The sedimentary organic carbon (Corg) stores of seagrass meadows – often referred to as “blue carbon” – can vary among seagrass species and habitats, with reports of up to 18-fold differences (Lavery et al., 2013). Ambiguity remains in the relative importance of the depositional environment and species characteristics contributing to this variability. Seagrasses occur in a variety of coastal habitats, ranging from highly depositional environments to highly exposed and erosional habitats (Carruthers et al., 2007). Since seagrass species differ in their biomass and canopy structure, and occur in a variety of habitat types, this raises the question of whether mud content can be used to predict Corg content within coastal sediments, or whether the species composition will significantly influence the soil Corg stores independently of the geomorphological nature of the habitat.

Geomorphological settings (i.e., topography and hydrology), soil characteristics (e.g., mineralogy and texture) and biological features (e.g., primary production and remineralization rates) control soil Corg storage in terrestrial ecosystems (Amundson, 2001; De Deyn et al., 2008; Jonsson and Wardle, 2009) and in mangrove and tidal salt marshes (Donato et al., 2011; Adame et al., 2013; Ouyang and Lee, 2014). While it is clear that habitat interactions have a large influence on stores of soil Corg, our understanding of the factors regulating this influence in seagrass meadows is limited (Nellemann et al., 2009; Duarte et al., 2010; Serrano et al., 2014).

The accumulation of Corg in seagrass meadows results from several processes: accretion (autochthonous plant and epiphyte production, and trapping of allochthonous Corg; Kennedy et al., 2010), erosion (e.g., export; Romero and Pergent, 1992; Hyndes et al., 2014) and decomposition (Mateo et al., 1997). Previous studies demonstrate that both autochthonous (e.g., plant detritus and epiphytes) and allochthonous (e.g., macroalgae, seston and terrestrial matter) sources contribute to the Corg pool in seagrass soils (Kennedy et al., 2010; Watanabe and Kuwae, 2015). Plant net primary productivity is a key factor controlling the amount of Corg potentially available for sequestration in seagrass ecosystems (Serrano et al., 2014), but the depositional environment is an important factor controlling Corg storage in coastal habitats (De Falco et al., 2004; Lavery et al., 2013).

Previous studies have shown a large variation in Corg stores among morphologically different seagrass species (Lavery et al., 2013; Rozaimi et al., 2013). Also, that Corg accumulates more in estuaries compared to coastal ocean environments (estimated at 81 and 45 Tg Corg yr-1, respectively; Nellemann et al., 2009). This is due largely to estuaries being highly depositional environments, receiving fine-grained particles from terrestrial and coastal ecosystems which enhance Corg accumulation (i.e., silt and clay sediments retain more Corg compared to sands; Keil and Hedges, 1993; Burdige, 2007) and preservation (i.e., reducing redox potentials and remineralization rates; Hedges and Keil, 1995; Dauwe et al., 2001; Burdige, 2007; Pedersen et al., 2011). The inputs of seagrass-derived Corg in the sedimentary pool could break the linear relationship among mud (i.e., silt and clay particles) and Corg contents typically found in terrestrial (Nichols, 1984; McGrath and Zhang, 2003) and marine sedimentary environments (Bergamaschi et al., 1997; De Falco et al., 2004). However, the amount of Corg that can be associated with mud particles is limited (Hassink, 1997), which could lead to a poor relationship between mud and soil Corg contents. Also, other factors found to play a key role in controlling soil Corg accumulation in terrestrial and coastal ecosystems, such as chemical stabilization of organic matter (Percival et al., 1999; Burdige, 2007), carbon in microbial biomass (Sparling, 1992; Danovaro et al., 1995), and soil temperature (Pedersen et al., 2011), could also influence Corg storage in seagrass meadows.

A significant relationship between mud and Corg contents would allow mud to be used as a proxy for Corg content, thereby enabling robust scaling up exercises at a low cost as part of blue carbon stock assessments. Furthermore, since most countries have conducted geological surveys within the coastal zone to determine sediment grain size, a strong, positive relationship between mud and Corg contents would allow the development of geomorphology models to predict blue carbon content within seagrass meadows, dramatically improving global estimates of blue carbon storage. The purpose of this study was therefore to test for relationships between Corg and mud contents within seagrass ecosystems and adjacent bare sediments.

