Sources of sediment carbon sequestered in restored seagrass meadows

Greiner, Jill T.; Wilkinson, Grace M.; McGlathery, K. J.; Emery, Kyle A.

doi:10.3354/meps11722

Cited by 37 publications

(43 citation statements)

References 54 publications

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“…). These findings align with other studies that have found largely allochthonous contributions to seagrass sediment OC stocks, with 50% or more of the carbon originating from non‐seagrass sources such as macroalgae, salt marsh vegetation, benthic microalgae, plankton, or terrestrial sources (Kennedy et al ; Miyajima et al ; Greiner et al ; Oreska et al ). While the isotope sources used here are distinct from one another, overlapping source signatures can make it challenging to decipher relative contributions to the sediments.…”

Section: Discussionsupporting

confidence: 91%

Reduced water motion enhances organic carbon stocks in temperate eelgrass meadows

Prentice

Hessing‐Lewis

Sanders‐Smith

et al. 2019

Limnology & Oceanography

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Organic carbon (OC) storage in coastal vegetated ecosystems is increasingly being considered in carbon financing and climate change mitigation strategies. However, spatial heterogeneity in these “blue carbon” stocks among and within habitats has only recently been examined, despite its considerable implications. Seagrass meadows have potential to store significant amounts of carbon in their sediments, yet studies comparing sediment OC content at regional and meadow scales remain sparse. Here, we collected sediment cores from six temperate eelgrass (Zostera marina) meadows on the coast of British Columbia, Canada, to quantify sediment OC stocks, accumulation rates, and sources, and to examine local and regional drivers of variability. Sediment OC content was highly variable—across all sites, stocks in the top 0–5 cm ranged from 83 to 1089 g OC m−2, while the 15–20 cm stocks exhibited a 24‐fold difference, from 59 to 1407 g OC m−2. Carbon accumulation rates ranged from 4 to 33 g OC m−2 yr−1. Isotopic mixing models revealed that sediment OC was primarily terrestrial carbon (41.3%) and canopy‐forming kelps (33.3%), with a smaller contribution of eelgrass (25.3%). Here, we show that regional variability in OC content exceeds meadow‐scale variability. This result is likely driven by landscape factors, most notably relative water motion, representing a more dominant control on seagrass OC accumulation than meadow‐scale factors such as canopy complexity. These findings elicit caution when scaling up seagrass meadow OC content and demonstrate that measures of the hydrodynamic environment could improve estimates of carbon storage in temperate soft sediment habitats.

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Section: Discussionsupporting

confidence: 91%

Reduced water motion enhances organic carbon stocks in temperate eelgrass meadows

Prentice

Hessing‐Lewis

Sanders‐Smith

et al. 2019

Limnology & Oceanography

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“…However, in their recent blue carbon review, Macreadie et al () only discuss microalgal carbon in the context of a regime shift from marsh or seagrass to microalgal production that results in less blue carbon storage—not coupling between these macrophytes and BMA that may increase BMA productivity and, therefore, SOC accumulation. By analyzing SOC at the meadow‐scale, our results confirm initial suggestions by Greiner et al () that BMA represents the dominant contributor to the SOC stock in this particular seagrass meadow. BMA production—not allochthonous POM trapping—likely accounts for much of the non‐seagrass SOC observed in seagrass meadows (Kennedy et al ).…”

Section: Discussionsupporting

confidence: 89%

“…The Zostera marina (eelgrass) meadow in South Bay, Virginia, U.S.A., is part of the Virginia Coast Reserve Long-Term Ecological Research (VCR-LTER) eelgrass restoration and represents the single largest, successfully restored seagrass meadow to date (Orth et al 2006Orth and McGlathery 2012). SOC profile comparisons confirm that this meadow now stores significantly more SOC than adjacent bare sites Greiner et al 2013) and that much of this SOC is non-seagrass in origin (Greiner et al 2016). However, the SOC is nonuniformly distributed.…”

