Coastal blue carbon ecosystems assessment in the Basque Coast.
Supervisors: Mireia Valle, Guillem Chust (AZTI)
Coastal Blue Carbon Ecosystems (BCEs) play a significant role in supporting biodiversity, water remediation, coastal stabilization, and carbon dioxide (CO2) sequestration. The carbon is stored in their living biomass (above-ground and below-ground) and sediments. However, these ecosystems are threatened by human activities and climate change. Effective management of BCEs, through conservation, rehabilitation, restoration, or creation, requires carbon stock estimations. This thesis aims to provide an initial estimate of the Blue Carbon (BC) stock in the Basque Autonomous Community (Spain), delimiting the sedimentary pool at its top 1 meter. To that end, it is essential to understand the distribution of organic carbon content (Corg).

The BCEs sampled to assess coastal BC in the Basque estuaries included saltmarshes, Zostera noltei seagrass meadows, and unvegetated intertidal flats. These ecosystems were classified into typologies: saltmarshes (Spartina spp., Halimione portulacoides, and Juncus maritimus dominated), Z. noltei seagrass meadows (low, medium, and high-density meadows), and unvegetated intertidal plains (muddy, muddy-sandy, sandy). Data from a summer 2023 sampling campaign in the Oka, Lea, and Bidasoa estuaries were collected for this study. Corg estimation and variability analysis were conducted using linear mixed models (LMMs), allowing the results to be generalized to all BCEs in the Basque Country. Improved cartography of BCE typologies across Basque estuaries enabled the estimation of BCEs surface area and, consequently, the determination of the total organic carbon stock (Corg stock) for the Basque coast.

Corg was found to vary across estuaries, ecosystems, typologies, and pools. Although J. maritimus dominated saltmarshes are known to be among the BCEs that accumulate the most organic carbon in their sediments, this study revealed that, in the middle of the estuaries, these types of saltmarshes have lower Corg than the other analysed saltmarshes.

The BCE organic carbon stock in the Basque Country was estimated to be 68,337 (59,924 – 75,325) Mg. If all this Corg stock were converted into CO2, it would represent about half of the direct emissions of the service sector in the Basque Country for one year (reference year 2019). Nevertheless, this estimate should be considered with caution, as the Corg values in the sediment top 30 cm, determined in this study, for J. maritimus dominated saltmarshes (which represent 28% of the total area of Basque BCEs), were all found to be lower than the values documented in the literature, which could have led to an underestimation.