Efficacy of seaweed-based carbon dioxide removal reduced by iron limitation and nutrient competition with phytoplankton.

Carbon dioxide removal (CDR) through seaweed cultivation has been proposed as a promising marine CDR approach due to its high afforestation potential and favorable carbon-to-nutrient ratios. However, recent studies suggest that the afforestation potential is constrained by iron limitation, and efficiency depends on relative stoichiometry with phytoplankton. Global CDR models overlook iron limitation and fail to capture how nutrient feedbacks with phytoplankton will reduce ocean carbon uptake. Here, an ocean biogeochemical model is used to assess how nutrient demand, affinity, and limitation influence the afforestation potential and CDR efficiency of seaweed cultivation. Iron limitation reduces afforestation potential by threefold after accounting for N and P limitation. Variations in nutrient affinity and demand cause large differences in CDR efficiency, from -43% to +78%. We show that if seaweed is less nutrient-efficient than phytoplankton, cultivation results in negative net CDR. Failing to account for these nutrient dynamics risks overestimating seaweed-based CDR efficacy.