The greenhouse gas offset potential from seagrass restoration

Awarding CO₂ offset credits may incentivize seagrass restoration projects and help reverse greenhouse gas (GHG) emissions from global seagrass loss. However, no study has quantified net GHG removal from the atmosphere from a seagrass restoration project, which would require coupled C_org stock and GHG flux enhancement measurements, or determined whether the creditable offset benefit can finance the restoration. We measured all of the necessary GHG accounting parameters in the 7-km² Zostera marina (eelgrass) meadow in Virginia, U.S.A., part of the largest, most cost-effective meadow restoration to date, to provide the first seagrass offset finance test-of-concept. Restoring seagrass removed 9,600 tCO₂ from the atmosphere over 15 years but also enhanced both CH₄ and N₂O production, releasing 950 tCO₂e. Despite tripling the N₂O flux to 0.06 g m^-2 yr^-1 and increasing CH₄ 8-fold to 0.8 g m^-2 yr^-1, the meadow now offsets 0.42 tCO₂e ha^-1 yr^-1, which is roughly equivalent to the seagrass sequestration rate for GHG inventory accounting but lower than the rates for temperate and tropical forests. The financial benefit for this highly successful project, $87 K at $10 MtCO₂e^-1, defrays ~10% of the restoration cost. Managers should also consider seagrass co-benefits, which provide additional incentives for seagrass restoration.