ABSTRACT

Using living corals collected from Okinawan coral reefs, laboratory experiments were performed to investigate the relationship between coral calcification and aragonite saturation state (Ω) of seawater at 25°C. Calcification rate of a massive coral Porites lutea cultured in a beaker showed a linear increase with increasing Ωaragonite values (1.08–7.77) of seawater. The increasing trend of calcification rate (c) for Ω is expressed as an equation, c = aΩ + b (a, b: constants). When Ω was larger than ∼4, the coral samples calcified during nighttime, indicating an evidence of dark calcification. This study strongly suggests that calcification of Porites lutea depends on Ω of ambient seawater. A decrease in saturation state of seawater due to increased pCO₂ may decrease reef-building capacity of corals through reducing calcification rate of corals.