The mitigation of the greenhouse gases is urgent task for the future climate change initiative. The Blue Carbon, which is carbon capture by marine living organisms, is recently focused as an important factor for the climate change initiative, because the Blue Carbon sequestrated in the sediment is considered to be separated from the atmosphere for thousands years.
　However, the contribution of Blue Carbon sequestration to atmospheric CO2 has been unclear because studies comprehensively analyzing the complex carbon-flows in coastal regions are few. Especially, the Blue Carbon contribution to atmospheric CO2 cannot be determined only from the sequestration rate of the Blue Carbon in submerged aquatic plants where their photosynthetic activity uses the dissolved inorganic carbon in water, not atmospheric CO2 directly.
　In turn, coastal regions have been thought to be a source of atmospheric CO2 due to mineralization of the organic matter supplied from land. In addition, most previous studies intended for estuaries having few vegetations, or mangroves and salt-marshes where the organic carbon decomposition is dominant in water. Thus, measurements in seagrasses, where the ecosystem production decreases the partial pressure of CO2 in water and would increase atmospheric CO2 influx, have been few.
　In this study, we performed in-situ carbon-flow measurements and analyzed the relationship between Blue Carbon sequestration and atmospheric CO2 flux in two seagrass meadows. The results indicate that autotrophic seagrass meadows would be an atmospheric CO2 sink, contrary to the former recognition. Furthermore, this result is indicated to be able to apply the global coastal regions from comparison with former studies.
　This study provokes a novel ecosystem functioning as “atmospheric CO2 sink” for the coastal vegetations, and such an ecosystem can be a basis for the climate change initiative.