Effect of elevated pCO₂ on photosynthesis and calcification of corals and interactions with seasonal change in temperature/irradiance and nutrient enrichment

TLDR

Net production and calcification in corals are thought to compete for the same internal supply of dissolved inorganic carbon (DIC). This study examined how elevated pCO₂ affects net production and calcification in a coral assemblage under near‑natural temperature, light, nutrient, and flow conditions. Experiments were carried out in summer and winter, including a nutrient enrichment treatment, to assess interactions between seasonal temperature/irradiance, elevated pCO₂, and the potential competition between photosynthesis and calcification. Elevated pCO₂ increased net photosynthetic production by ~3 % per μmol CO₂ kg⁻¹ h⁻¹, while calcification was strongly tied to aragonite saturation state and declined with both CO₂ and nutrient enrichment, supporting a DIC‑competition mechanism.

Abstract

An investigation was conducted to determine the effects of elevated pCO 2 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 pCO 2 . 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 pCO 2 . Net carbon production, NP C , increased with increased pCO 2 at the rate of 3 ± 2% (μmol CO 2 aq kg −1 ) −1 . Seasonal change of the slope NP C ‐[CO 2 aq] 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 CaCO 3 m −2 h −1 )(Ω a − 1), r 2 = 0.87, P = 0.0001. Both nutrient and CO 2 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 pCO 2 could be competition between photosynthesis and calcification for a limited supply of DIC.