading of CO 2 due to fossil fuel burning
already exceeds the permanent burial of OC in marine sediment in the second half
of the nineteenth century (Boden et al. 2011). However, in the global estimates of
carbon budgets discussed above, OC sequestration as a function of coastal vegetated
ecosystems is often overlooked.
Globally, the distribution of OC in surface marine sediments is not homogeneous. In several narrow regions, such as upwelling areas and anoxic basins, the
concentration of OC in sediments is exceptionally high. Excluding these areas, the
OC concentration in surface sediment rarely exceeds 2% of dry weight (Premuzic
T. Miyajima and M. Hamaguchi
Secondly, seagrass meadows have a filtering effect through which suspended
organic particles conveyed by the water current are trapped in the bundle of leaf
blades and accumulate in the sediment (Hendriks et al. 2008). This factor greatly
increases the supply rate of allochthonous OC per area of sediment.
Thirdly, seagrasses extend horizontal rhizomes outward, forming a network of rhizomes and fine roots that stabilizes the sediment surface under the meadow (Fonseca
1989; Gacia and Duarte 2001). Sediment stabilization by the belowground structure is
one of the most notable characteristics shared by all coastal vegetated ecosystems,
although the strength of this effect differs largely among species. The most extreme
known example may be the dense fine-root structure of the peat- forming mangrove
Rhizophora spp. (Fujimoto et al. 1999; Ono et al. 2015). No seagrass species produces
such a dense root/rhizome system that genuine peat is generated in the underlying
sediment. However, a few species, such as Mediterranean Posidonia oceanica and
subtropical Thalassia spp., can accumulate and stabilize fine sediment so effectively
using their robust root systems that mound-like microtopography is often generated
under their dense meadows (Wanless 1981; Lo Iacono et al. 2008).
2.3 Global Pattern of Sediment Organic Carbon
Accumulation
Before proceeding to the specific characteristics and functions of seagrass meadow
sediments, we briefly review carbon sequestration in marine sediments from a
global perspective in the following sections.
Most of the carbon on the Earth’s surface exists in the geosphere as crustal carbonates and sedimentary OC, with only a small fraction (<1%) circulating in the
biosphere (Table 2.2). The surface “reactive layer” of marine sediment represents
the boundary or interface zone connecting the carbon reservoirs in the biosphere
(biogeochemical carbon cycle) to those in the geosphere (geological carbon cycle).
Conversely, carbon may be returned from the geosphere to the biosphere, mainly
through volcanic eruption, tectonism, and fossil fuel mining. Due to the increasing
use of fossil fuels, the flux of carbon from the geosphere to the biosphere, particularly to the atmosphere (>8 Pg year
−1
), overwhelms the return flux of permanent
sequestration in marine sediment at present (~0.2 Pg year
−1
; Ciais et al. 2013). It has
been estimated that the annual atmospheric loading of CO 2 due to fossil fuel burning
already exceeds the permanent burial of OC in marine sediment in the second half
of the nineteenth century (Boden et al. 2011). However, in the global estimates of
carbon budgets discussed above, OC sequestration as a function of coastal vegetated
ecosystems is often overlooked.
Globally, the distribution of OC in surface marine sediments is not homogeneous. In several narrow regions, such as upwelling areas and anoxic basins, the
concentration of OC in sediments is exceptionally high. Excluding these areas, the
OC concentration in surface sediment rarely exceeds 2% of dry weight (Premuzic
T. Miyajima and M. Hamaguchi