19
much involved in the sequestration and storage of blue carbon, but their contributions to blue carbon sequestration and storage differ between the two types of plants.
Phytoplankton is a designation that encompasses all floating photosynthetic
organisms and therefore includes a wide variety of species. Although the functions
of individual species and taxonomic groups vary, the function of sequestering CO 2
through photosynthesis is common. Benthic plants are secured to and live on the
bottom of the ocean. Depending on the taxonomic group, the size and morphological characteristics of benthic plants ranges from a few micrometers to over 10 m.
Benthic algae avoid drifting away from the ocean floor by attaching to hard, rocky
substrates. Seaweeds are an especially large group of algae with a high ability to
sequester blue carbon. The main groups of seaweeds are the relatives of kelp (family
Succarinae, Eckloniae, etc.) and members of the Sargassum genus. Chapter 4
(Yoshida et al. 2018) provides details.
Neither neutrally buoyant phytoplankton nor macroalgae attached to rocks can
directly bury blue carbon in the sediment. However, because both phytoplankton
and macroalgae have high growth rates, their CO 2 uptake rates are very high (Chap.
4; Yoshida et al. 2018, Chap. 11; Kuwae et al. 2018). Because both phytoplankton
and attached macroalgae take up inorganic carbon in large quantities from the surrounding seawater, the CO 2 partial pressure of the seawater surrounding them
becomes lower than the CO 2 partial pressure in the atmosphere; hence, there is an
influx of CO 2 from the atmosphere (Chap. 6; Tokoro et al. 2018). The carbon sequestered by phytoplankton and macroalgae can contribute to blue carbon storage if it is
transported ex situ to shallow coastal areas or to the deep sea and subsequently
buried in the sediments (Krause-Jensen and Duarte 2016).
Seagrasses are taxonomically different from macroalgae. The seagrass species
are angiosperms that returned from the land to the ocean. Therefore, species of seagrass are classified differently from marine algae. Macroalgae are thought to have
first appeared about 3 billion years ago, whereas seagrasses are believed to have
evolved from terrestrial plants about 100 million years ago (Kato et al. 2003).
Whereas algae include about 10,000 species (Murphy et al. 2015), seagrasses are
divided into three families (four families if Posidoniaceae, the oldest lineage, is
considered) with about 60 species (Short et al. 2007). Seagrasses may seem like rare
plants because there are only 60 species of seagrasses on Earth, but in fact they are
distributed over most of the Earth, from the tropics to the subarctic regions. They are
absent only in polar regions (Fig. 1.3c). Among the 60 seagrass species, those distributed in the tropics and subtropics have lower shoot heights than those found in
temperate latitudes (Hemminga and Duarte 2000). The canopy heights of tropical
seagrasses are generally on the order of 10 cm, whereas those of temperate species
such as Zostera sp. are more than 1 m and sometimes more than 2 m when environmental conditions are favorable for their growth.
The largest difference between seagrasses and other marine plants is that seagrasses are flowering plants that bloom in the sea and produce seeds. They therefore
possess roots, stems and leaves; in addition, they inhabit areas with sandy or muddy
sediment, and their roots and rhizomes penetrate into the sediment. Bamboos are
analogous plants on land. Seagrasses extend their rhizomes in networks within the
1 Blue Carbon: Characteristics of the Ocean’s Sequestration and Storage Ability…
much involved in the sequestration and storage of blue carbon, but their contributions to blue carbon sequestration and storage differ between the two types of plants.
Phytoplankton is a designation that encompasses all floating photosynthetic
organisms and therefore includes a wide variety of species. Although the functions
of individual species and taxonomic groups vary, the function of sequestering CO 2
through photosynthesis is common. Benthic plants are secured to and live on the
bottom of the ocean. Depending on the taxonomic group, the size and morphological characteristics of benthic plants ranges from a few micrometers to over 10 m.
Benthic algae avoid drifting away from the ocean floor by attaching to hard, rocky
substrates. Seaweeds are an especially large group of algae with a high ability to
sequester blue carbon. The main groups of seaweeds are the relatives of kelp (family
Succarinae, Eckloniae, etc.) and members of the Sargassum genus. Chapter 4
(Yoshida et al. 2018) provides details.
Neither neutrally buoyant phytoplankton nor macroalgae attached to rocks can
directly bury blue carbon in the sediment. However, because both phytoplankton
and macroalgae have high growth rates, their CO 2 uptake rates are very high (Chap.
4; Yoshida et al. 2018, Chap. 11; Kuwae et al. 2018). Because both phytoplankton
and attached macroalgae take up inorganic carbon in large quantities from the surrounding seawater, the CO 2 partial pressure of the seawater surrounding them
becomes lower than the CO 2 partial pressure in the atmosphere; hence, there is an
influx of CO 2 from the atmosphere (Chap. 6; Tokoro et al. 2018). The carbon sequestered by phytoplankton and macroalgae can contribute to blue carbon storage if it is
transported ex situ to shallow coastal areas or to the deep sea and subsequently
buried in the sediments (Krause-Jensen and Duarte 2016).
Seagrasses are taxonomically different from macroalgae. The seagrass species
are angiosperms that returned from the land to the ocean. Therefore, species of seagrass are classified differently from marine algae. Macroalgae are thought to have
first appeared about 3 billion years ago, whereas seagrasses are believed to have
evolved from terrestrial plants about 100 million years ago (Kato et al. 2003).
Whereas algae include about 10,000 species (Murphy et al. 2015), seagrasses are
divided into three families (four families if Posidoniaceae, the oldest lineage, is
considered) with about 60 species (Short et al. 2007). Seagrasses may seem like rare
plants because there are only 60 species of seagrasses on Earth, but in fact they are
distributed over most of the Earth, from the tropics to the subarctic regions. They are
absent only in polar regions (Fig. 1.3c). Among the 60 seagrass species, those distributed in the tropics and subtropics have lower shoot heights than those found in
temperate latitudes (Hemminga and Duarte 2000). The canopy heights of tropical
seagrasses are generally on the order of 10 cm, whereas those of temperate species
such as Zostera sp. are more than 1 m and sometimes more than 2 m when environmental conditions are favorable for their growth.
The largest difference between seagrasses and other marine plants is that seagrasses are flowering plants that bloom in the sea and produce seeds. They therefore
possess roots, stems and leaves; in addition, they inhabit areas with sandy or muddy
sediment, and their roots and rhizomes penetrate into the sediment. Bamboos are
analogous plants on land. Seagrasses extend their rhizomes in networks within the
1 Blue Carbon: Characteristics of the Ocean’s Sequestration and Storage Ability…