Organic carbon sequestration and storage in vegetated coastal habitats along the western coast of the Arabian Gulf

Certain coastal ecosystems such as mangrove, saltmarsh and seagrass habitats have been identified as significant natural carbon sinks, through the sequestration and storage of carbon in their biomass and sediments, collectively known as 'blue carbon' ecosystems. These ecosystems can often thrive in extreme environments where terrestrial systems otherwise survive at the limit of their existence, such as in arid and desert regions of the globe. To further our understanding of the capability of blue carbon ecosystems to sequester and store carbon in such extreme climates, we measured carbon sediment stocks in 25 sites along the Western Arabian Gulf coast. While seagrass meadows and saltmarsh habitats were widely distributed along the coast, mangrove stands were much reduced as a result of anthropogenic pressures, with 90% of stands having been lost over the last century. Carbon stocks in 1 m deep surface sediments were similar across all three blue carbon habitats, with comparable stocks for saltmarsh (81 ± 22 Mg Corg ha−1), seagrass (76 ± 20 Mg Corg ha−1) and mangroves (76 ± 23 Mg Corg ha−1). We recorded a 38% decrease in carbon stocks between mature established mangrove stands (91 Mg Corg ha−1) and recently planted mangroves (56 Mg Corg ha−1). Mangroves also had the lowest carbon stock per total area owing to their very limited spatial coverage along the coast. The largest stock per total area belonged to seagrass beds as a result of their large spatial coverage within the Gulf. We employed 210Pb dating to determine the sediment accretion rates in each ecosystem and found mangrove habitats to be the most efficient carbon sequesters over the past century, with the highest carbon burial rate of the three ecosystems (19 g Corg m−2 yr−1), followed by seagrass (9 g Corg m−2 yr−1) and saltmarshes (8 g Corg m−2 yr−1). In this work, we describe a comprehensive comparison of sediment stocks in different blue carbon ecosystems within a single marine environment and across a large geographical area, and discuss our results in a global context for other blue carbon ecosystems in the dry tropics.