Data were compiled from a number of published and unpublished studies from Australia and Spain, in seagrass meadows across diverse habitats (Table 1). The study sites encompass monospecific and/or mixed meadows from a variety of temperate and tropical seagrass species of the genera Posidonia, Amphibolis, Zostera, Halophila and Halodule, and adjacent bare sediments, while including a variety of depositional environments (from estuarine to exposed coastal areas encompassing different water depths, from intertidal to the deep limit of seagrass distribution; Table 1). Data from 100 cores (79 from seagrass meadows and 21 from bare sediments) on sediment grain size, organic carbon (Corg) content and stable carbon isotope signatures of the Corg (δ13C) were explored in this study (N= 1345).

Sediment cores were sampled by means of percussion and rotation, or vibrocoring (ranging from 10 to 475 cm long). The core barrels consisted of PVC or aluminium pipes (50 to 90 mm inside diameter) with sharpened ends to cut fibrous material and minimize core shortening (compression) during coring (Serrano et al., 2012, 2014). All cores were sealed at both ends, transported vertically to the laboratory and stored at 5 ∘C before processing.

The cores were sliced at regular intervals, each slice and/or sample was weighed before and after oven drying to constant weight at 70 ∘C (DW), and subsequently sub-divided for analysis. The Corg elemental and isotopic composition of the organic matter was measured in milled subsamples from several slices along the cores. The sediment core sub-samples were acidified with 1 M HCl, centrifuged (3500 RPM; 5 min) and the supernatant with acid residues was removed using a pipette, then washed in deionized water, centrifuged again and the supernatant removed. The residual samples were re-dried (70 ∘C) before carbon elemental and isotopic analyses. The samples were encapsulated and the organic carbon elemental and isotopic composition was analyzed using an elemental analyzer interfaced with an isotope ratio mass spectrometer. Percentage Corg was calculated for the bulk (pre-acidified) samples. Carbon isotope ratios are expressed as δ values in parts per thousand (‰) relative to VPDB (Vienna Pee Dee Belemnite). For sediment grain size analysis, a Coulter LS230 laser-diffraction particle analyzer was used following digestion of the samples with 10 % hydrogen peroxide. The mud content in the sediments (< 63 µm) was determined, and expressed as a percentage of the bulk sample.

Pearson correlation analysis was used to test for significant relationships among Corg and mud contents, and Corg and δ13C signatures. Correlations between the variables studied were tested among seagrass species (nine categories) and bare sediments, seagrass genera (four categories), habitat geomorphology (coastal and estuarine habitats) and soil depth (in 1 to 10 and 11 to 110 cm thick deposits).

The soil organic carbon (Corg) and mud contents varied within the seagrass meadows and bare sediments studied in Australia and Spain. The soil Corg and mud contents were higher in seagrass meadows (average ± SE, 1.5 ± 0.2 % and 18 ± 2.4 %, respectively) compared to bare sediments (0.6 ± 0.1 % and 10.8 ± 1.2 %, respectively; Table 2). On average, seagrass meadows of the genera Amphibolis and Posidonia contained higher soil Corg (1.6 ± 0.1 %) and lower mud (7.2 ± 0.4) than meadows of Halophila, Halodule and Zostera (1.2 ± 0.2 % and 34.9 ± 5.4 %, respectively; Table 2). Overall, carbon isotopic ratios from sedimentary organic matter (δ13C) were similar between seagrass soils and bare sediments (-17.6 ± 0.3 ‰ and -17.3 ± 0.2 ‰, respectively). The Corg in soils from Posidonia and Amphibolis meadows were 13C-enriched (-15.5 ± 0.3 ‰) compared with seagrass soils from Halophila, Halodule and Zostera meadows (-20.7 ± 0.4 ‰; Table 2). The Corg content in soils from estuarine and coastal habitats were similar, while mud content in estuarine sediments was higher and δ13C values depleted when compared to coastal habitats (Table 2).

