Restoration history and its influence on 'blue carbon' storage in a Zostera marina (eelgrass) sediment carbon pool
The recognition that seagrass meadows store significant amounts of organic carbon has prompted interest in using ‘blue carbon’ offset credits to finance meadow conservation and restoration efforts. Estimates of the global seagrass carbon stock are sizeable. However, estimates for individual meadow carbon stocks are largely absent. Studies need to document and explain the variability in seagrass ‘blue carbon’ at regional spatial scales. This study investigated sediment carbon concentrations at sites distributed throughout a restored Zostera marina (eelgrass) meadow in Virginia—perhaps the largest, successfully restored seagrass meadow on the planet. Carbon concentrations measured at 66 sites were related to site meadow age, expansion history, and location relative to lagoon geomorphology. Z. marina existed in South Bay, Virginia, prior to the 1930s eelgrass pandemic in the North Atlantic but failed to recover naturally. Between 2000 and 2005, a coordinated restoration effort reseeded approximately 12 hectares in South Bay. Now, roughly a decade after restoration activities in South Bay ended, the plots have coalesced into a mature, naturally expanding meadow that covers more than 575 hectares. By measuring and mapping sediment carbon concentrations throughout the meadow, it is possible to calculate the total carbon storage attributable to the restoration.
Initial work at a mid-meadow site documented an organic carbon concentration comparable to those reported for natural seagrass meadows. However, sediment cores collected during this study in younger meadow areas with greater current exposure yielded significantly lower sediment carbon concentrations, approximately 3.8x10-3 g C cm-2 compared with 8.1x10-3 g C cm-2 near the meadow center. The mean across all sites was 5.5x10-3 g C cm-2, with a standard deviation of 1.8x10-3 g C cm-2. By interpolating measurements at sites, we were able to calculate total carbon storage for the sediment carbon pool: approximately 3,500 t C. In comparison, estimates based on scaled, mid-meadow measurements overestimated total carbon storage, ranging up to approximately 5,600 t C. However, several meadow-scale spatial patterns were also apparent that possibly explain observed differences at sites and might inform carbon accumulation models. Sediment carbon concentrations were highest immediately adjacent to the barrier island and declined towards the channel and inlets, suggesting that basin geomorphology controlled carbon accumulation at the meadow scale. Meadow expansion history had an observable but less pronounced effect on site sediment carbon concentrations.