55
Allochthonous OC transported to seagrass meadows as independent (i.e., mineral-free) organic particles (Category I) may be derived from either marine (I-1,2)
or terrestrial (I-3) primary production; OC derived from terrestrial plants, including
mangroves, is generally more structurally resistant against degradation compared
with OC of marine origin. Allochthonous organic particles are hydrodynamically
transported to coastal ecosystems, where they are effectively trapped and accumulated by seagrass blades (Dauby et al. 1995; Miyajima et al. 1998; Kennedy et al.
2004). Some invertebrate residents of seagrass meadows, such as bivalves and epibenthic particle feeders, actively capture floating nutrient-rich organic particles
such as living microalgae and coral mucus (Morimoto et al. 2017). A portion of the
suspended OC captured by these animals may be incorporated into sediments in
their excreta and carrion.
Mineral-associated allochthonous OC (Category II) can be supplied to seagrass
meadows as soil or sediment OC that has been resuspended and transported from
distant ecosystems including land, mangroves, and other soft-bottom communities,
including seagrass meadows. As explained in Sect. 2.4, mineral-associated soil
(sediment) organic matter (II-1) presumably originates from microbial secondary
products, such as extracellular polymeric materials (Decho 2000; Cyle et al. 2016).
Although it may not be intrinsically refractory, adsorption to a mineral surface
significantly retards remineralization. Clastic sediment particles generated through
physical weathering of sedimentary rocks, especially in tectonically active
continental margins, contain ancient organic matter (II-2), which may be transported
by rivers to coastal areas (Blair et al. 2003; Leithold et al. 2006; Rosenheim and
Galy 2012; Fig. 2.6). Such fossil organic matter, also referred to as petrogenic OC,
is generally highly altered and considered resistant against enzymatic decomposition,
but it is not totally undegradable (Petsch et al. 2001). Biogenic carbonate sediment
contains a significant amount of OC in a carbonate matrix (II-3) that may be
solubilized into dissolved OC upon acid decarbonation (Froelich 1980). This
fraction can be as large as 40% of the total OC in carbonate sediments of seagrass
meadows (Miyajima et al. 1998). This OC is putatively derived from tissues of
carbonate-bearing organisms such as reef corals, foraminifera, and calcareous algae
and is therefore generally labile once exposed to microbial hydrolytic enzymes.
Autochthonous organic matter produced within seagrass ecosystems (Category
III) is also an important source of OC stored in sediment. Organic matter is produced
by seagrasses, epiphytic algae, and epibenthic algae (including microphytobenthos).
Algal species in seagrass meadows are highly diverse, ranging from unicellular
diatoms and cyanobacteria to multicellular macroalgae. Microalgal primary
production in meadows can be comparable with that of seagrasses (Pollard and
Kogure 1993) and more important than the latter as an energy source for food webs
(Moncreiff and Sullivan 2001). The most important autochthonous source of
sedimentary OC storage can vary. However, it is often evident that the OC
concentration in the top few centimeters of seagrass meadow sediment is more or
less depleted compared with the subsurface layer (5–30 cm; Fig. 2.1f, Fig. 2.3c–g),
implying that belowground production by seagrasses (rhizomes, roots, and root
exudates), which is usually concentrated in the subsurface layer, is the predominant
source of autochthonous OC stored in the sediment.
2 Carbon Sequestration in Sediment as an Ecosystem Function of Seagrass Meadows
Allochthonous OC transported to seagrass meadows as independent (i.e., mineral-free) organic particles (Category I) may be derived from either marine (I-1,2)
or terrestrial (I-3) primary production; OC derived from terrestrial plants, including
mangroves, is generally more structurally resistant against degradation compared
with OC of marine origin. Allochthonous organic particles are hydrodynamically
transported to coastal ecosystems, where they are effectively trapped and accumulated by seagrass blades (Dauby et al. 1995; Miyajima et al. 1998; Kennedy et al.
2004). Some invertebrate residents of seagrass meadows, such as bivalves and epibenthic particle feeders, actively capture floating nutrient-rich organic particles
such as living microalgae and coral mucus (Morimoto et al. 2017). A portion of the
suspended OC captured by these animals may be incorporated into sediments in
their excreta and carrion.
Mineral-associated allochthonous OC (Category II) can be supplied to seagrass
meadows as soil or sediment OC that has been resuspended and transported from
distant ecosystems including land, mangroves, and other soft-bottom communities,
including seagrass meadows. As explained in Sect. 2.4, mineral-associated soil
(sediment) organic matter (II-1) presumably originates from microbial secondary
products, such as extracellular polymeric materials (Decho 2000; Cyle et al. 2016).
Although it may not be intrinsically refractory, adsorption to a mineral surface
significantly retards remineralization. Clastic sediment particles generated through
physical weathering of sedimentary rocks, especially in tectonically active
continental margins, contain ancient organic matter (II-2), which may be transported
by rivers to coastal areas (Blair et al. 2003; Leithold et al. 2006; Rosenheim and
Galy 2012; Fig. 2.6). Such fossil organic matter, also referred to as petrogenic OC,
is generally highly altered and considered resistant against enzymatic decomposition,
but it is not totally undegradable (Petsch et al. 2001). Biogenic carbonate sediment
contains a significant amount of OC in a carbonate matrix (II-3) that may be
solubilized into dissolved OC upon acid decarbonation (Froelich 1980). This
fraction can be as large as 40% of the total OC in carbonate sediments of seagrass
meadows (Miyajima et al. 1998). This OC is putatively derived from tissues of
carbonate-bearing organisms such as reef corals, foraminifera, and calcareous algae
and is therefore generally labile once exposed to microbial hydrolytic enzymes.
Autochthonous organic matter produced within seagrass ecosystems (Category
III) is also an important source of OC stored in sediment. Organic matter is produced
by seagrasses, epiphytic algae, and epibenthic algae (including microphytobenthos).
Algal species in seagrass meadows are highly diverse, ranging from unicellular
diatoms and cyanobacteria to multicellular macroalgae. Microalgal primary
production in meadows can be comparable with that of seagrasses (Pollard and
Kogure 1993) and more important than the latter as an energy source for food webs
(Moncreiff and Sullivan 2001). The most important autochthonous source of
sedimentary OC storage can vary. However, it is often evident that the OC
concentration in the top few centimeters of seagrass meadow sediment is more or
less depleted compared with the subsurface layer (5–30 cm; Fig. 2.1f, Fig. 2.3c–g),
implying that belowground production by seagrasses (rhizomes, roots, and root
exudates), which is usually concentrated in the subsurface layer, is the predominant
source of autochthonous OC stored in the sediment.
2 Carbon Sequestration in Sediment as an Ecosystem Function of Seagrass Meadows