Geophysical constraints for organic carbon sequestration capacity of Zostera marina seagrass meadows and surrounding habitats

Abstract

To elucidate the factors determining the organic carbon (OC) sequestration capacity of seagrass meadows, the distribution of OC and the fraction of seagrass-derived OC in sediments of the temperate cosmopolitan seagrass Zostera marina meadows and surrounding habitats were investigated in relation to physical properties of sedimentary materials. On average, seagrass meadow sediments showed OC levels twofold higher than other shallow nearshore habitats. However, offshore sediments often showed greater OC concentrations than average seagrass meadow sediments. According to estimations of OC sources based on carbon isotope ratios, 8-55% and 14-24% of OC in nonestuarine seagrass meadow sediments and < 30 m deep offshore sediments, respectively, were assigned to seagrass origin. The OC concentration in seagrass meadow and offshore sediments closely correlated to the specific surface area (SSA) of sediment (r 2 5 0.816 and 0.755, respectively; p < 0.0001),with an average OC loading per sediment surface area of approximately 60 lmol m 22. In seagrass meadow sediments, the fraction of seagrass-derived OC was also greater in samples with a larger SSA, and the seagrassderived OC occurred preferentially in sediment grains that had a specific gravity exceeding 2.0, namely, in a form closely associated with sediment minerals. The OC concentration, the fraction of seagrass-derived OC, and the SSA were positively correlated to the logarithm of areal extent of individual seagrass meadows (p < 0.01). These findings suggest that the OC sequestration capacity of nearshore vegetated habitats is under the primary control of geophysical constraints such as sediment supply rate and depositional conditions. Vegetated shallow coastal ecosystems, including intertidal salt marshes, mangroves, and seagrass meadows have been ranked among the most efficient biotic systems for accumulating organic carbon (OC) on an areal basis (McLeod et al. 2011;Fourqurean et al. 2012). It is estimated that these ecosystems may contribute almost half of OC burial in the global ocean even though they cover < 2% of the ocean surface (Duarte et al. 2005). Recent interest has focused on the potential to incorporate these ecosystems, called "blue forests," into policies for reducing carbon dioxide (CO 2 ) emissions. At the same time, there is increased concern about the possibility of CO 2 emissions caused by the decline of blue forest ecosystems, including the seagrass meadows (Pendleton et al. 2012;Grimsditch et al. 2013).High rates of OC accumulation in seagrass meadows are likely the result of specific ecosystem functions such as (1) extremely high primary productivity of seagrasses and associated microalgae, (2) efficient trapping of organic particles within the meadow sediment via its flow-regulation and bottom-stabilization effects, and (3) slowness of remineralization of OC within the meadow sediment due to the anoxic conditions that prevail (Duarte et al. 2013). Most of the OC stored in seagrass meadows exists as detrital OC derived from seagrasses...

A Synthesis of Blue Carbon Stocks, Sources, and Accumulation Rates in Eelgrass (Zostera marina) Meadows in the Northeast Pacific

“…Our isotopic data suggest that the organic carbon that ends up in eelgrass meadow sediments is largely derived from noneelgrass sources, which aligns with previous findings that upward of 50% of carbon in seagrass sediments may be allochthonous in origin (Kennedy et al, ; Miyajima et al, ; Oreska et al, ; Prentice et al, ). While we did not have enough replicates of sediment isotope values or enough localized signatures of potential carbon sources to determine the relative proportion of carbon sources using an isotope mixing model, the biplot shows a clear separation of signatures in the sediments from those of Z. marina across various regions (Figure ).…”

Modeled CO2 Emissions from Coastal Wetland Transitions to Other Land Uses: Tidal Marshes, Mangrove Forests, and Seagrass Beds

“…the mineral fraction within sediments may also influence the probability of CO 2 emissions (Miyajima et al, 2017).…”

“…While a wide range of evidence suggests that a high proportion of the organic carbon that enters the marine environment is remineralized and that high concentration of CO 2 in coastal waters, and associated degassing, is due to rapid mineralization of C derived from coastal plant communities, some proportion of this sediment carbon may be transported to deep water, offshore environments, and thus may be returned to anoxic conditions (Baldock et al, 2004;Cai, 2011;Blair and Aller, 2012;Miyajima et al, 2017). Additionally, refixation of CO 2 by other primary producers (e.g., phytoplankton, macroalgae), which may depend on the level of nutrients and light available to support production, are also likely to be important factors determining CO 2 emissions (Maher and Eyre, 2012).…”

Seagrass community-level controls over organic carbon storage are constrained by geophysical attributes within meadows of Zanzibar, Tanzania

“…Once the flow-regime becomes energetic enough to create sediment 10 limitation, properties of the sediment can again outweigh plant traits to limit OC storage even under meadows with traits conducive to OC storage. Of course, this hypothesis needs to be rigorously tested but the modulation of trait effects by geophysical properties provides hints that different OC stabilization mechanisms are potentially operating within the different environments (Belshe et al, 2017;Burdige, 2007;Lutzow et al, 2006;Miyajima et al, 2017), and the persistence of seagrass sediment OC is a whole-ecosystem property (Lehmann and Kleber, 2015;Schmidt et al, 2011). 15…”