Partitioning the sources of sediment organic carbon in South African seagrass meadows

Abstract

In the sediments of seagrass meadows, allochthonous carbon sources can account for 50–90% of sediment organic carbon (SOC), in  which non-seagrass carbon is derived from external organic matter advected into the meadow’s sediments or trapped by the seagrass  canopy. Identifying the SOC origin is essential to accurately estimate the climate change mitigation potential of seagrass ecosystems, yet  the ratio of allochthonous to autochthonous SOC in South African seagrass populations has not been investigated. In this study, we  measured the SOC and carbon isotope ratio (δ13C) in Zostera capensis seagrass meadows in the Berg River and Breede River estuaries  and applied Bayesian mixing models to disentangle the SOC contributory sources. Sediment organic carbon differed not only between  estuaries, but also showed variability within each estuary, with autochthonous carbon accounting for between ~35% (SD 0.23) and ~42.3%  (SD 0.18) of total meadow SOC. Other sources of SOC included macroalgae, salt marsh and seagrass species. Our work highlights  the importance of accounting for allochthonous carbon in disentangling the drivers of high variability of carbon stocks in estuarine  environments and contributes to improving the accuracy of South African seagrass carbon stock estimates.