We sampled 43 stations along the Saudi coast of the Red Sea, including 27 seagrass meadows and 16 mangrove stands (Figure 1) on four coastal cruises onboard the R/V Thuwal on February 2016, January 2017, March 2017, and July 2017 (for further details on cruises and survey methods see Garcias-Bonet and Duarte, 2017; Anton et al., 2018; Duarte et al., 2018; Sea et al., 2018). In addition, stations located in the Central Red Sea were sampled using outboard motor boats on November 2014, February 2016, January 2017, March 2017, and July 2017. The sampled seagrass and mangrove stations encompass a latitudinal range of 10 and 8.9°, respectively. At each station, we collected triplicate sediment samples using cylindrical plastic cores (inner diameter = 9.5 cm, length = 30 cm) containing at least 10 cm of sediment. We standardized the sediment sampling to the top 10 cm because most of the sampled seagrass meadows held thin sediment layers. We collected sediment samples from each seagrass species present at each station. The samples were transported back to the laboratory and processed immediately after collection.

Sediment samples were carefully extruded from the cylindrical cores, and the sediment was cut in three layers: from the surface to 1, 1–5, and 5–10 cm below the sediment surface. Seagrass and mangrove tissues were removed from sediment samples after which sediment samples were dried at 60°C (Ryba and Burgess, 2002) until constant weight.

The sediment bulk density was calculated as the mass of dry solids in a given volume of sediment (expressed in g DW sediment cm^-3). Dried sediments were ground using mortar and pestle, and a sediment subsample (1 g DW sediment) was used to estimate the sediment carbonate and inorganic carbon (C_inorg) concentrations using a calcimeter as described by Müller and Gastner (1971). Sediment bulk density, carbonate concentration, and C_inorg concentration were analyzed on triplicate for each sediment horizon at each station. The remaining sediment was acidified in order to remove carbonates (Hedges and Stern, 1984) before analyzing the organic carbon (C_org) and nitrogen (N) concentrations and their stable isotopic compositions (δ¹³C and δ¹⁵N). Specifically, we added chloride (1M HCl) to each sediment sample until no bubble formation was detectable, then the sediment samples were rinsed, centrifuged and re-dried overnight. The use of a weak HCl solution (up to 2M) has been shown to be free of biases in analysis of δ¹³C and δ¹⁵N in sediments (Kennedy et al., 2005).

Elemental C_org and N concentrations of sediment samples were analyzed with an Organic Elemental Analyzer Flash 200 (Thermo Fisher Scientific, Waltham, MA, United States). The accuracy was <0.2 and <0.1% for C and N, respectively. The δ¹³C and δ¹⁵N analyses of sediment samples were performed by elemental analysis isotope ratio mass spectrometry. Two replicates of C_org and N concentrations and their stable isotopic compositions were analyzed for each sediment horizon. Specifically, analyses of samples collected on November 2014 were performed on a Delta V mass spectrometer with Conflo III coupled to a Costech 4010 elemental analyzer (Thermo Scientific, Waltham, MA, United States) at the UH Hilo Analytical Laboratory (United States). Analyses of samples collected from February 2016 to July 2017 were performed on a Carlo Elba NC1500 (Milan, Italy) elemental analyzer along with a Delta Plus XP (Thermo-Finnigan, Bremen, Germany) mass spectrometer (EA-IRMS) at the Stable Isotope Laboratory of the Andalusian Institute of Earth Sciences (CSIC-UGR, Spain). Commercial CO₂ and N₂ were used as the internal standard for the C and N isotopic analyses. For δ¹³C analysis, we used internal standards of -30.63 and -11.65‰ (V-PDB), and for δ¹⁵N analysis, we used internal standards of -1.02 and +16.01‰ (AIR). The analytical precision was better than ± 0.1‰ for δ¹⁵N and δ¹³C, calculated from standards systematically interspersed in sample batches and after correcting for the daily drift of the mass spectrometer. The standard for reporting C measurements is V-PDB (Vienna-PDB) and for N measurements the standard is atmospheric nitrogen (AIR). The stable isotopic composition is reported as δ values:

where R = ¹³C/¹²C for δ¹³C, and R = ¹⁵N/¹⁴N for δ¹⁵N values.

