263
considering the water temperature, salinity, outflow–inflow of heat and substances
between the grids. To more accurately reproduce this system in detail, it is better for
the computational grid to be smaller, but in that case the computation time becomes
very long. In coastal areas, the size of the computational grid is often 1 km horizontally and 10–20 layers vertically.
The flow model used here to trace eelgrass-derived organic carbon is the Seto
Inland Sea Model developed by the National Research Institute of Fisheries
Engineering of Japan (Nakayama et al. 2009). The fundamental equation of the
model uses motion, continuity, state of density, and transport of heat and salts (for
details, see Nakayama et al. 2009). In this model, hydrodynamics in the sea area can
be calculated by considering the tidal flow, density flow, and wind-induced flow.
Using this model, the drifting leaves and suspended particles that flowed out of
eelgrass beds were traced.
9.5.1.2 Outflow, Sedimentation, and Decomposition
We took two approaches to represent the behavior of eelgrass (drifting leaves) that
flowed out of the eelgrass bed based on the Seto Inland Sea Model. One was to
evaluate its behavior in the material circulation model by assuming drifting leaves
are passive suspended organic matter (behavior of fine particles in water coincide
with the behavior of water molecules), and the other was to track the drifting leaves
as is (floating and settling while being reduced in size due to fragmentation and/or
decomposition). By combining these two approaches, Sugimatsu et al. (2015)
developed a drifting leave carbon circulation model that quantitatively expresses the
material circulation including the behavior of the drifting leaves and decomposition
process of the leaves after settling. Figure 9.11 shows the schematic view of the
model for estimating the fate of drifting leaves and suspended particles.
Drifting leaves that flow out of the eelgrass bed begin to settle after drifting near
the surface layer for a certain period; after settling on the seabed, they are fragmented into suspended organic matter. According to an experiment by Hamaguchi
et al. (2012), in which eelgrass leaves were floated in a 10-ton running water tank to
determine the number of days before settlement began, the drift period was 25.7 days
and the settling velocity was 21.1 mm s
−1
.
The leaves that settle on the sea bottom are fragmented and resuspended into the
water column as particulate organic matter, or they are decomposed and resuspended as dissolved inorganic matter. The fragmentation rate of the leaves and the
decomposition rate of the particulate organic matter can be obtained from the
decomposition experiment using litter bags described in Sect. 9.2.2. The leaves
comprise three components with different decomposition rates, with 42% being
labile, 52% semi-labile, and 6% refractory organic matter. Because the labile
organic matter has already been decomposed within the eelgrass bed, the suspended
organic matter that flows out is assumed to be composed only of the semi-labile and
refractory organic matter. Dissolved organic carbon (DOC) was not measured in the
decomposition experiment above, so the model did not directly deal with
9 Quantifying the Fate of Captured Carbon: From Seagrass Meadows to the Deep Sea
considering the water temperature, salinity, outflow–inflow of heat and substances
between the grids. To more accurately reproduce this system in detail, it is better for
the computational grid to be smaller, but in that case the computation time becomes
very long. In coastal areas, the size of the computational grid is often 1 km horizontally and 10–20 layers vertically.
The flow model used here to trace eelgrass-derived organic carbon is the Seto
Inland Sea Model developed by the National Research Institute of Fisheries
Engineering of Japan (Nakayama et al. 2009). The fundamental equation of the
model uses motion, continuity, state of density, and transport of heat and salts (for
details, see Nakayama et al. 2009). In this model, hydrodynamics in the sea area can
be calculated by considering the tidal flow, density flow, and wind-induced flow.
Using this model, the drifting leaves and suspended particles that flowed out of
eelgrass beds were traced.
9.5.1.2 Outflow, Sedimentation, and Decomposition
We took two approaches to represent the behavior of eelgrass (drifting leaves) that
flowed out of the eelgrass bed based on the Seto Inland Sea Model. One was to
evaluate its behavior in the material circulation model by assuming drifting leaves
are passive suspended organic matter (behavior of fine particles in water coincide
with the behavior of water molecules), and the other was to track the drifting leaves
as is (floating and settling while being reduced in size due to fragmentation and/or
decomposition). By combining these two approaches, Sugimatsu et al. (2015)
developed a drifting leave carbon circulation model that quantitatively expresses the
material circulation including the behavior of the drifting leaves and decomposition
process of the leaves after settling. Figure 9.11 shows the schematic view of the
model for estimating the fate of drifting leaves and suspended particles.
Drifting leaves that flow out of the eelgrass bed begin to settle after drifting near
the surface layer for a certain period; after settling on the seabed, they are fragmented into suspended organic matter. According to an experiment by Hamaguchi
et al. (2012), in which eelgrass leaves were floated in a 10-ton running water tank to
determine the number of days before settlement began, the drift period was 25.7 days
and the settling velocity was 21.1 mm s
−1
.
The leaves that settle on the sea bottom are fragmented and resuspended into the
water column as particulate organic matter, or they are decomposed and resuspended as dissolved inorganic matter. The fragmentation rate of the leaves and the
decomposition rate of the particulate organic matter can be obtained from the
decomposition experiment using litter bags described in Sect. 9.2.2. The leaves
comprise three components with different decomposition rates, with 42% being
labile, 52% semi-labile, and 6% refractory organic matter. Because the labile
organic matter has already been decomposed within the eelgrass bed, the suspended
organic matter that flows out is assumed to be composed only of the semi-labile and
refractory organic matter. Dissolved organic carbon (DOC) was not measured in the
decomposition experiment above, so the model did not directly deal with
9 Quantifying the Fate of Captured Carbon: From Seagrass Meadows to the Deep Sea