The role of seagrass-tidal marsh ecosystem connectivity in blue carbon sequestration

Abstract:

Blue carbon (C) ecosystems – seagrass meadows, tidal marshes, and mangrove forests – are responsible for nearly 50% of the ocean’s C burial. In addition to storing self-produced (autochthonous) C, blue C ecosystems store large amounts of imported (allochthonous) C, and can export large proportions of their own biomass. In temperate regions, seagrasses can release as much as 90% of their annually produced leaf biomass, which may be captured and buried in situ, trapped by adjacent ecosystems, assimilated by fauna, exported to deeper ocean systems, and/or remineralized back to the atmosphere as CO₂. However, C produced in one ecosystem and sequestered over the long term in an adjacent ecosystem has received little attention in carbon accounting schemes. The barrier island system of the Virginia Coast Reserve Long Term Ecological Research Site, USA, is home to mainland and fringing tidal marsh systems and has a history of complete seagrass loss in the 1930s, and now hosts the largest successful seagrass restoration site in the world, and provides the ideal setting for testing the impacts of connectivity between seagrass meadows and tidal marshes and C sequestration. Using an MBACI design, we assessed if seagrass C from restored meadows is stored in adjacent tidal marshes. We sampled marsh sediments from before (10-15 cm depth interval) and after (0-5 cm depth interval) the time of seagrass restoration along transects in back-barrier marshes adjacent to the largest restored seagrass meadow and one further from a smaller seagrass meadow. Sediments were analyzed for organic C, nitrogen, organic matter, particle size, C source (via bulk isotope analysis and Bayesian mixing models), and age (lead-210 dating). Based on these results, we calculated C stocks and sequestration rates, and estimated the contribution of seagrass biomass to sediment C stocks for before and after seagrass restoration at each site across the two islands. The outcomes from this study can improve C accounting measures and benefit conservation and restoration of blue C ecosystems by providing quantitative evidence of C transfer and preservation between blue C ecosystems connected in the landscape.