Then, we calculated the weighted average per replica taking into account the depth of each horizon. Moreover, we calculated inorganic C (C_inorg), organic C (C_org), and N stocks integrated over the top 10 cm taking into account the sediment C_inorg, C_org, and N concentrations and the bulk density for each sediment horizon. We only integrated the stocks over the analyzed top 10 cm of sediment, despite the general recommendation for sediment stock analysis to sample the top 1 m (Macreadie et al., 2014), because our analyses were restricted by the available sediment thickness at seagrass stations. In many reef lagoons, particularly in the southern Red Sea, sampled seagrass meadows were composed of thin sediment layers, often just barely over 10 cm thick, overlaying carbonate rock. It is, therefore, more appropriate, for comparative purposes, to compare the top 10 cm stock than to extrapolate to 1 m, beyond the sediment thickness present in many of the meadows sampled here. The thickness we used, however, is the same as in a recent study on C_org, N, and phosphorous sediment stocks in Danish seagrass meadows (Kindeberg et al., 2018).

We used recently reported stable isotopic compositions of primary producers in the Red Sea (Duarte et al., 2018) to assess their contribution to the organic matter accumulation in seagrass and mangrove sediments. Specifically, we compiled available δ¹³C and δ¹⁵N values of macroalgae blades, seagrass leaves, halophytes leaves, mangrove leaves, and seston sampled along the Red Sea (Duarte et al., 2018).

We complemented the compilation of stable isotopic composition values of primary producers in the Red Sea reported by Duarte et al. (2018) by sampling and analyzing the δ¹³C of microbial mats found in Central Red Sea mangrove stands, which was included in the larger data set. Microbial mats were sampled from the sediment near a mangrove stand at the ‘Ibn Sina field research station and nature conservation area’ in the Central Red Sea (22° 20′ 25.032″ N; 39° 5′ 17.411″ E) in April 2018 by carefully collecting the upper layer with a spatula. The δ¹³C was analyzed by cavity ring-down spectroscopy (CM-CRDS G2201-I, Picarro, Santa Clara, CA, United States) attached to a combustion module (Costech Analytical Technologies, Inc., Valencia, CA, United States) at the Tarek Ahmed Juffali Research Chair in Red Sea Ecology Laboratory (KAUST, Saudi Arabia). Microbial mat samples were dried, acidified and encapsulated in tin boats. We used two CO₂ gas Stable Isotope Calibration Standards UN1956 (δ¹³C = 25 and -40‰, Air Liquide America Specialty Gases, Plumsteadville, PA, United States) and three certified solid C standards from the Reston Stable Isotope Laboratory [United States Geological Survey (USGS), Reston, VA, United States]: USGS62 (Caffeine, δ¹³C = -14.79‰), USGS40 (glutamic acid, δ¹³C = -26.39‰), and USGS41a (L- glutamic acid enriched in ¹³C, δ¹³C = +36.55‰). The analytical precision and accuracy of δ¹³C measurements were ± 0.13 and ± 0.69‰, respectively.

We calculated the difference in δ¹³C and δ¹⁵N between sediments and the overlying vegetation for each station, by subtracting the δ¹³C and δ¹⁵N of primary producers from the δ¹³C and δ¹⁵N of sediments, respectively.

Finally, the relative contribution of different primary producers as potential sources of organic matter in seagrass and mangrove sediments was estimated using Bayesian mixing models (Parnell et al., 2010; Phillips et al., 2014). We run the mixing models separately for each type of sediment and we only included as potential sources those primary producers for which both δ¹³C and δ¹⁵N values were available (i.e., macroalgae blades, seagrass leaves, halophytes leaves, and mangrove leaves).

