256
were lost first and then the fragmentation rapidly progressed. Although the contribution of grazers to the fragmentation process was expected to be very large, few grazers were present in the 1-mm mesh bag in this experiment. In addition, fragmentation
progressed even in the 30-μm mesh bag, which meiofauna cannot invade. Therefore,
these findings suggest that, after a certain period, fragmented leaves can decompose
rapidly without the intervention of grazers.
9.2.3 Decomposition of Leaves
Organic matter is decomposed and mineralized by microorganisms, but the rates
differ depending on the organic matter components. Starch, saccharides, and proteins are decomposed rapidly, cellulose is somewhat slower, and lignin is slow
(Klap et al. 2000; Hemminga and Duarte 2000). With regard to decomposition rates
of organic matter, three components are often considered: labile organic matter with
rapid decomposition rates, semi-labile organic matter with intermediate decomposition rates, and refractory organic matter that is difficult to decompose and is continuously present as organic matter over a prolonged period. Eelgrass is made up of
various components, and it contains a fixed amount of semi-labile organic matter
such as cellulose and hemicellulose constituting the cell wall and refractory organic
matter such as lignin that adheres and solidifies the cells (Opsahl and Benner 1993;
Klap et al. 2000).
Figure 9.5 presents the results of the leave decomposition experiments, showing
the change in the residual amount of the leaves. At the beginning of the experiment,
the decomposition rate was fast because the labile organic matter decomposed first.
About 20 days later, the decomposition rate slowed as the semi-labile organic matter decomposed. If the decomposition experiment was continued, only refractory
organic matter would eventually remain. We often refer to the components that are
not decomposed after more than a year as refractory organic matter (Opsahl and
Benner 1993). Organic carbon compounds in eelgrass accumulate in bottom sediments or in the deep sea while they are decomposing, whereas a portion are not
decomposed and are stored for a prolonged period. In this field experiment, the
decomposition rates of labile and semi-labile organic matter were 0.116 (day
−1
) and
0.004 (day
−1
), respectively. In addition, the percentage of labile, semi-labile, and
refractory organic matter of the leaves were 42%, 52%, and 6%, respectively
(Tarutani et al. 2012). These values were obtained while assuming the value of the
general lignin content (6%) was the proportion of refractory organic matter.
According to a review by Enriquez et al. (1993), the decomposition rate of eelgrass (Z. marina) was 0.0007–0.0357, and the average was 0.0114 (day
−1
). However,
these values were obtained without distinguishing between labile and semi-labile
organic matter. Based on the field experiment presented here, the decomposition
rate obtained without distinction between labile and semi-labile organic matter was
K. Abo et al.
were lost first and then the fragmentation rapidly progressed. Although the contribution of grazers to the fragmentation process was expected to be very large, few grazers were present in the 1-mm mesh bag in this experiment. In addition, fragmentation
progressed even in the 30-μm mesh bag, which meiofauna cannot invade. Therefore,
these findings suggest that, after a certain period, fragmented leaves can decompose
rapidly without the intervention of grazers.
9.2.3 Decomposition of Leaves
Organic matter is decomposed and mineralized by microorganisms, but the rates
differ depending on the organic matter components. Starch, saccharides, and proteins are decomposed rapidly, cellulose is somewhat slower, and lignin is slow
(Klap et al. 2000; Hemminga and Duarte 2000). With regard to decomposition rates
of organic matter, three components are often considered: labile organic matter with
rapid decomposition rates, semi-labile organic matter with intermediate decomposition rates, and refractory organic matter that is difficult to decompose and is continuously present as organic matter over a prolonged period. Eelgrass is made up of
various components, and it contains a fixed amount of semi-labile organic matter
such as cellulose and hemicellulose constituting the cell wall and refractory organic
matter such as lignin that adheres and solidifies the cells (Opsahl and Benner 1993;
Klap et al. 2000).
Figure 9.5 presents the results of the leave decomposition experiments, showing
the change in the residual amount of the leaves. At the beginning of the experiment,
the decomposition rate was fast because the labile organic matter decomposed first.
About 20 days later, the decomposition rate slowed as the semi-labile organic matter decomposed. If the decomposition experiment was continued, only refractory
organic matter would eventually remain. We often refer to the components that are
not decomposed after more than a year as refractory organic matter (Opsahl and
Benner 1993). Organic carbon compounds in eelgrass accumulate in bottom sediments or in the deep sea while they are decomposing, whereas a portion are not
decomposed and are stored for a prolonged period. In this field experiment, the
decomposition rates of labile and semi-labile organic matter were 0.116 (day
−1
) and
0.004 (day
−1
), respectively. In addition, the percentage of labile, semi-labile, and
refractory organic matter of the leaves were 42%, 52%, and 6%, respectively
(Tarutani et al. 2012). These values were obtained while assuming the value of the
general lignin content (6%) was the proportion of refractory organic matter.
According to a review by Enriquez et al. (1993), the decomposition rate of eelgrass (Z. marina) was 0.0007–0.0357, and the average was 0.0114 (day
−1
). However,
these values were obtained without distinguishing between labile and semi-labile
organic matter. Based on the field experiment presented here, the decomposition
rate obtained without distinction between labile and semi-labile organic matter was
K. Abo et al.