Coccolithophore Growth and Calcification in an Acidified Ocean: Insights From Community Earth System Model Simulations

Anthropogenic CO₂ emissions are inundating the upper ocean, acidifying the water, and altering the habitat for marine phytoplankton. These changes are thought to be particularly influential for calcifying phytoplankton, namely, coccolithophores. Coccolithophores are widespread and account for a substantial portion of open ocean calcification; changes in their abundance, distribution, or level of calcification could have far-reaching ecological and biogeochemical impacts. Here, we isolate the effects of increasing CO₂ on coccolithophores using an explicit coccolithophore phytoplankton functional type parameterization in the Community Earth System Model. Coccolithophore growth and calcification are sensitive to changing aqueous CO₂. While holding circulation constant, we demonstrate that increasing CO₂ concentrations cause coccolithophores in most areas to decrease calcium carbonate production relative to growth. However, several oceanic regions show large increases in calcification, such the North Atlantic, Western Pacific, and parts of the Southern Ocean, due to an alleviation of carbon limitation for coccolithophore growth. Global annual calcification is 6% higher under present-day CO₂ levels relative to preindustrial CO₂ (1.5 compared to 1.4 Pg C/year). However, under 900 μatm CO₂, global annual calcification is 11% lower than under preindustrial CO₂ levels (1.2 Pg C/year). Large portions of the ocean show greatly decreased coccolithophore calcification relative to growth, resulting in significant regional carbon export and air-sea CO₂ exchange feedbacks. Our study implies that coccolithophores become more abundant but less calcified as CO₂ increases with a tipping point in global calcification (changing from increasing to decreasing calcification relative to preindustrial) at approximately ∼600 μatm CO₂.