Differences in C_inorg concentration, C_org concentration, N concentration, C/N ratio, bulk density, δ¹³C, δ¹⁵N, C_org stock, N stock, and C_inorg stock between seagrass and mangrove sediments were tested by the parametric unpaired t-test, when data follow the normality assumption, and the non-parametric Mann–Whitney test, when data did not follow the normality assumption. Data normality was checked by the Shapiro–Wilk test. Relationships between latitude and sediment parameters were assessed with linear regression analysis. Differences in the δ¹³C of seagrass and mangrove sediments and the δ¹³C of the overlying plants were tested by Wilcoxon matched-pairs signed rank test. Statistical analyses were performed using JMP (SAS Institute, Inc., United States) and PRISM (GraphPad Software, Inc., United States) statistical software; while Bayesian mixing models, used to constrain the likely contribution of different primary producers to seagrass and mangrove sediments, were run using the stable isotope mixing model (simmr) package (Parnell, 2016) in RStudio 1.2.1335 (RStudio Team, 2018).

The data set is available in the data repository Pangaea: Garcias-Bonet et al. (2018). Data set on carbon and nitrogen concentrations, stocks and stable isotope compositions (δ13C, δ15N) in Red Sea seagrass and mangrove sediments. PANGAEA. https://doi.pangaea.de/10.1594/PANGAEA.895644.

The C_inorg concentration of seagrass and mangrove sediments was 0.09 ± 0.005 and 0.08 ± 0.007 g C_inorg g DW sediment^-1, respectively (Table 1); with no significant differences between sediment types (Mann–Whitney test, U = 183, p > 0.05).

The C_org and N concentrations of seagrass and mangrove sediments ranged broadly across the Red Sea, particularly for mangrove sediments. In seagrass sediments, C_org and N concentrations ranged from 0.1 to 1.69% DW and from 0.01 to 0.24% DW, respectively. In mangrove sediments, C_org and N concentrations ranged from 0.46 to 6.6% DW and from 0.03 to 0.64% DW, respectively (Figure 2 and Table 1). The C_org concentration was higher in mangrove (1.85 ± 0.42%) than in seagrass (0.59 ± 0.05%) sediments (Mann–Whitney test, U = 71, p < 0.0001). However, seagrass and mangrove sediments did not have significantly different N concentrations (0.11 ± 0.01 and 0.18 ± 0.04%, respectively; Mann–Whitney test, U = 169, p > 0.05). The sediment N concentration increased linearly with that of C_org in seagrass (Y = 0.13X + 0.02, R² = 0.51, p < 0.0001, Figure 3A), and mangrove (Y = 0.08X + 0.02, R² = 0.92, p < 0.0001, Figure 3B) sediments, with significant difference in the slopes between seagrass and mangrove sediments (ANCOVA, F_2,142 = 13.34, p < 0.0001). N concentration tended to be higher, for a given C_org concentration, in seagrass compared to mangrove sediments. This implies significantly higher C/N ratios for mangrove (11.10 ± 0.95) than for seagrass (6.37 ± 0.31) sediments (Figure 2 and Table 1, Mann–Whitney test, U = 22, p < 0.0001). Bulk density was higher in seagrass (1.16 ± 0.04 g DW sediment cm^-3) compared to mangrove (0.96 ± 0.04 g DW sediment cm^-3) sediments (Figure 2 and Table 1, Unpaired t-test, t₄₅ = 3.182, p < 0.01).

There was, as hypothesized, a trend for N concentration to decrease from south to north (Figure 4) in seagrass (Y = -0.005X + 0.22, R² = 0.08; p < 0.01), although this relationship was not significant for mangrove sediments (R² = 0.05; p > 0.05), which showed high variability in sediment N concentrations across mangrove stands for any one latitude (Figure 4). However, C_org concentration did not show any latitudinal trend in seagrass (R² = 0.01; p > 0.05) and mangrove (R² = 0.01; p > 0.05) sediments. Yet, the C/N ratio increased from north to south for both seagrass (Y = 0.27X + 0.28, R² = 0.23; p < 0.0001) and mangrove (Y = 0.75X - 5.52, R² = 0.58; p < 0.0001) sediments (Figure 4).

