Potential blue carbon in the fringe of Southern European Kelp forests.

Blue Carbon encompasses the organic carbon sequestered and stored by coastal and marine ecosystems, including seaweed forests. This study aims to quantify the potential Blue Carbon storage and sequestration rates of subtidal kelp forests in Northern Portugal, focusing on the most dominant species Laminaria hyperborea and Saccorhiza polyschides. Through in-situ measurements of forest extension, biomass, growth, and carbon content, we determined that these kelp forests store approximately 16.48 Gg of carbon in aboveground biomass, covering an area of 5189 hectares. The estimated carbon sequestration rate is 1903 Mg C year, which is lower than the 3717 Mg C year sequestered by saltmarshes and seagrasses in mainland Portugal (available data). However, when normalised by area, kelp forests sequester carbon at rates that equal or exceed those of saltmarshes and seagrasses, underscoring their significance for regional carbon management. Despite this, export assumptions significantly influence total carbon flux estimates, and large portions of the Portuguese coastline remain understudied. Expanding in-situ assessments, combined with regional modelling would improve Blue Carbon estimates and lay the groundwork for a future national-scale assessment. Given their potential role in climate mitigation, conservation and restoration efforts should be prioritized to protect and enhance the Blue Carbon capacity of kelp forests and associated habitats.