Seagrass blue carbon spatial patterns at the meadow-scale

Abstract

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.

Non‐seagrass carbon contributions to seagrass sediment blue carbon

“…). Past studies have observed OM and SOC concentration peaks between 3 cm and 6 cm below the sediment–water interface in this meadow, the zone of maximum Z. marina root and rhizome biomass (Cole and McGlathery ; Greiner et al ; Oreska et al ). The sediment samples collected in this study exactly captured this 3–6 cm bed depth interval.…”

“…Oakes and Connolly ). We considered using the C : N ratio as a third tracer but ultimately excluded it due to possible preferential loss of N in different bed locations (Oreska et al ). Absent a third tracer, we were not able to simultaneously quantify the macroalgal contribution, which Greiner et al () found to be negligible (∼ 3%), or include phytoplankton, which was likely a minor SOC contributor due to very low concentrations and productivity in the water column (McGlathery et al ).…”

“…However, the SOC is nonuniformly distributed. Percent OM and SOC concentrations decline with distance from the adjacent barrier island (Oreska et al )—a spatial distribution that suggests “outwelling” from the island's fringing marsh as a possible carbon source vector. The South Bay meadow is part of a coupled seagrass‐marsh system, where marsh scarp erosion potentially supplies sediment to the seagrass bed (McGlathery et al ; Fig.…”

Increased current flow enhances the risk of organic carbon loss from Zostera marina sediments: Insights from a flume experiment

“…Hydrodynamic exposure could partially explain the variability in seagrass carbon storage seen globally (Samper‐Villarreal et al ; Mazarrasa et al ; Oreska et al ), as exposure to wind and currents largely governs the process of sedimentation (Winterwerp and van Kesteren ). To what degree currents affect the sedimentary carbon stocks in seagrass meadows is not known, although such data are critical for understanding the carbon sequestration and storage efficiency in coastal environments.…”

Reduced water motion enhances organic carbon stocks in temperate eelgrass meadows

“…Yet, many regions and habitats, including the Pacific coast of North America, have only recently been explored for their blue carbon potential. The scarcity of data on the spatial heterogeneity in carbon stocks and accumulation rates creates challenges for scaling up carbon estimates and incorporating blue carbon into such climate policies (Lavery et al ; Oreska et al ; Mazarrasa et al ). Furthermore, the mechanisms that regulate spatial variability across different scales are only beginning to be understood (Samper‐Villarreal et al ; Kindeberg et al ; Mazarrasa et al ; Santos et al ).…”