Seagrass sediments contained higher C_inorg stocks integrated over the top 10 cm than mangrove sediments (Figure 5 and Table 1, Mann–Whitney test, U = 143, p < 0.05). Mangrove C_org stocks exceeded, on average, twofold those in seagrass meadows (Figure 5 and Table 1, Mann–Whitney test, U = 8, p < 0.0001). However, mangrove sediments contained similar N stocks than seagrass sediments (Figure 5 and Table 1, Mann–Whitney test, U = 136, p > 0.05). Sediment N stocks decreased from south to north in seagrass (Y = -8.81 + 342.4, R² = 0.31; p < 0.0001) and in mangrove (Y = -13.25X + 456.6, R² = 0.56; p < 0.0001) sediments (Figure 4). However, C_org stocks did not show any latitudinal trend in seagrass (R² = 0.06; p > 0.05) and mangrove (R² = 0.05; p > 0.05) sediments.

The isotopic signature of C_org in seagrass sediments was enriched in ¹³C relative to that of mangrove sediments (δ¹³C = -13.36 ± 0.40 and -21.52 ± 0.57‰, respectively; Mann–Whitney test, U = 7, p < 0.0001), while sediment N isotopic signatures were comparable between the two habitats (δ¹⁵N = 0.91 ± 0.24 and 1.21 ± 0.33‰ in seagrass and mangrove, respectively; Figure 6 and Table 1). C_org isotope signatures in seagrass sediments were independent of latitude in seagrass and mangrove sediments, while N isotopic composition became lighter from south to north in both seagrass (Y = -0.21X + 5.99, R² = 0.26; p < 0.0001) and mangrove (Y = -0.24X + 6.78, R² = 0.16; p < 0.01) sediments (Figure 4).

The δ¹³C (± SD) of Red Sea primary producers compiled here ranged broadly from -26.59 ± 1.06‰ in mangrove leaves to -7.73 ± 1.48‰ in seagrass leaves, with intermediate values for seston, halophytes leaves, microbial mats, and macroalgae blades (Table 2). Similarly, the δ¹⁵N (± SD) of Red Sea primary producers ranged from 0.20 ± 2.94‰ in seagrass leaves to 4.19 ± 2.91‰ in halophytes leaves, while mangrove leaves and macroalgae blades had similar mean values (1.72 ± 2.30 and 1.74 ± 1.59‰, respectively; Table 2). No δ¹⁵N data available for seston and microbial mats.

When comparing the stable isotope compositions of sediments and the overlying vegetation found at each station, seagrass sediments showed lighter C_org isotopic composition than the leaves of overlying seagrass plants (mean δ¹³C = -13.57 ± 0.40 and -7.69 ± 0.16‰, respectively; Wilcoxon matched-pairs signed rank test, p < 0.0001), resulting in an average shift of -6.17 ± 0.4‰ between the two compartments. Mangrove sediments tended to have isotopically heavier δ¹³C than the overlying mangrove leaves at each station (mean δ¹³C = -21.52 ± 0.57 and -26.64 ± 0.20‰, respectively; Wilcoxon matched-pairs signed rank test, p < 0.0001), resulting in an average shift of +5.12 ± 0.45‰ between the two compartments (Supplementary Figures S1, S2). However, the δ¹³C of seagrass sediments did not differ significantly from that of overlying macroalgae at each station with mean δ¹³C = -13.44 ± 0.45‰ (Wilcoxon matched-pairs signed rank test, p > 0.05; difference -0.93 ± 0.96‰).

Seagrass and mangrove sediments and overlying plant canopies at each station had similar mean N isotopic compositions (δ¹⁵N = 0.91 ± 0.24 and 1.21 ± 0.33‰ in seagrass and mangrove sediments, respectively; δ¹⁵N = -0.25 ± 0.56 and 1.37 ± 0.52‰ in seagrass and mangrove leaves, respectively; Supplementary Figure S1).

