ABSTRACT: To date, the impact of future increases in carbon availability on marine primary producers is uncertain due to the diversity of carbon acquisition strategies and differences in the ability of species to adapt to concurrent changes in other environmental factors. The aim of this study was to compare the CO₂ uptake rates of intertidal brown algae inhabiting different shore levels on the Irish west coast to increased atmospheric CO₂ levels with the use of an infra-red gas analyser. Considering the 5 species studied, 4 (Pelvetia canaliculata, Ascophyllum nodosum, Sargassum muticum, Saccharina latissima) showed a significant linear relationship between CO₂ uptake and CO₂ concentration with only Fucus spiralis exhibiting no increase. The lower shore species (S.muticum, S. latissima) exhibited statistically significant increases in V_max between CO₂ treatments and, in addition, a much larger affinity for carbon. The relative increase in carbon uptake and the slope of the CO₂ concentration versus CO₂ uptake curve (C_α) oscillated over the annual cycle for A. nodosum and S. muticum, and was also dependent on site in the case of A. nodosum, highlighting the importance of ambient environmental conditions on the relationship between availability and uptake. The affinity for CO₂ was correlated with in situ temperature in both species, suggesting a seasonal strategy of photosynthetic up-regulation. In the case of A. nodosum, the results indicated that future impacts of increased CO₂ would be greatest during the spring growth period. Future studies into the impact of high CO₂ levels on macroalgae need to consider the seasonal regulation of photosynthesis, as this will strongly influence the response observed.