Assessment of blue carbon sequestration potential in the Basque estuaries: insights for climate change mitigation

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Blue carbon ecosystems (BCEs) such as saltmarshes and seagrass meadows, have been recently found to importantly contribute to global regulation services through their ability to sequester and store carbon. They also support a key set of services such as habitat provision, flood and coastal protection, enhancement of water quality and various tangible and intangible cultural services. These multiple co-benefits associated with BCEs underscores the importance of integrating nature-based solutions into climate change mitigation and adaptation strategies. This research aims to assess blue carbon sequestration potential in the Basque estuaries by estimating current blue carbon stock and provide insights for climate change mitigation management strategies involving conservation and restoration of BCEs. To do so, we performed a field campaign in summer 2022 where we sampled 3 BCEs habitat types (saltmarshes, Zostera noltei seagrass meadows and non-vegetated intertidal flats) in 3 different estuaries. In each estuary, we sampled 3 plots for each habitat type. In the plots, we collected 3 sediment cores to estimate total organic carbon (TOC), organic matter and grain size of the sediment; and 3 vegetation quadrants (in vegetated plots of saltmarshes and seagrass) to estimate TOC stored in above- and below-ground biomass. Additionally, we performed a literature review to determine BCEs mean organic carbon value and be able to compare it to that found in our assessment. Existing cartography of BCEs was also compiled to be able to make spatial estimations and identify BCEs hotspots. In the case of Zostera noltei seagrass, a spatio-temporal analysis was performed examining the trends in its distribution. We compared available detailed cartographies from 2008, 2012 and 2021 and a significant increase of the area was detected. Results showed that BCEs from the Basque estuaries might play an important role in mitigating residual anthropogenic carbon emissions.