107
Environment, Japan. The last and most detailed survey before 2000 was conducted
in the early 1990s (the 4th National Survey). No similar nationwide survey has been
conducted until recently, and the area surveyed in the 4th National Survey
(201,212 ha) has been used as the total area of seagrass and macroalgal bed in
Japan, often even now. Given that more than 20 years has passed since the survey
was conducted, an update is needed to determine the current nationwide status of
seagrass and macroalgal beds.
Against this background, a survey project was conducted by the Fisheries Agency
of Japan (2009–2013) that attempted to evaluate the nationwide area of seagrass and
macroalgal beds in Japan based on an analysis of satellite images (Hori et al. 2014).
The area of each type of seagrass and macroalgal beds was estimated from an analysis of images taken by the Earth Observation Satellite ALOS of the Japan Aerospace
Exploration Agency starting in 2000 (Hori et al. 2014). Here, we use the area
obtained by this project to estimate the carbon sequestration by seagrass and
macroalgae.
In the survey project in 2009–2013, estimated areas were 61,667 ha of eelgrass
bed, 88,041 ha of Sargassum bed, 63,120 ha of warm-temperate kelp bed, and
20,356 ha of cold-temperate kelp bed. These values are similar to the areas reported
in the previous survey in the 4th National Survey (49,464 ha, 85,682 ha, 64,483 ha,
and 35,724 ha, respectively; Environment Agency, Marine Parks Center of Japan
1994), although larger differences were observed in eelgrass and cold-temperate
kelp beds. However, there were differences in methodology and processes used for
the estimations between the 4th National Survey and the latest survey by the
Fisheries Agency, so it is not practical to simply compare the area values obtained
in the two surveys. In the 4th National Survey, the area was estimated only for seagrass and macroalgal beds with an area of at least 1 ha and a depth of less than 20 m,
whereas higher resolution data from satellite images were obtained in the survey led
by the Fisheries Agency.
Figure 4.3 shows the coastal area of Japan divided into seven regions according
to the characteristics of the water temperature environment, which is mainly influenced by warm and cold ocean currents. It also shows the area of each type of seagrass and macroalgal bed in each region, which were obtained in the latest survey
by the Fisheries Agency. As can be readily seen, each type of seagrass and macroalgal bed exhibits a very characteristic geographic distribution pattern along the
Japanese coastline.
More eelgrass beds are distributed in the coastal areas of Hokkaido and the Seto
Inland Sea than in other regions (Fig. 4.3). Generally, seagrasses including Z.
marina grow rhizomes in sediment composed of sand and mud. Therefore, it is
difficult for them to form and maintain beds in environments where the bottom sediment is disturbed by strong waves and currents. In Hokkaido, where relatively natural coastlines remain highly intact, many lagoons are sheltered from the waves of
the open sea and suitable for eelgrass bed development. For example, there are vast
eelgrass beds in Lake Furen and Lake Akkeshi, which are large lagoons in eastern
Hokkaido. The Seto Inland Sea is the largest semi-enclosed sea area in Japan and
with many islands, so there are many places that escape the influence of intense
4 Carbon Sequestration by Seagrass and Macroalgae in Japan: Estimates and Future…
Environment, Japan. The last and most detailed survey before 2000 was conducted
in the early 1990s (the 4th National Survey). No similar nationwide survey has been
conducted until recently, and the area surveyed in the 4th National Survey
(201,212 ha) has been used as the total area of seagrass and macroalgal bed in
Japan, often even now. Given that more than 20 years has passed since the survey
was conducted, an update is needed to determine the current nationwide status of
seagrass and macroalgal beds.
Against this background, a survey project was conducted by the Fisheries Agency
of Japan (2009–2013) that attempted to evaluate the nationwide area of seagrass and
macroalgal beds in Japan based on an analysis of satellite images (Hori et al. 2014).
The area of each type of seagrass and macroalgal beds was estimated from an analysis of images taken by the Earth Observation Satellite ALOS of the Japan Aerospace
Exploration Agency starting in 2000 (Hori et al. 2014). Here, we use the area
obtained by this project to estimate the carbon sequestration by seagrass and
macroalgae.
In the survey project in 2009–2013, estimated areas were 61,667 ha of eelgrass
bed, 88,041 ha of Sargassum bed, 63,120 ha of warm-temperate kelp bed, and
20,356 ha of cold-temperate kelp bed. These values are similar to the areas reported
in the previous survey in the 4th National Survey (49,464 ha, 85,682 ha, 64,483 ha,
and 35,724 ha, respectively; Environment Agency, Marine Parks Center of Japan
1994), although larger differences were observed in eelgrass and cold-temperate
kelp beds. However, there were differences in methodology and processes used for
the estimations between the 4th National Survey and the latest survey by the
Fisheries Agency, so it is not practical to simply compare the area values obtained
in the two surveys. In the 4th National Survey, the area was estimated only for seagrass and macroalgal beds with an area of at least 1 ha and a depth of less than 20 m,
whereas higher resolution data from satellite images were obtained in the survey led
by the Fisheries Agency.
Figure 4.3 shows the coastal area of Japan divided into seven regions according
to the characteristics of the water temperature environment, which is mainly influenced by warm and cold ocean currents. It also shows the area of each type of seagrass and macroalgal bed in each region, which were obtained in the latest survey
by the Fisheries Agency. As can be readily seen, each type of seagrass and macroalgal bed exhibits a very characteristic geographic distribution pattern along the
Japanese coastline.
More eelgrass beds are distributed in the coastal areas of Hokkaido and the Seto
Inland Sea than in other regions (Fig. 4.3). Generally, seagrasses including Z.
marina grow rhizomes in sediment composed of sand and mud. Therefore, it is
difficult for them to form and maintain beds in environments where the bottom sediment is disturbed by strong waves and currents. In Hokkaido, where relatively natural coastlines remain highly intact, many lagoons are sheltered from the waves of
the open sea and suitable for eelgrass bed development. For example, there are vast
eelgrass beds in Lake Furen and Lake Akkeshi, which are large lagoons in eastern
Hokkaido. The Seto Inland Sea is the largest semi-enclosed sea area in Japan and
with many islands, so there are many places that escape the influence of intense
4 Carbon Sequestration by Seagrass and Macroalgae in Japan: Estimates and Future…