Some abiotic conditions are well known to play disproportionately large roles in shaping contemporary assemblages, yet their roles may not continue to have similar magnitudes of effect into the future. We tested whether forecasted levels of CO2 could alter the strength of influence of an abiotic factor (i.e., light intensity) well known for its strength of influence on the subtidal ecology of photosynthetic organisms. We investigated these dynamics in two subtidal algal species that form contrasting associations with kelp forests, one negatively associated with kelp canopies (turf-forming brown algae, Feldmannia spp.) and the other positively associated with kelp as understory (calcifying red crustose algae, Lithophyllum sp.). Using an experimental approach, we assessed the independent and combined effects of [CO2] (control and elevated) and light (shade, low ultraviolet B [UVB], full light) on growth, recruitment, and relative electron transport rate (rETR). Under control [CO2], the effects of light corresponded to the relative light environments of the two groups of algae. The influence of light on the percentage cover and biomass of understory crusts was substantially reduced under elevated [CO2], which caused crusts to grow less. While elevated [CO2] had the opposite effect of positively influencing turf cover and biomass, it had the same effect of reducing the structuring effects of light and UVB. Hence, if we are to predict the ecological consequences of future CO2 conditions, the role of contemporary processes cannot be assumed to produce similar effects relative to other processes, which will change with a changing climate.