Mean δ¹³C and δ¹⁵N values of seagrass and mangrove sediments laid within the convex hull defined by δ¹³C and δ¹⁵N mean values of primary producers (Figure 7). The Bayesian mixing models identified different primary producers as main sources of organic matter in seagrass and mangrove sediments (Figure 8 and Table 3). Seagrass leaves and macroalgae blades were the major potential contributors in seagrass sediments (mean ± SD proportion = 0.43 ± 0.08 and 0.37 ± 0.12, respectively); while mangrove and halophytes leaves had a minor contribution. Mangrove leaves were the main potential contributors in mangrove sediments (mean ± SD proportion = 0.56 ± 0.08), with minor contributions of macroalgae blades, seagrass leaves and halophytes leaves (Figure 8 and Table 3).

Seagrass and mangrove sediments in the Red Sea are carbonate-rich compared to previous reports (Mazarrasa et al., 2015; Saderne et al., 2018). Red Sea seagrass C_inorg stocks in the top 10 cm of sediment (10861 ± 686.6 g C m^-2) are higher than previous global seagrass estimates (6540 ± 240 g C m^-2 in 10 cm of sediment, Mazarrasa et al., 2015) and previous estimates of central Red Sea seagrass sediments (5700 ± 300 g C m^-2 in 10 cm of sediment, Saderne et al., 2018). Mangrove C_inorg stocks in the top 10 cm of sediment (8644 ± 882.9 g C m^-2) are equally large, and comparable to previous data from the central Red Sea (8900 ± 400 g C m^-2 in 10 cm of sediment, Saderne et al., 2018). However, seagrass and mangrove sediments were depleted in C_org and N relative to sediments in these coastal habitats elsewhere (Donato et al., 2011; Fourqurean et al., 2012; Atwood et al., 2017), consistent with previous reports pointing at low concentrations of C_org in Central Red Sea mangrove (Almahasheer et al., 2017) and seagrass (Serrano et al., 2018) sediments. The low concentration of C_org in Red Sea Blue Carbon habitats is partially attributable to the absence of rivers in the Red Sea, thereby limiting the supply of allochthonous C to these sediments (Almahasheer et al., 2017; Serrano et al., 2018). Indeed, arid marine ecosystems are poorly represented in global syntheses of the C_org stocks in Blue Carbon habitats (Almahasheer et al., 2017). Red Sea mangrove C_org stocks integrated over the top 10 cm of sediment (14.5 ± 1.4 Mg C ha^-1) are lower than mean values for mangrove sediments at the latitudinal range encompassed by the Red Sea (20 to 31 Mg C ha^-1 in 10 cm of sediment, Atwood et al., 2017). Similarly, Red Sea seagrass C_org stocks integrated over the top 10 cm of sediment (7.2 ± 0.4 Mg C ha^-1) are lower than mean global values (ranging from 11.5 to 82.9 Mg C ha^-1 in 10 cm of sediment, Fourqurean et al., 2012). However, these global values are heavily skewed toward carbon-dense Mediterranean Posidonia oceanica meadows, and therefore likely overestimate global mean C_org stocks in seagrass sediments (e.g., Lavery et al., 2013). Yet, our estimates on seagrass C_org stocks are twofold higher than recently reported stocks for seagrasses in the Central Red Sea (3.4 ± 0.3 Mg C ha^-1 in 10 cm of sediment, Serrano et al., 2018). These results confirm that the general tendency for mangrove sediments to be more organic carbon-rich than seagrass sediments (McLeod et al., 2011; Duarte et al., 2013) also applies to the arid sediments of the Red Sea, lacking riverine inputs. Since available data are reported as stocks over the top 1 m of sediment, we recalculated the fraction corresponding to 10 cm of sediment to allow comparison. This may generate uncertainties, as this assumes that the C_org content is relatively uniform, but is also consistent with the fact that literature estimates for seagrass sediments were mostly based on extrapolating from sediment corers well below 1 m (Fourqurean et al., 2012; Mazarrasa et al., 2015).