mentioning

confidence: 99%

Non‐seagrass carbon contributions to seagrass sediment blue carbon

Oreska

Wilkinson

McGlathery

et al. 2017

Limnology & Oceanography

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Non‐seagrass sources account for ∼ 50% of the sediment organic carbon (SOC) in many seagrass beds, a fraction that may derive from external organic matter (OM) advected into the meadow and trapped by the seagrass canopy or produced in situ. If allochthonous carbon fluxes are responsible for the non‐seagrass SOC in a given seagrass bed, this fraction should decrease with distance from the meadow perimeter. Identifying the spatial origin of SOC is important for closing seagrass carbon budgets and “blue carbon” offset‐credit accounting, but studies have yet to quantify and map seagrass SOC stocks by carbon source. We measured sediment δ13C, δ15N, and δ34S throughout a large (6 km2), restored Zostera marina (eelgrass) meadow and applied Bayesian mixing models to quantify total SOC contributions from possible autotroph sources, Z. marina, Spartina alterniflora, and benthic microalgae (BMA). Z. marina accounted for < 40% of total meadow SOC, but we did not find evidence for outwelling from the fringing S. alterniflora salt‐marsh or OM advection from bare subtidal areas. S. alterniflora SOC contributions averaged 10% at sites both inside and outside of the meadow. The BMA fraction accounted for 51% of total meadow SOC and was highest at sites furthest from the bare subtidal‐meadow edge, indicative of in situ production. 210Pb profiles confirmed that meadow‐enhanced sedimentation facilitates the burial of in situ BMA. Deducting this contribution from total SOC would underestimate total organic carbon fixation within the meadow. Seagrass meadows can enhance BMA burial, which likely accounts for most of the non‐seagrass SOC stored in many seagrass beds.

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“…Greiner et al [44] found that only half of the sediment C org at an interior site in the South Bay meadow derived from vascular plants. The C:N ratio in the C org hotspot in the southeast part of the meadow (Fig 4) conforms more closely to the range observed for seston and macroalgae than for Z .…”

Section: Discussionmentioning

confidence: 99%

Seagrass blue carbon spatial patterns at the meadow-scale

2017

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Most information on seagrass carbon burial derives from point measurements, which are sometimes scaled by meadow area to estimate carbon stocks; however, sediment organic carbon (Corg) concentrations may vary with distance from the meadow edge, resulting in spatial gradients that affect the accuracy of stock estimates. We mapped sediment Corg concentrations throughout a large (6 km2) restored seagrass meadow to determine whether Corg distribution patterns exist at different spatial scales. The meadow originated from ≤1-acre plots seeded between 2001 and 2004, so we expected Corg to vary spatially according to the known meadow age at sample sites and with proximity to the meadow edge. Applying spatial autoregressive models allowed us to control for spatial autocorrelation and quantify the relative effects of edge proximity and age on Corg concentrations. We found that edge proximity, not age, significantly predicted the meadow-scale Corg distribution. We also evaluated relationships between Corg and a variety of specific explanatory variables, including site relative exposure, shoot density, sediment grain size, and bathymetry. Factors known to affect carbon burial at the plot-scale, such as meadow age and shoot density, were not significant controls on the meadow-scale Corg distribution. Strong correlations between Corg, grain size, and edge proximity suggest that current attenuation increases fine-sediment deposition and, therefore, carbon burial with distance into the meadow. By mapping the sediment Corg pool, we provide the first accurate quantification of an enhanced carbon stock attributable to seagrass restoration. The top 12 cm of the bed contain 3660 t Corg, approximately 1200 t more Corg than an equal area of bare sediment. Most of that net increase is concentrated in a meadow area with low tidal current velocities. Managers should account for the effects of meadow configuration and current velocity when estimating seagrass blue carbon stocks. Our results suggest that a large, contiguous meadow should store more blue carbon than an equal area of small meadow patches.

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Sources of sediment carbon sequestered in restored seagrass meadows

Cited by 37 publications

References 54 publications

Reduced water motion enhances organic carbon stocks in temperate eelgrass meadows

Reduced water motion enhances organic carbon stocks in temperate eelgrass meadows

Non‐seagrass carbon contributions to seagrass sediment blue carbon

Seagrass blue carbon spatial patterns at the meadow-scale

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