On the Atlantic pelagic Sargassum's role in carbon fixation and sequestration

Abstract

The extensive blooms of the pelagic Sargassum in the Atlantic raised the question of whether this brown seaweed may play an important role in climate change mitigation through carbon fixation and carbon sequestration, as argued in several recent papers. Using simple calculations and published values on Sargassum coverage, biomass density, carbon/biomass ratio, primary productivity, and carbon sequestration efficiency, we show that the total carbon stock in pelagic Sargassum of the entire Atlantic, even during the peak month, is unlikely to exceed 3.61 × 10−3 Pg C, and carbon fixation cannot exceed 6.0 million tons C month−1. While the carbon fixation estimate represents an upper bound, it is still <0.2% of carbon fixation by phytoplankton in the Atlantic Ocean. The carbon stock estimate is 2000 times lower than predicted using a machine learning model in another recent paper. In contrast, carbon sequestration by Sargassum appears significant locally within the Atlantic Sargassum belt. The analysis further suggests that, while the Atlantic pelagic Sargassum may play an important role in affecting local carbon budget and carbon sequestration, its contribution to either carbon stock or carbon sequestration at a global scale may be insignificant. This, however, does not diminish the importance of Atlantic pelagic Sargassum in many other aspects.