ABSTRACT. – This study aimed to evaluate experimentally the effect of high pCO₂/low pH on a simple biotic assemblage including grazer molluscs (Patella ulyssiponensis and Gibbula umbilicalis) and the biofilm as food source through the use of δ¹³C and δ¹⁵N. Organisms were exposed for 3 months at ambient (400 μatm) and elevated (1120 μatm) pCO₂. A pronounced δ13C decrease of Gibbula umbilicalis was pointed out when exposed to high pCO₂ while the biofilm did not show any isotopic deviation between both treatments. From these results (1) high pCO₂ may affect the qualitative and/or quantitative characteristics of biofilm-herbivore trophic link (2) coexisting gastropods can respond differently in term of feeding activity and/or assimilated food source when submitted to higher pCO₂.