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

Abstract

Valuing sedimentary ‘blue carbon’ stocks of seagrass meadows requires exclusion of allochthonous recalcitrant forms of carbon, such as black carbon (BC). Regression models constructed across a Southeast Asian tropical estuary predicted that carbon stocks within the sandy meadows of coastal embayments would support a modest but not insignificant amount of BC. We tested the prediction across three coastal meadows of the same region: one patchy meadow located close to a major urban centre and two continuous meadows contained in separate open embayments of a rural marine park; all differed in fetch and species. The BC/total organic carbon (TOC) fractions in the urban and rural meadows with small canopies were more than double the predicted amounts, 28 ± 1.6% and 36 ± 1.5% (±95% confidence intervals), respectively. The fraction in the rural large-canopy meadow remained comparable to the other two meadows, 26 ± 4.9% (±95% confidence intervals) but was half the amount predicted, likely owing to confounding of the model. The relatively high BC/TOC fractions were explained by variability across sites of BC atmospheric supply, an increase in loss of seagrass litter close to the exposed edges of meadows and sediment resuspension across the dispersed patchy meadow.

Inorganic and black carbon hotspots constrain blue carbon mitigation services across tropical seagrass and temperate tidal marshes

“…Interestingly, the proportional response of BC with TOC is statistically identical to that found across a rural meadow dominated by the same seagrass species (Enhalus spp.) at Limau Limauan (P = 0.64, reanalysed from supplementary material attached to Gallagher et al, 2019). Although overall, Middle Bank supported a larger BC/TOC fraction than Limau Limauan (26.1% ± 4.9 CI 95%).…”

“…With aeolian deposition, sedimentary contents of BC across a meadow or wetland is independent on its TOC. The extent of residual variance in BC is likely affected by different accretion processes that may vary over larger distances than those areas restricted across embayments (Chew and Gallagher, 2018;Gallagher et al, 2019;Sun et al, 2008). Whereas sedimentary BC supplied from soil washout increases with TOC, where any the BC intercept represents the underlying remnant of BC invariance from additional aeolian deposition (Chew and Gallagher, 2018).…”

“…Traditional assessments have failed to remove allochthonous organic recalcitrants from the equation (Chew and Gallagher, 2018;Chuan et al, 2020;Gallagher, 2014;Gallagher, 2015;Gallagher, 2017;Gallagher et al, 2020 in press;Gallagher et al, 2019). These stable organic forms are not produced by the blue carbon ecosystem and deposition by the ecosystem does not afford additional protection from remineralization.…”

“…The meadow is affected by both nearby Sumatran peat fires and urban pollution. Furthermore, its patchy seascape and coastal exposure are known to lead to greater amounts of exported allochthonous litter and fine organic clay particles that can elevate BC fractions (Gallagher et al, 2019).…”

Blue carbon sequestration dynamics within tropical seagrass sediments: long-term incubations for changes over climatic scales

“…protection component that will likely reduce the generality of the result across meadows. In the more exposed and turbulent meadows, seagrass sediments are composed of larger, sandier particles (Gallagher et al 2019). Such particles lack the necessary cohesiveness, surface area, and fine pore structures required to exclude bacterial influence (Burdige 2007).…”