Carbon stocks of coastal macroalgal beds in the SW Atlantic

Abstract Macroalgae beds are extensive and productive marine benthic vegetated habitats globally, occurring in nearly 28% of coastal habitats worldwide. These marine forests may be important coastal carbon sinks but there are very limited in-situ assessments of their carbon stocks. In this study, we assessed the blue carbon stocks (aboveground and belowground) of typical coastal macroalgae beds in the SW Atlantic coast by comparing shallow (< 2 m) subtidal environments with and without the presence of macroalgae. We estimated a total ecosystem carbon stock (TECS) of 1.64 ± 0.24 Mg C ha−1 on macroalgae beds, which was on average 30% higher than areas without macroalgae (1.14 ± 0.16 Mg C ha−1). The differences between areas were attributed to the aboveground carbon stock in macroalgal biomass (0.49 ± 0.23 Mg C ha−1), which corresponded to nearly 30% of the total stocks. Sargassum sp. (Phaeophyceae) was the most representative macroalgae on the studied areas (59%), and contributed to a higher proportion of the aboveground carbon stocks. Using satellite imagery, we estimated an area of 527 ha of macroalgae beds along a stretch of 410 km in the Eastern Marine Ecoregion of Brazil, representing a TECS of 869 ± 128 Mg C. Nearly 45% of these macroalgae beds are currently within coastal marine protected areas in the region, and although they hold a limited value for climate mitigation, they remain largely unrecognized in the conservation planning in the SW Atlantic.