123
continue research on spatiotemporal changes in production of each seagrass and
macroalgal bed to improve the accuracy of estimated values.
4.4 Conclusion: Future Issues and Concerns
In this chapter, we estimated the area of the main seagrass and macroalgal beds (i.e.,
Amamo-ba, Garamo-ba, Arame-ba, and Kombu-ba) in Japan to be approximately
230,000 ha. Through the production of seaweed and macroalgae in these beds,
about 4.7 million tons CO 2 /year was estimated to be sequestered annually. For comparison, Japan’s reported forested area is about 25 million ha and absorbed approximately 49.9 million tons of carbon dioxide in 2014 (Forestry Agency 2015). On a
unit area basis, the carbon sequestration function of seagrass and macroalgal beds is
higher than that of forests, most likely because the turnover rates of seagrasses and
macroalgae are larger than those of trees. Therefore, the production of seagrass and
macroalgal beds is higher than that of trees.
Japan was the fifth largest greenhouse gas emitting country in the world in 2014,
and in the same year, it emitted approximately 1.3 billion tons of carbon dioxide
(Ministry of the Environment 2016). In comparison, the production of seagrass and
macroalgae (4.7 million tons CO 2 /year) may be considered small. However, among
industrial sectors, the amounts of carbon dioxide emitted annually by the agriculture and fisheries sectors are approximately 5.65 million tons and 5.74 million tons,
respectively (National Institute for Environmental Studies 2010). Each of these
amounts is approximately equal to the amount of carbon dioxide estimated to be
sequestered by seagrass and macroalgae.
However, for seagrass and macroalgal beds to be recognized as carbon sinks that
contribute to measures to reduce greenhouse gas emissions, more fundamental
research is needed. Seagrass and macroalgae produced within a bed follow numerous fates, for example, decomposition, consumption by animals, sedimentation at
the sea bottom, and outflow from coastal areas (Fig. 4.1) (Abo et al. 2018). In Japan,
however, few studies have quantified each carbon flow path. Particularly,
sedimentation at the sea bottom needs to be quantitatively evaluated from the viewpoint of contribution to carbon storage. In particular for macroalgae, research on a
large scale, along with the development of effective equipment and technology, is
necessary to elucidate the outflow and transportation from the bed to sedimentation
in the deep sea. Also, it is becoming clear that DOM released by seagrasses and
macroalgae, which was not discussed in detail in this chapter, should not be ignored
(Krause- Jensen and Duarte 2016). It is necessary to understand the quantitative values and dynamics of DOM in the ocean to further justify the function of carbon
sequestration and the storage potential of seagrass and macroalgae.
Although seagrass and macroalgal beds are expected to contribute to the mitigation of global warming through sequestration and storage of organic carbon, future
concerns about the beds themselves exist because they are extremely vulnerable to
global warming. Since the latter half of the 1990s, Iso-yake (or sea desertification),
4 Carbon Sequestration by Seagrass and Macroalgae in Japan: Estimates and Future…
continue research on spatiotemporal changes in production of each seagrass and
macroalgal bed to improve the accuracy of estimated values.
4.4 Conclusion: Future Issues and Concerns
In this chapter, we estimated the area of the main seagrass and macroalgal beds (i.e.,
Amamo-ba, Garamo-ba, Arame-ba, and Kombu-ba) in Japan to be approximately
230,000 ha. Through the production of seaweed and macroalgae in these beds,
about 4.7 million tons CO 2 /year was estimated to be sequestered annually. For comparison, Japan’s reported forested area is about 25 million ha and absorbed approximately 49.9 million tons of carbon dioxide in 2014 (Forestry Agency 2015). On a
unit area basis, the carbon sequestration function of seagrass and macroalgal beds is
higher than that of forests, most likely because the turnover rates of seagrasses and
macroalgae are larger than those of trees. Therefore, the production of seagrass and
macroalgal beds is higher than that of trees.
Japan was the fifth largest greenhouse gas emitting country in the world in 2014,
and in the same year, it emitted approximately 1.3 billion tons of carbon dioxide
(Ministry of the Environment 2016). In comparison, the production of seagrass and
macroalgae (4.7 million tons CO 2 /year) may be considered small. However, among
industrial sectors, the amounts of carbon dioxide emitted annually by the agriculture and fisheries sectors are approximately 5.65 million tons and 5.74 million tons,
respectively (National Institute for Environmental Studies 2010). Each of these
amounts is approximately equal to the amount of carbon dioxide estimated to be
sequestered by seagrass and macroalgae.
However, for seagrass and macroalgal beds to be recognized as carbon sinks that
contribute to measures to reduce greenhouse gas emissions, more fundamental
research is needed. Seagrass and macroalgae produced within a bed follow numerous fates, for example, decomposition, consumption by animals, sedimentation at
the sea bottom, and outflow from coastal areas (Fig. 4.1) (Abo et al. 2018). In Japan,
however, few studies have quantified each carbon flow path. Particularly,
sedimentation at the sea bottom needs to be quantitatively evaluated from the viewpoint of contribution to carbon storage. In particular for macroalgae, research on a
large scale, along with the development of effective equipment and technology, is
necessary to elucidate the outflow and transportation from the bed to sedimentation
in the deep sea. Also, it is becoming clear that DOM released by seagrasses and
macroalgae, which was not discussed in detail in this chapter, should not be ignored
(Krause- Jensen and Duarte 2016). It is necessary to understand the quantitative values and dynamics of DOM in the ocean to further justify the function of carbon
sequestration and the storage potential of seagrass and macroalgae.
Although seagrass and macroalgal beds are expected to contribute to the mitigation of global warming through sequestration and storage of organic carbon, future
concerns about the beds themselves exist because they are extremely vulnerable to
global warming. Since the latter half of the 1990s, Iso-yake (or sea desertification),
4 Carbon Sequestration by Seagrass and Macroalgae in Japan: Estimates and Future…