The relationships between the variables studied (i.e., %Corg, %mud, and δ13C signatures of sedimentary Corg) among different species and habitat geomorphologies, and among different soil depths were explored in Figs. 1 to 3, and Table 3. When accounting for the whole data set (up to 475 cm long cores), the Corg content increased with increasing mud content in bare sediments (R2= 0.78) and at species level, except for Posidonia oceanica (i.e., Corg content decreased with increasing mud content; R2= 0.15) and Amphibolis griffithii (i.e., no relationship was found, R2 = 0.05; Table 3). Although most of the correlations at species level were significant, they only explain 2 to 39 % of the variance in trends described, except for Halophila ovalis (91 %; Table 3). In particular, Posidonia meadows (P. australis, P. sinuosa and P. oceanica) had the lower correlation values (R2 ranged from 0.02 to 0.15). When combining mud and Corg contents in seagrass meadows of the colonizing and opportunistic genera Halophila, Halodule and Zostera (Kilminster et al., 2015), a relatively high correlation was found (R2= 0.56; Fig. 1), while soil Corg and mud contents in persistent genera were only slightly positively correlated in combined Amphibolis spp. and not correlated in Posidonia spp. meadows (Fig. 1).

The relationships between soil Corg and mud contents within different core depths (from 1 to 10 cm thick deposits, and from 11 to up to 110 cm thick deposits) for bare sediments and each group of seagrass species were explored in Fig. 2. The Corg content increased with increasing mud content in bare sediments for both 1 to 10 cm thick (R2= 0.74) and 11 to 110 cm thick (R2= 0.81) soils. When combining mud and Corg contents in seagrass meadows of the genera Halophila, Halodule and Zostera, a higher correlation was found for deeper core sections (11 to 110 cm-thick; R2 = 0.74) compared to top core sections (1 to 10 cm-thick; R2= 0.17). For combined Amphibolis and Posidonia species, soil Corg and mud contents were only slightly positively correlated in deeper Amphibolis spp. sections (11 to 110 cm-thick; R2= 0.23) and not correlated in Posidonia spp. meadows (Fig. 2). The classification of habitats based on geomorphology (i.e., coastal and estuarine) showed a lack of correlation between soil Corg and mud contents in coastal ecosystems, and a poor correlation in estuarine ecosystems (R2= 0.14; Fig. 3 and Table 3).

The relationships between soil %Corg and δ13C signatures were poor for all individual Amphibolis and Posidonia species studied (R2 ranging from 0.09 to 0.3; Table 3), and for combined Amphibolis spp. (Fig. 1), with a tendency of Corg-rich soils being enriched in 13C (Fig. 1). In contrast, %Corg and δ13C signatures were not correlated in any of the small and fast-growing Halodule, Zostera, Halophila meadows studied (Table 3), neither individually nor when combined (Fig. 1 and Table 3). A lack of correlation between soil %Corg and δ13C signatures was also found in bare sediments adjacent to seagrass meadows (Fig. 3 and Table 3).

Overall mud content is a poor predictor of soil Corg in seagrass meadows and care should be taken in its use as a cost-effective proxy or indicator of Corg for scaling-up purposes in the emerging field of blue carbon science. Although we describe some promise for opportunistic and early colonizing Halophila, Halodule and Zostera meadows (i.e., mud content explained 34 to 91 % of variability in Corg content) and in bare sediments adjacent to seagrass meadows (explaining 78 % of the variability), mud is not a universal proxy for blue carbon content and therefore should not be applied generally across all seagrass habitats. In particular, mud content only explained 5 to 32 % of soil Corg content in Amphibolis spp. meadows and 2 to 15 % of soil Corg content in Posidonia spp. meadows, and therefore, mud content is not a good proxy for blue carbon content in these meadows.

