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

Abstract

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 (δ 13 C and δ 15 N), 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 δ 15 N 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.

Stunted Mangrove Trees in the Oligotrophic Central Red Sea Relate to Nitrogen Limitation

“…N limitation, thus, is reflected in low mangrove canopy heights, while keeping N levels above minimum requirements for functional mangrove leaves. In addition, the variation in sediment nitrogen concentration could have been partially affected by the variation in sediment bulk density, which ranged from 0.7 to 1.2 g cm −3 across our study sites (Garcias-Bonet et al, 2019). Bulk density has been shown to negatively correlate with soil nitrogen in coastal biogenic habitats (Fourqurean et al, 2012), although we did not find a correlation between bulk density and the concentration of N in the sediment or the mangrove leaves (linear regression; p = 0.122 and 0.874, respectively).…”

“…Therefore, these two nutrients (P and Fe) are likely to be less available in seagrass compared to mangrove sediments in the Red Sea. In addition, sediments on the fringing mangroves in the Red Sea are more depleted in N relative to organic carbon than seagrass sediments (Garcias-Bonet et al, 2019), providing further evidence for inorganic N to be a limiting source for mangroves in the basin. Nevertheless, the mean Fe concentration in the Red Sea mangrove sediment was 0.58 ± 0.85%, which is much lower than the mean for marine sediment (4.1% DW; Bowen, 1979), and within the lower end of the global range for mangrove sediments (0.14-14.6% DW; Billah et al, 2014), highlighting the overall extremely oligotrophic conditions for mangroves in the Red Sea.…”

“…The Red Sea is characterized by a strong nutrient gradient, with nutrient-rich Indian Ocean waters entering the Red Sea in the south and waters becoming progressively oligotrophic toward the north due to the absence of riverine nutrient inputs (Raitsos et al, 2015). In fact, this latitudinal nutrient gradient is reflected in a decrease in sediment N stocks in mangroves and seagrass meadows from south to north and a subsequent increase in the sediment C/N ratio (Garcias-Bonet et al, 2019). Similarly, the stable N isotopic composition of Red Sea mangrove leaves and sediments, which become lighter toward the north, suggests a change in N sources from nitrates in the south to atmospheric N fixation in the north (Duarte et al, 2018;Garcias-Bonet et al, 2019).…”

“…In fact, this latitudinal nutrient gradient is reflected in a decrease in sediment N stocks in mangroves and seagrass meadows from south to north and a subsequent increase in the sediment C/N ratio (Garcias-Bonet et al, 2019). Similarly, the stable N isotopic composition of Red Sea mangrove leaves and sediments, which become lighter toward the north, suggests a change in N sources from nitrates in the south to atmospheric N fixation in the north (Duarte et al, 2018;Garcias-Bonet et al, 2019). Therefore, nutrient limitation of mangrove stands is expected to increase from south to north along the Red Sea.…”

Warming enhances carbon dioxide and methane fluxes from Red Sea seagrass (<i>Halophila stipulacea</i>) sediments

“…Our chosen study sites were located in an enclosed lagoon with a high abundance of mangrove forests, leading to the conclusion that mangroves might be a major source of organic matter for our study sites. However, a recent study applying stable isotope mixing models found the major contributors to the organic matter in seagrass sediments in the Red Sea to be seagrass leaves and macroalgae blades, with contributions of 43 % and 37 %, respectively (Garcias-Bonet et al, 2019a).…”