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Vegetated coastal ecosystems along the Red Sea and Arabian Gulf coasts of Saudi Arabia thrive in an extremely arid and oligotrophic environment, with high seawater temperatures and salinity. Mangrove, seagrass and saltmarsh ecosystems have been shown to act as efficient sinks of sediment organic carbon, earning these vegetated ecosystems the moniker ‘blue carbon’ ecosystems. However, their role as nitrogen and phosphorus (N and P) sinks remains poorly understood. In this study, we examine the capacity of blue carbon ecosystems to trap and store nitrogen and phosphorous in their sediments in the central Red Sea and Arabian Gulf. We estimated the N and P stocks (in 0.2 m thick-sediments) and accumulation rates (for the last century based on ^210 Pb and for the last millennia based on ^14 C) in mangrove, seagrass and saltmarsh sediments from eight locations along the coast of Saudi Arabia (81 cores in total). The N and P stocks contained in the top 20 cm sediments ranged from 61 g N m ^−2 in Red Sea seagrass to 265 g N m ^−2 in the Gulf saltmarshes and from 70 g P m ^−2 in Red Sea seagrass meadows and mangroves to 58 g P m ^−2 in the Gulf saltmarshes. The short-term N and P accumulation rates ranged from 0.09 mg N cm ^−2 yr ^−1 in Red Sea seagrass to 0.38 mg N cm ^−2 yr ^−1 in Gulf mangrove, and from 0.027 mg P cm ^−2 yr ^−1 in the Gulf seagrass to 0.092 mg P cm ^−2 yr ^−1 in Red Sea mangroves. Short-term N and P accumulation rates were up to 10-fold higher than long-term accumulation rates, highlighting increasing sequestration of N and P over the past century, likely due to anthropogenic activities such as coastal development and wastewater inputs.

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 C _org ha ^−1 ), seagrass (76 ± 20 Mg C _org ha ^−1 ) and mangroves (76 ± 23 Mg C _org ha ^−1 ). We recorded a 38% decrease in carbon stocks between mature established mangrove stands (91 Mg C _org ha ^−1 ) and recently planted mangroves (56 Mg C _org 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 ^210 Pb 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 C _org m ^−2 yr ^−1 ), followed by seagrass (9 g C _org m ^−2 yr ^−1 ) and saltmarshes (8 g C _org 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.