A tenet of carbon cycling within the coastal ocean is that fine-grained sediments (i.e., mud) have higher Corg contents. The positive relationship found between mud and Corg contents in coastal bare sediments (explaining 78 % of the variability) is in agreement with previous studies (e.g., Bergamaschi et al., 1997; De Falco et al., 2004), and is related to their larger surface areas compared to coarse-grained sediments, providing more binding sites for Corg on the surface of minerals (Keil and Hedges, 1993; Mayer, 1994a, b; Galy et al., 2007; Burdige, 2007). In addition, the predominance of fine sediments reduces oxygen exchange and results in low sediment redox potentials and remineralization rates, contributing to the preservation of sedimentary Corg after burial (Hedges and Keil, 1995; Bergamaschi et al., 1997; Dauwe et al., 2001; Burdige 2007; Pedersen et al., 2011). However, the maximum capacity of a given soil to preserve Corg by their association with clay and silt particles is limited (i.e., mud-Corg saturation; Hassink, 1997). The results obtained showed that bare sediment samples with relatively high Corg contents (i.e., > 4 % Corg) and relatively low mud contents were also 13C-depleted (Fig. 1), suggesting significant contributions of soil Corg from allochthonous sources (e.g., terrestrial and sestonic; Kennedy et al., 2010). This could have disrupted the correlation found between soil Corg and mud contents in the bare sediments studied.

Mud is not a universal proxy for soil Corg content in seagrass meadows, which could be mainly explained by additional inputs of seagrass-derived Corg and/or allochthonous Corg to the sedimentary Corg pool, obviating the linear relationship between mud and Corg contents found in the absence of vegetation. The δ13C values indicated that both seagrass-Corg and non-seagrass-derived Corg (i.e., epiphytes, algae, seston or terrestrial matter) were buried in the soils of all studied meadows, but are consistent with a model of increasing capture of seagrass-derived Corg at meadows formed by persistent, high-biomass seagrasses (i.e., genera Posidonia and Amphibolis) relative to opportunistic, low-biomass seagrasses (i.e., genera Halophila, Halodule and Zostera).

On one hand, the soil δ13C signatures measured in these long-living and large seagrass meadows (averaging -15 ± 0.2 ‰ in both cases) were closer to the δ13C signatures of Posidonia and Amphibolis tissues (ranging from -8 to -14 ‰; Hyndes and Lavery, 2005; Hindell et al., 2004; Cardona et al., 2007; Fourqurean et al., 2007; Collier et al., 2008; Kennedy et al., 2010; Hanson et al., 2010; Serrano et al., 2016) than to δ13C values of algae or terrestrial organic matter (ranging from -18 to -32 ‰; e.g., Smit et al., 2006; Cardona et al., 2007; Kennedy et al., 2010; Hanson et al., 2010; Deudero et al., 2011). The poor relationship between mud and soil Corg contents in Amphibolis soils could be explained by samples with relatively high Corg contents (i.e., > 2.5 % Corg) and relatively low mud contents, as a result of both the contribution of seagrass-derived Corg (i.e., 13C-enriched) and Corg from allochthonous sources (i.e., 13C-depleted; Fig. 1). In Posidonia soils, the poor relationship between mud and soil Corg contents could be explained by samples with relatively high Corg contents (i.e., > 10 % Corg) and relatively low mud contents, as a result of the contribution of seagrass-derived Corg (i.e., 13C-enriched; Fig. 1). The contribution of seagrass-derived Corg (i.e., root, rhizome and sheath detritus) in Posidonia soils play a much larger role than the accumulation of fine, organic-rich allochthonous particles.

On the other hand, the soil δ13C signatures measured in Halodule, Halophila and Zostera meadows (averaging -21 ± 0.4 ‰) were more similar to δ13C values of algae or terrestrial organic matter than to δ13C values of their seagrass tissues (ranging from -10 and -14 ‰; e.g., Hemminga and Mateo, 1996; Kennedy et al., 2010; Hanson et al., 2010). The positive relationship between mud and soil Corg contents in Halodule, Halophila and Zostera soils could be explained by their relatively high mud content and 13C-depleted Corg, indicating that allochthonous Corg inputs and mud content play a major role in soil Corg accumulation in these opportunistic and early-colonizing seagrasses. However, the relatively high Corg contents found with relatively low mud contents (i.e., mud-Corg saturation) disrupted the correlation found between soil Corg and mud contents in these meadows (Corg > 1 % in samples with 0–20 % mud; Corg > 2 % in samples with 20–70 % mud and Corg > 3.5 in samples with 70–100 % mud; Fig. 1).

