Diatoms are silicifying phytoplankton contributing about one quarter to primary production on Earth. Ocean acidification (OA) could alter the competitiveness of diatoms relative to other taxa and/or lead to shifts among diatom species. In spring 2016, we set up a plankton community experiment at the coast of Gran Canaria (Canary Islands, Spain) to investigate the response of subtropical diatom assemblages to elevated seawater pCO₂. Therefore, natural plankton communities were enclosed for 32 days in in situ mesocosms (∼8 m³ volume) with a pCO₂ gradient ranging from 380 to 1140 μatm. Halfway through the study we added nutrients to all mesocosms (N, P, Si) to simulate injections through eddy-induced upwelling which frequently occurs in the region. We found that the total diatom biomass remained unaffected during oligotrophic conditions but was significantly positively affected by high CO₂ after nutrient enrichment. The average cell volume and carbon content of the diatom community increased with CO₂. CO₂ effects on diatom biomass and species composition were weak during oligotrophic conditions but became quite strong above ∼620 μatm after the nutrient enrichment. We hypothesize that the proliferation of diatoms under high CO₂ may have been caused by a fertilization effect on photosynthesis in combination with reduced grazing pressure. Our results suggest that OA in the subtropics may strengthen the competitiveness of (large) diatoms and cause changes in diatom community composition, mostly under conditions when nutrients are injected into oligotrophic systems.