The Dual Climate Role of Seagrass Meadows in Arcachon Bay

Vegetated coastal ecosystems such as seagrass meadows are increasingly recognized as key contributors to climate regulation. Acting as blue carbon sinks, they exhibit carbon burial rates up to 30–50 times higher than those of terrestrial forest soils, thereby storing large amounts of organic carbon in their sediments. However, these benefits may be partly offset by the release of greenhouse gases (GHG), particularly methane (CH₄) and nitrous oxide (N₂O), which possess a much higher global warming potential than CO₂. Comparing seagrass meadows to bare sediments helps identify their specific role as blue carbon ecosystems. In this study, we assessed carbon accumulation rates (CAR) together with in situ fluxes of CH₄ and N₂O in Zostera noltii seagrass meadows and adjacent bare sediments within Arcachon Bay, France, which hosts Europe’s largest Z. noltii meadows. Our results demonstrate 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 28.7 ± 0.3% 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. noltii meadows still provide a net climate benefit 2.2 times greater than bare sediments, emphasizing their significant contribution to climate regulation. These findings highlight the dual nature of seagrass meadows as both carbon sinks and GHG sources. Incorporating GHG flux estimates into blue carbon assessments will improve global model predictions and guide better conservation and restoration strategies.