Blue Carbon Benefits of Seagrass Restoration

In recent years, studies have noted that seagrass meadow collapse results in a greenhouse gas (GHG) flux, because the Corg sequestered in plant biomass and bed sediment (SOC) is rapidly remineralized and outgassed to the atmosphere absent seagrass bed stabilization. Given the estimated size of the existing seagrass ‘blue carbon’ sequestered stock and the global meadow loss rate, the global CO2 flux from seagrass habitat conversion is sufficiently large to warrant concern about climate impacts. Observers have, therefore, called for incentivizing seagrass conservation and restoration through the allocation of voluntary offset-credits. The new Methodology for Tidal Wetland and Seagrass Restoration provides a framework that the Verified Carbon Standard (VCS) can use to quantify and award offsets to applicant seagrass projects. However, absent a seagrass offset-credit case study, prospective projects do not have a benchmark for expectations about the offset-credit return from seagrass restoration. Despite extensive research on seagrass carbon burial, questions remain about how best to account for seagrass bed accretion, scale SOC measurements to estimate meadow-scale stocks, identify SOC sources (i.e. allochthonous Corg), account for possible trace gas increases, and demonstrate that restoration projects are ‘additional,’ especially in regions with pre-existing meadows. These questions can be addressed by studying the recent Zostera marina (eelgrass) restoration in the Virginia Coast Reserve (VCR). The large (>6 km2), successfully-restored meadow in South Bay, VA, provides an ideal test case for quantifying the net GHG benefit that can be achieved through restoration.

The South Bay meadow restoration has resulted in the net removal of almost 10,000 tCO2 from the atmosphere since 2001. This benefit derives in large part from canopy-sedimentation effects that enhance the burial of seagrass Corg and in situ benthic microalgae at mid-meadow sites. Measuring and mapping SOC concentrations, organic matter stable isotope ratios, and sediment grain size throughout this meadow revealed SOC concentration gradients resulting from hydrodynamic ‘edge effects.’ Sediment fractionates by size as it is advected into the meadow, resulting in more fine-grained deposition at interior meadow sites. These sites, therefore, accumulate more SOC from seagrass and from microalgae than sites closer to the meadow perimeter, irrespective of site meadow age. Measuring seasonal trace gas fluxes confirmed that seagrass presence also increases the release of both CH4 and N2O, but the enhanced release rates have a marginal effect on the net GHG benefit.

The continued natural expansion of the restored Z. marina meadows in the VCR suggests that the GHG benefit resulting from the restoration effort will continue to increase, even absent continued broadcast seeding. As a consequence, a new VCR restoration effort that applies for VCS offset-credits may not qualify as ‘additional.’ Species distribution models applied to natural eelgrass recruitment data suggest that total eelgrass area within the VCR will eventually increase from 25 to approximately 34 km2. However, applying the same machine learning models to survival data from the eelgrass restoration seed plots suggests that additional restoration effort could, potentially, triple the total restored area in the VCR. If coastal managers decide to undertake this additional restoration, VCS offset-credits can partially defray the cost.