124
which is the sudden and unusual disappearance and decline of macroalgal beds, has
been expanding, mainly in the coastal areas of western Japan. A reported 18,538 ha
of macroalgal beds have disappeared (Akimoto and Matsumura 2010) since then.
Assuming that the disappearance caused by Iso-yake has occurred mainly in
Garamo-ba and Arame-ba, the carbon sequestration function by these macroalgal
beds, which corresponds to about 300,000–500,000 tons CO 2 /year, has already been
lost in coastal areas (Remember though, as noted previously in this chapter, the latest seagrass and macroalgal bed area shown in Fig. 4.3 cannot be simply compared
with an area from past surveys because of differences in methodologies).
The expansion of Iso-yake has occurred concurrently with the rise of seawater
temperature in coastal areas. Along the northern coast of Kyushu, massive deaths of
Ecklonia and Eisenia were recorded due to abnormally high water temperature over
a wide area in the summer of 2013 (Yatsuya et al. 2014b). Grazing activity by herbivorous fish and invertebrates such as sea urchins, which is a major factor for Isoyake, has also been suggested to be associated with an increase in water temperature.
In the coastal areas of Kyushu and Shikoku, in addition to the shrinking of the
macroalgal beds due to Iso-yake, changes in the constituent species of Garamo-ba
have been occurring as invasive tropical Sargassum species have been replacing
temperate species (Yoshimura et al. 2010; Tanaka et al. 2012). In the tropical and
subtropical macroalgal bed ecosystem, the grazing pressure of herbivorous animals
tends to be large and the biomass of macroalgae tends to be small. The expansion of
Iso-yake and changes in the constituent species of the beds may indicate that tropicalization of the temperate coastal zone in Japan is ongoing.
In local areas where fishery grounds have been devastated due to Iso-yake, countermeasures such as the principal method of protecting the macroalgal beds from
grazing have been implemented. For example, activities related to the removal and
control of sea urchins in Iso-yake zones have been conducted by fishermen, and
successful recovery of beds has been observed in some areas (Fisheries Agency
2015). To make these measures against Iso-yake sustainable, it will be necessary to
provide social incentives to these activities. If macroalgal beds are institutionally
recognized as a sink of carbon dioxide similar to forests, fishermen’s activities to
combat Iso- yake may be viewed as having the same significance as forest maintenance activities, some of which have already been approved as contributing to carbon sequestration.
On the other hand, if global warming progresses according to current predictions, changes in seagrass and macroalgal beds will unfortunately become irreversible in the long run. If that becomes the case, it will then be necessary to take the
viewpoint of adaptive use of the ecosystem services provided by ‘new’ coastal ecosystems. Tropical and subtropical seagrass and macroalgal beds can be used as a
model for this scenario, but it will require the comparison of various ecological
functions including carbon sequestration and storage with temperate seagrass and
macroalgal beds.
G. Yoshida et al.
which is the sudden and unusual disappearance and decline of macroalgal beds, has
been expanding, mainly in the coastal areas of western Japan. A reported 18,538 ha
of macroalgal beds have disappeared (Akimoto and Matsumura 2010) since then.
Assuming that the disappearance caused by Iso-yake has occurred mainly in
Garamo-ba and Arame-ba, the carbon sequestration function by these macroalgal
beds, which corresponds to about 300,000–500,000 tons CO 2 /year, has already been
lost in coastal areas (Remember though, as noted previously in this chapter, the latest seagrass and macroalgal bed area shown in Fig. 4.3 cannot be simply compared
with an area from past surveys because of differences in methodologies).
The expansion of Iso-yake has occurred concurrently with the rise of seawater
temperature in coastal areas. Along the northern coast of Kyushu, massive deaths of
Ecklonia and Eisenia were recorded due to abnormally high water temperature over
a wide area in the summer of 2013 (Yatsuya et al. 2014b). Grazing activity by herbivorous fish and invertebrates such as sea urchins, which is a major factor for Isoyake, has also been suggested to be associated with an increase in water temperature.
In the coastal areas of Kyushu and Shikoku, in addition to the shrinking of the
macroalgal beds due to Iso-yake, changes in the constituent species of Garamo-ba
have been occurring as invasive tropical Sargassum species have been replacing
temperate species (Yoshimura et al. 2010; Tanaka et al. 2012). In the tropical and
subtropical macroalgal bed ecosystem, the grazing pressure of herbivorous animals
tends to be large and the biomass of macroalgae tends to be small. The expansion of
Iso-yake and changes in the constituent species of the beds may indicate that tropicalization of the temperate coastal zone in Japan is ongoing.
In local areas where fishery grounds have been devastated due to Iso-yake, countermeasures such as the principal method of protecting the macroalgal beds from
grazing have been implemented. For example, activities related to the removal and
control of sea urchins in Iso-yake zones have been conducted by fishermen, and
successful recovery of beds has been observed in some areas (Fisheries Agency
2015). To make these measures against Iso-yake sustainable, it will be necessary to
provide social incentives to these activities. If macroalgal beds are institutionally
recognized as a sink of carbon dioxide similar to forests, fishermen’s activities to
combat Iso- yake may be viewed as having the same significance as forest maintenance activities, some of which have already been approved as contributing to carbon sequestration.
On the other hand, if global warming progresses according to current predictions, changes in seagrass and macroalgal beds will unfortunately become irreversible in the long run. If that becomes the case, it will then be necessary to take the
viewpoint of adaptive use of the ecosystem services provided by ‘new’ coastal ecosystems. Tropical and subtropical seagrass and macroalgal beds can be used as a
model for this scenario, but it will require the comparison of various ecological
functions including carbon sequestration and storage with temperate seagrass and
macroalgal beds.
G. Yoshida et al.