The results obtained showed a tendency for high-biomass and persistent meadows (i.e., Posidonia and Amphibolis) to accumulate higher Corg stores and seagrass-derived Corg compared to ephemeral and low-biomass meadows (i.e., Halophila, Halodule and Zostera), suggesting that factors (biotic and abiotic) affecting the production, form and preservation of Corg within habitats exert a significant influence on soil Corg content (Lavery et al., 2013; Serrano et al., 2014, 2016). The above- and below-ground biomass in meadows of the genus Posidonia (averaging 535 and 910 g DW m-2, respectively) is up to 2-fold higher than in Amphibolis meadows (averaging 641 and 457 g DW m-2, respectively) and 4 to 18-fold higher than in small and opportunistic seagrasses of the genera Halophila, Halodule and Zostera (125 and 49 g DW m-2, on average; respectively; Duarte and Chiscano, 1999; Paling and McComb, 2000). Indeed, larger seagrasses tend to have larger and more persistent rhizomes, constituted by more refractory forms of Corg, more prone to be preserved in soils than simpler, more labile forms of Corg such as seston and algal detritus which are more suitable to experience remineralization during early diagenesis (Henrichs, 1992; Burdige, 2007). In addition, the larger size of detritus within Amphibolis and Posidonia meadows compared to Halophila, Halodule and Zostera meadows could also contribute to the larger accumulation of Corg in the former, since decay rates of seagrass detritus increase with decreasing particle size due to larger surfaces available for microbial attack (Harrison, 1989). Differences in above- and below-ground biomass and recalcitrance between Posidonia and Amphibolis spp. could explain the larger contribution of seagrass-derived Corg (i.e., 13C-enriched) in the former, thereby obviating the linear relationship between mud and Corg contents (Fig. 1).

The soil Corg content tends to decrease with soil depth and ageing in seagrass ecosystems (e.g., Serrano et al., 2012), thereby the persistence of discrete organic detritus within upper soil horizons could lead to organic matter concentrations above those levels explained by the association with clay and silt particles, as previously demonstrated for terrestrial soils (Mayer and Xing, 2001; Gami et al., 2009). The organic matter preserved in most marine sediments is intimately associated with mineral surfaces (i.e., selective preservation by sorption of organic matter into minerals; Keil et al., 1994) and therefore the correlation between soil Corg and mud contents in seagrass meadows could vary as a function of soil depth and ageing. The results obtained show that soil depth is not an important factor when attempting to predict soil Corg content based on mud content in bare sediments (i.e., R2 > 0.74 for all core depths explored; 1 to 110, 1 to 10, and 11 to 110 cm thick; Fig. 2). However, a clearer pattern appeared when exploring the correlation between soil Corg and mud contents in top 10 cm and within 11–110 cm soil depths of combined Halodule, Halophila and Zostera species (R2= 0.17 and R2= 0.74, respectively). These results suggest that the relatively small below-ground biomass of these species (i.e., organic detritus) only has an impact on the expected positive correlation between soil Corg and mud content within the top 10 cm, while the correlation for deeper soil depths (11–110 cm) improved (R2= 0.74) compared to the whole data set (1 to 110 cm thick; R2= 0.56). For combined Amphibolis and Posidonia species, the results obtained show that soil depth is not an important factor when attempting to predict soil Corg content based on mud content (i.e., R2 < 0.2 in all cases; 1 to 110, 1 to 10, and 11 to 110 cm thick; Fig. 2). These results suggest that the relatively large below-ground biomass of these species (i.e., organic detritus) has an impact on the expected positive correlation between soil Corg and mud content within all depths studied.

