Abstract
Seagrass ecosystems have recently been identified for their role in climate change mitigation due to their globally-significant carbon sinks; yet, the capacity of seagrasses to sequester carbon has been shown to vary greatly among seagrass ecosystems. The recalcitrant nature of seagrass tissues, or the resistance to degradation back into carbon dioxide, is one aspect thought to influence sediment carbon stocks. In this study, a global survey investigated how the macromolecular chemistry of seagrass leaves, sheaths/stems, rhizomes and roots varied across 23 species from 16 countries. The goal was to understand how this seagrass chemistry might influence the capacity of seagrasses to contribute to sediment carbon stocks. Three non-destructive analytical chemical analyses were used to investigate seagrass chemistry: thermogravimetric analysis (TGA) and solid state 13C-NMR and infrared spectroscopy. A strong latitudinal influence on carbon quality was found, whereby temperate seagrasses contained 5% relatively more labile carbon, and tropical seagrasses contained 3% relatively more refractory carbon. Sheath/stem tissues significantly varied across taxa, with larger morphologies typically containing more refractory carbon than smaller morphologies. Rhizomes were characterized by a higher proportion of labile carbon (16% of total organic matter compared to 8–10% in other tissues); however, high rhizome biomass production and slower remineralization in anoxic sediments will likely enhance these below-ground tissues' contributions to long-term carbon stocks. Our study provides a standardized and global dataset on seagrass carbon quality across tissue types, taxa and geography that can be incorporated in carbon sequestration and storage models as well as ecosystem valuation and management strategies.
Highlights
Seagrasses are considered a biological or ecological group of marine vascular flowering plants that have evolved from three to four separate lineages over the past 70–100 million years (Les et al, 1997)
A principal components analysis (PCA) of all samples showed that climatic region driven by organic matter (OM) (56.7%) and tissue type driven by TI3 (25.6% of OM) explained most of the variation in thermogravimetric analysis (TGA) results across the seagrass samples as PC1 and PC2, respectively (Figure 2)
Investigation of the variation in climatic zone and taxa for each tissue type revealed that differences in temperate and tropical TGA signatures were consistent and explained most of the variation across all tissue types, which was driven by OM content (PC1 = 46.2–70.1%; Figure 5; Table 3)
Summary
Seagrasses are considered a biological or ecological group of marine vascular flowering plants that have evolved from three to four separate lineages over the past 70–100 million years (Les et al, 1997). Seagrasses have a relatively low diversity (∼60 species within 12 genera), they do span a range of sizes, morphologies and life-cycle characteristics (ephemeral vs persistent, or r- and K-strategies) (Carruthers et al, 2007). These difference are important for colonization succession of bare sediments (Rasheed, 2004) and in some cases, the particle trapping and accumulation of sediment carbon (Serrano et al, 2016). Despite litter quality being a main factor controlling global C remineralization and cycling in terrestrial ecosystems (Couteaux et al, 1995; Silver and Miya, 2001), The influence of litter quality in C sequestration remains comparably understudied for seagrass blue carbon ecosystems (Cebrián et al, 1997; Cebrián, 1999)
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