Carbon and Nitrogen Concentrations, Stocks, and Isotopic Compositions in Red Sea Seagrass and Mangrove Sediments

Coastal vegetated ecosystems are intense global carbon (C) sinks; however, seagrasses and mangroves in the Central Red Sea are depleted in organic C (C-org). Here, we tested whether C-org depletion prevails along the Red Sea, or if sediment C-org and nitrogen (N) stocks reflect the latitudinal productivity gradient of the Red Sea. We assessed C-org and N concentrations, stocks, isotopic compositions (delta C- 13 and delta N- 15), and the potential contribution of primary producers to the organic matter accumulation in seagrass and mangrove sediments along the Eastern coast of the Red Sea. Sediment C-org concentration was higher in mangroves than seagrasses, while N concentrations were similar, resulting in higher C/N ratios in mangrove than seagrass sediments. Mangrove C-org stocks (integrated over the top 10 cm) were twofold higher than those of seagrasses. N concentrations and stocks decreased from south to north in seagrass sediments matching the productivity gradient while C-org concentrations and stocks were uniform. The delta N-15 decreased from south to north in seagrass and mangrove sediments, reflecting a shift from nitrate and nitrite as N sources in the south, to N-2 fixation toward the north. Stable isotope mixing models showed that seagrass leaves and macroalgae blades were the major contributors to the organic matter accumulation in seagrass sediments; while mangrove leaves were the major contributors in mangrove sediments. Overall, vegetated sediments in the Red Sea tend to be carbonate-rich and depleted in C-org and N, compared to coastal habitats elsewhere. Specifically, mean C-org stocks in Red Sea seagrass and mangrove sediments (7.2 +/- 0.4 and 14.5 +/- 1.4 Mg C ha(-1), respectively) are lower than previously reported mean global values. This new information of Blue Carbon resources in the Red Sea provides a background for Blue Carbon programs in the region while also helping to balance global estimates.