Nearshore Macroalgae Cultivation for Carbon Sequestration by Biomass Harvesting: Evaluating Potential and Impacts With an Earth System Model

Abstract This study assesses an ocean‐based carbon dioxide removal (CDR) approach: Nearshore Macroalgae Aquaculture for Carbon Sequestration (N‐MACS). By cultivating macroalgae in nearshore areas, N‐MACS aims to sequester atmospheric through subsequent carbon storage. Using an Earth System Model of intermediate complexity, we evaluate its CDR potential and impacts on the global carbon cycle, marine biogeochemistry, and ecosystems. Our results show that N‐MACS could sequester 0.7–1.1 gigatonnes of carbon annually. However, it significantly reduces marine phytoplankton productivity due to nutrient uptake and canopy shading. The CDR efficiency—defined as the increase in air–sea flux relative to carbon fixed in harvested macroalgal biomass—is approximately 65%, with primary offsets from ocean carbon backflux and suppressed phytoplankton growth. Reduced surface NPP lowers carbon export from the euphotic zone, increasing dissolved oxygen and decreasing denitrification in modern oxygen minimum zones. Biogeochemical effects linked to phosphorus removal via harvesting persist for centuries after N‐MACS ends.