C_org and N sediment stocks were closely coupled, reflecting the stoichiometric compositions in overlying vegetation and suggesting that N supply constrains sediment C_org stocks. However, mangrove sediments were depleted in N, relative to C_org, compared to seagrass sediments. This is consistent with the generally higher C/N ratio of mangrove leaves compared to seagrass leaves (Cebrián and Duarte, 1995). As the C/N ratio of organic matter is a robust predictor of decomposition rates (Enríquez et al., 1993), high C/N ratios in mangrove leaves and sediments, compared to those in seagrass meadows, imply lower decomposition rates and, hence, higher accumulation rates in mangrove sediments relative to seagrass sediments.

The results confirmed that the south to north gradient of decreasing nutrient inputs characteristic of the Red Sea (Raitsos et al., 2015) is reflected in a decrease in sediment N stocks and an increase in sediment C/N ratios from south to north. The gradient is also reflected in a tendency for the δ¹⁵N isotopic ratio in sediments to decrease from south to north. This gradient suggests a shift from nitrate and nitrite delivered from the incoming Indian Ocean as N sources in the south, to nitrogen fixation, characterized by lower δ¹⁵N (Peterson and Fry, 1987; Papadimitriou et al., 2005; Garcias-Bonet et al., 2016), in the absence of allochthonous sources of reactive N, toward the north. However, there was no such latitudinal trend in C_org stocks in seagrass and mangrove sediments, indicating that the higher nutrient supply in the southern Red Sea does not result in higher sediment C_org stocks. This could be explained by lower sediment C/N ratios in the Southern Red Sea resulting in higher decomposition rates than in the comparatively N-depleted sediments in the Northern Red Sea.

Mean δ¹³C of Red Sea primary producers were reported to range from highly negative for mangrove leaves to the heaviest values for seagrass leaves, with intermediate values for halophyte leaves, macroalgae blades, and seston (Duarte et al., 2018). In addition, sampled microbial mats showed intermediate mean δ¹³C (-15.03 ± 0.25‰), similar to previously reported values for these communities (between -14.4 and -13‰, Oakes and Eyre, 2014).

The difference in δ¹³C between seagrass and mangrove sediments and their leaves suggests that other sources of carbon than habitat-forming primary producers contribute to the sediment C_org stocks. Stable isotope mixing models showed that in Red Sea seagrass sediments both seagrass leaves and macroalgae blades were the major contributors to the accumulated organic matter, accounting for 80% together; while in Red Sea mangrove sediments, mangrove leaves were the major contributors, with a 56%. The important contribution of sources other than the habitat-forming primary producers to sediment organic matter is consistent with previous results. For instance, Kennedy et al. (2010) reported that half of the C_org in seagrass sediment stocks derived from other primary producers than seagrass. Other potential sources not included in the mixing models could also contribute to the accumulated organic matter in seagrass and mangrove sediments. For instance, previous results for mangrove sediments elsewhere, pointed at an important role of microbial mats as a source of C_org to underlying sediments and mangrove food webs (Newell et al., 1995; Bouillon et al., 2008; Kristensen et al., 2008). Moreover, the high correlation observed between pairs of sources in both types of sediment indicated that the mixing model could not successfully distinguish between them. This is the case of macroalgae blades and seagrass leaves in seagrass sediments, which were strongly negatively correlated (-0.96); indicating that if macroalgae blades contributed to seagrass sediments at the top of their outcome probability range, most likely seagrass leaves contributed at the bottom of their probability range. Similarly, in mangrove sediments, we observed two pairs of sources strongly negatively correlated: macroalgae blades – seagrass leaves (-0.86) and halophytes leaves – mangrove leaves (-0.80).

Nitrogen sources to sediments include nitrate advected with the inflow of Indian Ocean waters (δ¹⁵N = + 6‰, Brandes et al., 1998), and N fixation (δ¹⁵N near 0, Peterson and Fry, 1987). The δ¹⁵N of sediment N stocks in the south is consistent with a dominance of nitrate as the main source of N, while the δ¹⁵N of sediment N stocks in the north points at a prevalence of N fixation. However, due to diagenetic processes, the isotopic signature of sediment organic matter might change during decomposition (Macko et al., 1994). Therefore, the identification of potential sources and contributions to seagrass and mangrove sediments based on stable isotopes needs to be interpreted with care.