111
of a shoot with a needle, and the shoot is then collected after a certain period of
time. The pinhole-free leaves inside the recovered shoots are new leaves that formed
during this period. Because the outermost leaf of the shoot has almost stopped
growing when the pinhole was made, the pinhole on this outer leaf is considered to
have not moved. Using this position as a starting point, the new formation of leaf
tissue of each leaf is estimated from the movement of the pinhole (Fig. 4.4). The
sum of the weight of newly formed tissues on each leaf with pinholes and new
leaves is considered to be the production by the shoot within the period. With this
method, however, production may be underestimated by missing the maturation of
leaf tissue above the pinholes. Therefore, Short and Duarte (2001) recommended an
alternative method to evaluate production based on the formation rate of new leaves.
In this method, a period of observation (days) is divided by the number of newly
formed leaves within the period to obtain the plastochrone interval (days) (Fig. 4.4).
The plastochrone interval is therefore an interval between consecutive new leaf formations, and the reciprocal of this value shows the leaf formation rate, that is, the
number of newly formed leaves in a certain period (generally, per day). By dividing
the weight of an individual fully-grown leaf (usually using the third to fourth leaf
from the inside, which is the longest in the shoot) by the plastochrone interval, the
daily leaf production of the shoot can be approximated.
In the case of Z. marina, the nodes of the rhizomes are formed at the growth point
at the same rate as the leaves, so the production of the underground part can be estimated in the same way. By dividing the weight of a fully-grown internode with its
root by the plastochrone interval, the daily underground production of the shoot can
be estimated. The sum of the production of the aboveground part (leaves) and underground part (rhizome and roots) is the total production of the shoot. Furthermore, by
multiplying the mean density of shoots, the production amount per unit area can be
calculated.
4.3.1.2 Production of Eelgrass Along the Coast of Japan
Zostera marina exhibits seasonal changes in its growth. In Japan, vigorous growth
and active vegetative propagation generally occur after spring, and shoot size, shoot
density, and biomass reach their annual peak in late spring to summer. After summer, shoot density decreases, and from late autumn to early winter, Z. marina exhibits senescence, with the shortest annual shoot size and smallest biomass.
Shoot density in the peak growing season of eelgrass beds in Japan has been
reported to be from less than 100 shoots/m
2
to 1500 shoots/m
2
, and the biomass of
the aboveground part from about 50 g dry weight/m
2
to about 700 g dry weight/m
2
(Hasegawa et al. 2013; Fig. 4.5). The modes for the frequency distributions of these
values were found to be about 200 shoots/m
2
and 100–200 g dry weight/m
2
, respectively, but large variations exist among beds. Also, no clear relationship has been
found between shoot density and aboveground biomass (Hasegawa et al. 2013). The
lack of a relationship between density and biomass seems to be due to the fact that
the morphological variation of Z. marina is extremely large among the beds; for
4 Carbon Sequestration by Seagrass and Macroalgae in Japan: Estimates and Future…
of a shoot with a needle, and the shoot is then collected after a certain period of
time. The pinhole-free leaves inside the recovered shoots are new leaves that formed
during this period. Because the outermost leaf of the shoot has almost stopped
growing when the pinhole was made, the pinhole on this outer leaf is considered to
have not moved. Using this position as a starting point, the new formation of leaf
tissue of each leaf is estimated from the movement of the pinhole (Fig. 4.4). The
sum of the weight of newly formed tissues on each leaf with pinholes and new
leaves is considered to be the production by the shoot within the period. With this
method, however, production may be underestimated by missing the maturation of
leaf tissue above the pinholes. Therefore, Short and Duarte (2001) recommended an
alternative method to evaluate production based on the formation rate of new leaves.
In this method, a period of observation (days) is divided by the number of newly
formed leaves within the period to obtain the plastochrone interval (days) (Fig. 4.4).
The plastochrone interval is therefore an interval between consecutive new leaf formations, and the reciprocal of this value shows the leaf formation rate, that is, the
number of newly formed leaves in a certain period (generally, per day). By dividing
the weight of an individual fully-grown leaf (usually using the third to fourth leaf
from the inside, which is the longest in the shoot) by the plastochrone interval, the
daily leaf production of the shoot can be approximated.
In the case of Z. marina, the nodes of the rhizomes are formed at the growth point
at the same rate as the leaves, so the production of the underground part can be estimated in the same way. By dividing the weight of a fully-grown internode with its
root by the plastochrone interval, the daily underground production of the shoot can
be estimated. The sum of the production of the aboveground part (leaves) and underground part (rhizome and roots) is the total production of the shoot. Furthermore, by
multiplying the mean density of shoots, the production amount per unit area can be
calculated.
4.3.1.2 Production of Eelgrass Along the Coast of Japan
Zostera marina exhibits seasonal changes in its growth. In Japan, vigorous growth
and active vegetative propagation generally occur after spring, and shoot size, shoot
density, and biomass reach their annual peak in late spring to summer. After summer, shoot density decreases, and from late autumn to early winter, Z. marina exhibits senescence, with the shortest annual shoot size and smallest biomass.
Shoot density in the peak growing season of eelgrass beds in Japan has been
reported to be from less than 100 shoots/m
2
to 1500 shoots/m
2
, and the biomass of
the aboveground part from about 50 g dry weight/m
2
to about 700 g dry weight/m
2
(Hasegawa et al. 2013; Fig. 4.5). The modes for the frequency distributions of these
values were found to be about 200 shoots/m
2
and 100–200 g dry weight/m
2
, respectively, but large variations exist among beds. Also, no clear relationship has been
found between shoot density and aboveground biomass (Hasegawa et al. 2013). The
lack of a relationship between density and biomass seems to be due to the fact that
the morphological variation of Z. marina is extremely large among the beds; for
4 Carbon Sequestration by Seagrass and Macroalgae in Japan: Estimates and Future…