Abstract. Anthropogenic emissions of CO₂ are leading to an acidification of the oceans by 0.4 pH units in the course of this century according to the more severe model scenarios. The excess of CO₂ could notably affect the benthic communities of calcifiers and macrophytes in different aspects (photosynthesis, respiration and calcification). Seaweeds are key species of nearshore benthic ecosystems of the Baltic Sea. They frequently are the substratum of fouling epibionts like bryozoans and tubeworms. Most of those species secrete calcified structures and could therefore be impacted by the seawater pCO₂. On the other hand, the biological activity of the host may substantially modulate the pH and pCO₂ conditions in the thallus boundary layer where the epibionts live. The aim of the present study was to test the sensitivity of seaweed macrofouling communities to higher pCO₂ concentrations. Fragments of the macroalga Fucus serratus bearing the calcifiers Spirorbis spirorbis (Annelida) and Electra pilosa (Bryozoa) and the non-calcifier Alcyonidium gelatinosum (Bryozoa) were maintained for 30 days under three pCO₂ conditions: natural 460 ± 59 μatm and enriched 1193 ± 166 μatm and 3150 ± 446 μatm. Our study showed a significant reduction of growth rates and recruitment of Spirorbis individuals only at the highest pCO₂. At a finer temporal resolution, the tubeworm recruits exhibited enhanced calcification of 40% during irradiation hours compared to dark hours, presumably due to the effect of photosynthetic and respiratory activities of the host alga on the carbonate system. Electra colonies showed significantly increased growth rates at 1193 μatm. No effect on Alcyonidium colonies growth rates was observed. Those results suggest a remarkable resistance of the algal macro-epibiontic communities to the most elevated pCO₂ foreseen in year 2100 for open ocean (~1000 μatm) conditions possibly due to the modulation of environmental conditions by the biological activities of the host alga.