Predicting the impacts of ocean acidification in coastal habitats is complicated by bio-physical feedbacks between organisms and carbonate chemistry. Daily changes in pH and other carbonate parameters in coastal ecosystems, associated with processes such as photosynthesis and respiration, often greatly exceed global mean predicted changes over the next century. We assessed the strength of these feedbacks under projected elevated CO₂ levels by conducting a field experiment in 10 macrophyte-dominated tide pools on the coast of California, USA. We evaluated changes in carbonate parameters over time and found that under ambient conditions, daytime changes in pH, pCO₂, net ecosystem calcification (NEC), and O₂ concentrations were strongly related to rates of net community production (NCP). CO₂ was added to pools during daytime low tides, which should have reduced pH and enhanced pCO₂. However, photosynthesis rapidly reduced pCO₂ and increased pH, so effects of CO₂ addition were not apparent unless we accounted for seaweed and surfgrass abundances. In the absence of macrophytes, CO₂ addition caused pH to decline by ∼0.6 units and pCO₂ to increase by ∼487 µatm over 6 hr during the daytime low tide. As macrophyte abundances increased, the impacts of CO₂ addition declined because more CO₂ was absorbed due to photosynthesis. Effects of CO₂addition were, therefore, modified by feedbacks between NCP, pH, pCO₂, and NEC. Our results underscore the potential importance of coastal macrophytes in ameliorating impacts of ocean acidification.

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