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 Vmax 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 (Ca) 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.