56
The relative contributions of different OC sources to the sedimentary OC pool in
seagrass meadows can vary widely depending on their ecological and oceanographic settings. In typical temperate seagrass meadows growing on clastic sediment, approximately half of the total OC stored in sediment is derived from primary
production by seagrasses (Kennedy et al. 2010; Miyajima et al. 2015; Oreska et al.
2018), and most exists as mineral-associated OC (Miyajima et al. 2017). On the
other hand, in estuarine Z. marina meadows (e.g., Fig. 2.4b; see also Watanabe and
Kuwae 2015) and mangrove-fringed tropical seagrass meadows (e.g., Fig. 2.4h; see
also Hemminga et al. 1994; Chen et al. 2017), the majority of sediment OC is often
of terrestrial or mangrove origin, as suggested by a low δ
13
C, and is present as
mineral-free detrital particles (see Sect. 2.9 for the use of δ
13
C as a source proxy). In
tropical and subtropical seagrass meadows on carbonate sediments, mineral-free
OC and mineral-associated OC contribute roughly equally to the sediment OC stock
(e.g., Fig. 2.4e, f), and in the absence of OC input from mangroves, mineral-free OC
exhibits an elevated δ
13
C (≥ −15‰), indicating that it is almost exclusively derived
from seagrasses and possibly microphytobenthos (Fig. 2.4e, f). Carbonate sediment
slowly undergoes dissolution via reaction with CO 2 produced by microbial respiration (Eldridge and Morse 2000; Hu and Burdige 2007), which may inhibit accumulation of OC on carbonate mineral surfaces and consequently result in a low relative
abundance of mineral-associated OC compared with clastic sediment. Although OC
loading in carbonate seagrass meadow sediments is apparently very high (Fig. 2.5b),
this could be partly explained by the coexistence with mineral-free OC. Another
remarkable case involves meadows of P. oceanica, a large Mediterranean seagrass
species. This species forms a robust belowground structure of rhizomes and roots,
which can persist in the sediment for thousands of years after death (Mateo et al.
1997; Lo Iacono et al. 2008). In this case, belowground seagrass production is
apparently the most important OC source to the sediment by far (Serrano et al.
2016). A similar but less extensive belowground structure formed by rhizomes and
roots is also found in seagrass meadows of subtropical Thalassia spp. (Wanless
1981; Fig. 2.1e).
As such, the provenance of OC in seagrass meadow sediments can differ greatly
depending on sediment mineralogy, seagrass species composition, and connectivity
with neighboring habitats. Estimating global carbon inventory without considering
habitat-specific variability of OC sources is likely to introduce a large bias in the
case of seagrass meadow sediments.
2.9 Methods for Provenance Analysis of Sediment OC
Many seagrass species do not leave any visible remains that persist long term in
sediments (Reich et al. 2015). Most OC in seagrass meadow sediments is either
mineral-adsorbed OC or amorphous detrital OC, and the morphological
characteristics of these types of organic matter provide few clues as to their origin.
Three methods are currently used to infer the primary producers that originally
produced OC: isotopic signatures, molecular biomarkers, and environmental DNA
T. Miyajima and M. Hamaguchi
The relative contributions of different OC sources to the sedimentary OC pool in
seagrass meadows can vary widely depending on their ecological and oceanographic settings. In typical temperate seagrass meadows growing on clastic sediment, approximately half of the total OC stored in sediment is derived from primary
production by seagrasses (Kennedy et al. 2010; Miyajima et al. 2015; Oreska et al.
2018), and most exists as mineral-associated OC (Miyajima et al. 2017). On the
other hand, in estuarine Z. marina meadows (e.g., Fig. 2.4b; see also Watanabe and
Kuwae 2015) and mangrove-fringed tropical seagrass meadows (e.g., Fig. 2.4h; see
also Hemminga et al. 1994; Chen et al. 2017), the majority of sediment OC is often
of terrestrial or mangrove origin, as suggested by a low δ
13
C, and is present as
mineral-free detrital particles (see Sect. 2.9 for the use of δ
13
C as a source proxy). In
tropical and subtropical seagrass meadows on carbonate sediments, mineral-free
OC and mineral-associated OC contribute roughly equally to the sediment OC stock
(e.g., Fig. 2.4e, f), and in the absence of OC input from mangroves, mineral-free OC
exhibits an elevated δ
13
C (≥ −15‰), indicating that it is almost exclusively derived
from seagrasses and possibly microphytobenthos (Fig. 2.4e, f). Carbonate sediment
slowly undergoes dissolution via reaction with CO 2 produced by microbial respiration (Eldridge and Morse 2000; Hu and Burdige 2007), which may inhibit accumulation of OC on carbonate mineral surfaces and consequently result in a low relative
abundance of mineral-associated OC compared with clastic sediment. Although OC
loading in carbonate seagrass meadow sediments is apparently very high (Fig. 2.5b),
this could be partly explained by the coexistence with mineral-free OC. Another
remarkable case involves meadows of P. oceanica, a large Mediterranean seagrass
species. This species forms a robust belowground structure of rhizomes and roots,
which can persist in the sediment for thousands of years after death (Mateo et al.
1997; Lo Iacono et al. 2008). In this case, belowground seagrass production is
apparently the most important OC source to the sediment by far (Serrano et al.
2016). A similar but less extensive belowground structure formed by rhizomes and
roots is also found in seagrass meadows of subtropical Thalassia spp. (Wanless
1981; Fig. 2.1e).
As such, the provenance of OC in seagrass meadow sediments can differ greatly
depending on sediment mineralogy, seagrass species composition, and connectivity
with neighboring habitats. Estimating global carbon inventory without considering
habitat-specific variability of OC sources is likely to introduce a large bias in the
case of seagrass meadow sediments.
2.9 Methods for Provenance Analysis of Sediment OC
Many seagrass species do not leave any visible remains that persist long term in
sediments (Reich et al. 2015). Most OC in seagrass meadow sediments is either
mineral-adsorbed OC or amorphous detrital OC, and the morphological
characteristics of these types of organic matter provide few clues as to their origin.
Three methods are currently used to infer the primary producers that originally
produced OC: isotopic signatures, molecular biomarkers, and environmental DNA
T. Miyajima and M. Hamaguchi