Habitat conditions in seagrass meadows not only influence the amount of Corg accumulation through detrital plant inputs, but the capacity of the plant canopies to retain particles (Gacia et al., 1999). The amount of fine suspended particles available for burial varies among sites, driven by geomorphological features (e.g., run-off, hydrodynamic energy and water depth), while meadow structure (i.e., density, cover and morphology of the canopy) constrains their capacity to accumulate sediment particles (Hendriks et al., 2010; Peralta et al., 2008). Although the number of cores and species studied in coastal and estuarine ecosystems was unbalanced (i.e., Amphibolis and Posidonia dominate in coastal habitats and Halophila, Halodule, Zostera dominate in estuarine habitats), the lack of, or poor correlations found within estuarine and coastal ecosystems, precludes the general use of mud as a predictor of blue carbon content based on habitat geomorphology (Fig. 3). Seagrass meadows and bare sediments in environments conducive for depositional processes (i.e., estuaries) accumulated up to 4-fold higher amounts of mud compared to other coastal ecosystems, but the saturation of mud with Corg and the large contribution of seagrass detritus into the sedimentary Corg pool (13C-enriched soils) in some study sites disrupted the positive relationship expected between mud and soil-Corg contents. In estuarine ecosystems, soil Corg originated from both mud inputs linked to allochthonous-Corg via deposition from upstream transport (e.g., Aller, 1998) and seagrass inputs (i.e., in samples with Corg > 5 %; Fig. 3). The insignificant relationship between mud and soil Corg contents in coastal habitats could be explained by their relatively low mud content and the accumulation of seagrass-derived Corg, in particular in samples with Corg > 5 % (Fig. 3).

In sum, mud is not a universal proxy for blue carbon content in seagrass ecosystems and should not be applied generally across all habitat and vegetation types. Overall, the positive relationship between mud and Corg contents found in bare sediments and in opportunistic and/or low biomass seagrass meadows (i.e., genera Zostera, Halodule and Halophila) allow mud to be used as a proxy for Corg content in these ecosystems, thereby enabling robust scaling up exercises (i.e., benefiting from existing geological surveys and models) at low cost as part of blue carbon stock assessment programs. However, mud content is not a good predictor of Corg content in highly productive meadows such as those constituted by P. oceanica in the Mediterranean Sea and P. australis, P. sinuosa and Amphibolis spp. in Australia. Analyses of soil grain size (i.e., %mud) could constitute a relatively cheap method to estimate soil organic carbon content in seagrass ecosystems, particularly dry and wet sieving using standard geological sieves (Erftemeijer and Koch, 2001). These could be used to cheaply quantify mud content as a proxy for carbon, particularly in student projects, citizen science and in countries where funding for science is limited and they do not have access to higher technology methods or cannot afford to pay for analysis. In addition, since most countries have conducted geological surveys within the coastal zone to determine sediment grain size (e.g., Passlow et al., 2005), a strong, positive relationship between mud and Corg contents could allow the development of geomorphology models to predict blue carbon content within seagrass meadows, dramatically improving global estimates of blue carbon storage. Indeed, maps of soil grain-size could be obtained using remote sensing (Rainey et al., 2003; De Falco et al., 2010), opening new opportunities for scaling exercises.

Previous studies suggested that the relationship between organic matter and the sediment matrix is best seen with clay-sized fractions (< 0.004 mm; Bergamaschi et al., 1997; De Falco et al., 2004). However, the grain size cut-off selected in this study (mud, < 0.063 mm) is more representative of the bulk soil and their Corg content (Pedrosa-Pàmies et al., 2013) and therefore a higher correlation is expected when comparing bulk soil Corg with a larger and more representative fraction of the sediment (i.e., including the silt fraction, 0.004–0.063 mm, also provides binding sites for Corg; Burdige, 2007). Other biological, chemical and geological factors not explored in detail in this study may also play a key role in Corg storage, and ultimately in the relationship between soil Corg and mud contents. For example, the effects of habitat geomorphology (e.g., hydrodynamic energy, terrestrial mud and Corg inputs, export of seagrass biomass) and species identity (e.g., variation in terms of productivity, oxygen exposure and recalcitrance of Corg stores, and plant influence on sediment retention) within both coastal and estuarine environments, are among the factors identified in this study which might explain significant variation in the Corg stores of meadows in relatively similar exposure conditions (Serrano et al., 2016). Other factors found to play a key role in controlling soil Corg accumulation in terrestrial ecosystems, such as chemical stabilization of organic matter (Percival et al., 1999; Galy et al., 2008) and microbial biomass carbon (Danovaro et al., 1994), could also influence Corg storage in seagrass ecosystems. Further studies are needed to identify the influences of these other factors on Corg storage in seagrass meadows, and in addition to the mud content, other characteristics should be taken into account when attempting to obtain robust estimates of Corg stores within coastal areas.

The raw data compiled for this study was published in ECU Research Online Portal (10.4225/75/56c55ab91d417).