Mechanisms for inorganic carbon acquisition in macroalgal assemblages today could indicate how coastal ecosystems will respond to predicted changes in ocean chemistry due to elevated carbon dioxide (CO2). We identified the proportion of noncalcifying macroalgae with particular carbon use strategies using the natural abundance of carbon isotopes and pH drift experiments in a kelp forest. We also identified all calcifying macroalgae in this system; these were the dominant component of the benthos (by % cover) at all depths and seasons while cover of noncalcareous macroalgae increased at shallower depths and during summer. All large canopy-forming macroalgae had attributes suggestive of active uptake of inorganic carbon and the presence of a CO2 concentration mechanism (CCM). CCM species covered, on average, 15–45% of the benthos and were most common at shallow depths and during summer. There was a high level of variability in carbon isotope discrimination within CCM species, probably a result of energetic constraints on active carbon uptake in a low light environment. Over 50% of red noncalcifying species exhibited values below −30‰ suggesting a reliance on diffusive CO2 uptake and no functional CCM. Non-CCM macroalgae covered on average 0–8.9% of rock surfaces and were most common in deep, low light habitats. Elevated CO2 has the potential to influence competition between dominant coralline species (that will be negatively affected by increased CO2) and noncalcareous CCM macroalgae (neutral or positive effects) and relatively rare (on a % cover basis) non-CCM species (positive effects). Responses of macroalgae to elevated CO2 will be strongly modified by light and any responses are likely to be different at times or locations where energy constrains photosynthesis. Increased growth and competitive ability of noncalcareous macroalgae alongside negative impacts of acidification on calcifying species could have major implications for the functioning of coastal reef systems at elevated CO2 concentrations.