A Global Assessment of the Chemical Recalcitrance of Seagrass Tissues: Implications for Long-Term Carbon Sequestration

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.

A Synthesis of Blue Carbon Stocks, Sources, and Accumulation Rates in Eelgrass (Zostera marina) Meadows in the Northeast Pacific

“…First, it is important to understand how carbon flows among coastal ecosystems to gain perspective on seascape connectivity and to better inform conservation of nearshore habitats (Hyndes et al, , ). Second, different sources of carbon may vary in their persistence in the sediments, as some sources (e.g., seston, macroalgae) are more labile and can be remineralized at a faster rate than recalcitrant sources such as terrestrial carbon or seagrass tissues (Holmer et al, ; Mazarrasa et al, ; Trevathan‐Tackett, Macreadie, et al, ). Finally, the relative contribution of allochthonous versus autochthonous carbon is an important component of carbon accounting, as projects may not receive credit for allochthonous carbon unless it would have been returned to the atmosphere in the baseline scenario (Emmer et al, ; Needelman et al, ).…”

High Seasonal Variability in Sediment Carbon Stocks of Cold‐Temperate Seagrass Meadows

“…The carbon stock variability was negatively linked to the net community production and although this seems contradictory, the high biomass production in summer and autumn (Baden & Pihl, ) and the associated build‐up of autochthonous and allochthonous organic matter will cause a high detritus pool subjected to an internal cycling by the microbial community (Trevathan‐Tackett et al, ), resulting in higher respiration rates and hence a low or negative NCP. This might explain the negative correlation between NCP and TOC stocks as the increased sedimentary TOC during summer and autumn is a consequence of the high detritus formation, which to a large extent has been consumed by the microbial community, leading to both a negative NCP and high organic matter accumulation.…”

Green turtle diet is dominated by seagrass in the Western Indian Ocean except amongst gravid females

“…It follows that intestinal microflora typical of individuals that feed on seagrass likely differs from that of algivores and they are less efficient at digesting algae and vice versa (Bjorndal et al 1991), but diets comprising large amounts of both seagrass and algae are also sometimes reported (e.g., López-Mendilaharsu et al 2005;Shimada et al 2014;Whiting et al 2014). Stable isotope studies have been used to show that individuals in the same area can have long-term dietary preferences (Thomson et al 2018). On the other hand, abrupt shifts between algae, seagrass and mangrove diets have been observed by examining the guts of individual green turtles (Brand et al 1999;Arthur et al 2009) and may simply reflect the food items available that yield the highest nutritional value with minimal search and handling costs (Bjorndal 1997).…”