Photosynthetic Responses of Turf‐forming Red Macroalgae to High CO 2 Conditions

Seaweeds are important components of near‐shore ecosystems as primary producers, foundation species, and biogeochemical engineers. Seaweed communities are likely to alter under predicted climate change scenarios. We tested the physiological responses of three perennial, turf‐building, intertidal rhodophytes, Mastocarpus stellatus, Osmundea pinnatifida, and the calcified Ellisolandia elongata , to elevated p CO 2 over 6 weeks. Responses varied between these three species. E. elongata was strongly affected by high p CO 2 , whereas non‐calcified species were not. Elevated p CO 2 did not induce consistent responses of photosynthesis and respiration across these three species. While baseline photophysiology differed significantly between species, we found few clear effects of elevated p CO 2 on this aspect of macroalgal physiology. We found effects of within‐species variation in elevated p CO 2 response in M. stellatus , but not in the other species. Overall, our data confirm the sensitivity of calcified macroalgae to elevated p CO 2 , but we found no evidence suggesting that elevated p CO 2 conditions will have a strong positive or negative impact on photosynthetic parameters in non‐calcified macroalgae.