Specific growth rate of bloom-forming (pale green) and nonbloom-forming (light brown) macroalgae differ in their response to elevated pCO2. Under carbon enrichment, with pCO2 levels ranging from 280 to 2,000 µatm, growth rates increase strongly in bloom-forming macroalgae, whereas perennial, nonbloom-forming species show a unimodal response to elevated pCO2 levels, with the highest growth rates around ~700 µatm pCO2.