An investigation was conducted to determine the effects of elevated pCO₂ on the net production and calcification of an assemblage of corals maintained under near-natural conditions of temperature, light, nutrient, and flow. Experiments were performed in summer and winter to explore possible interactions between seasonal change in temperature and irradiance and the effect of elevated pCO₂. Particular attention was paid to interactions between net production and calcification because these two processes are thought to compete for the same internal supply of dissolved inorganic carbon (DIC). A nutrient enrichment experiment was performed because it has been shown to induce a competitive interaction between photosynthesis and calcification that may serve as an analog to the effect of elevated pCO₂. Net carbon production, NP_C, increased with increased pCO₂ at the rate of 3 ± 2% (μmol CO₂aq kg⁻¹)⁻¹. Seasonal change of the slope NP_C-[CO₂aq] relationship was not significant. Calcification (G) was strongly related to the aragonite saturation state Ω _a . Seasonal change of the G-Ω _a relationship was not significant. The first-order saturation state model gave a good fit to the pooled summer and winter data: G = (8 ± 1 mmol CaCO₃ m⁻² h⁻¹)(Ω _a − 1), r ² = 0.87, P = 0.0001. Both nutrient and CO₂ enrichment resulted in an increase in NP_C and a decrease in G, giving support to the hypothesis that the cellular mechanism underlying the decrease in calcification in response to increased pCO₂ could be competition between photosynthesis and calcification for a limited supply of DIC.