Carbon stocks of coastal seagrass in Southeast Asia may be far lower than anticipated when accounting for black carbon

Abstract

Valuing the sedimentary blue carbon stocks of seagrass meadows in mitigating greenhouse gas emissions requires the exclusion of allochthonous recalcitrant forms, such as black carbon (BC) from the stock assessment. Regression models constructed across a tropical estuary predicted that carbon sinks within the more abundant sandy meadows of coastal bays likely support a significant but modest BC fraction. We tested the prediction by measuring BC fractions of total organic carbon (TOC) across three coastal meadows of the same region. One patchy meadow was located close to a major urban centre while the remaining two continuous meadows where contained in separate open embayments of a rural marine park, differing in fetch and species. In all cases, the BC/TOC fractions were significantly greater than predicted constituting a major component of the organic carbon content, 28% {+/-} 1.6, and 26% {+/-} 4.9 to 36% {+/-} 1.5 ({+/-}95% confidence intervals) for urban and marine park meadows respectively. The higher BC/TOC fractions were explained by site-specific variability in BC atmospheric supply, patchy coverage, and a presumed increase in the loss of seagrass litter, as determined by the canopy height and proximity to the meadows exposed edge.