Growth of several green and red algae was consistently enhanced under high CO2 conditions, but was statistically different only in the intertidal green alga Ulva lactuca and in the red alga Chondrus crispus only under elevated temperatures. The corresponding photosynthetic rates exhibited the same trend, but differences were insignificant. The two brown algae Fucus vesiculosus and Laminaria hyperborea showed opposite responsestowards CO2 increase: While Laminaria hyperborea enhanced growth andphotosynthesis under elevated CO2 conditions, Fucus vesiculosus unexpectedly had lower growth and photosynthesis rates under this condition.The elemental composition (C/N/P) of Fucus vesiculosus in different CO2 conditions did not change and thus no changes in the nutritional quality of the brown seaweed were detected. Accordingly, there were noimplications for the common mesoherbivore Idotea baltica. The isopod feeding on algae from different CO2 treatments did not show differences in food consumption and respiration rates.