Evaluation of the net climate benefits of seagrass meadows at a regional scale

Coastal ecosystems like seagrass meadows play a crucial role in climate regulation by acting as blue carbon sinks with carbon burial rates up to 30–50 times higher than terrestrial forest soils. However, greenhouse gas (GHG) emissions, such as methane and nitrous oxide, may offset part of these benefits. Comparing seagrass meadows to bare sediments helps identify their specific role as blue carbon ecosystems. By assessing carbon accumulation rates (CAR) and GHG fluxes in Zostera noltei seagrass-covered and bare sediments in the Europe’s largest Zostera noltei meadows (Arcachon bay), we showed that seagrass areas exhibit CAR values that are 2.5 times higher, highlighting their superior carbon burial capacity. However, GHG emissions from seagrass meadows were 3.5 times higher than those from bare sediments, offsetting 29% of the estimated net climate benefit provided by carbon burial across the bay. Hydrodynamic conditions influence this balance: sheltered areas promote organic matter accumulation but also amplify GHG emissions. Despite these trade-offs, Z. noltei meadows still provide a net climate benefit 2.2 times greater than bare sediments, emphasizing their significant contribution to climate regulation. Incorporating GHG flux estimates into blue carbon assessments will improve global model predictions and guide better conservation and restoration strategies.