Abstract
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 210Pb and for the last millennia based on 14C) 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.