Limnol. Oceanogr., 44(3), 1999, 716–720

The addition of 2 mM bicarbonate to aquaria containing tropical ocean water and branches of Porites porites caused a doubling of the skeletal growth rate of the coral. Nitrate or ammonium addition (20 mM) to oligotrophic seawater caused a significant reduction in coral growth, but when seawater containing the extra bicarbonate was supplemented with combined nitrogen, no depression of the higher growth rate was evident. We infer that (1) the present dissolved inorganic carbon (DIC) content of the ocean limits coral growth, (2) this limitation is exacerbated by nitrate and ammonium, and (3) adding DIC increases coral calcification rates and confers protection against nutrient enrichment. Dissolved inorganic carbon (DIC) is used by all hard corals for growth of CaCO3 skeletons. Hermatypic corals (reefbuilding corals that harbor zooxanthellae) also require DIC for photosynthesis. Until recently, the general view has been that, at concentrations greater than 2 mM, DIC in seawater is in ample supply compared with other essential solutes such as nitrate and phosphate and trace elements such as iron, zinc, and silicon. There is, however, a growing awareness (Raven 1993) that the supply of DIC limits photosynthesis in planktonic algae, seagrasses (Beer and Rehnberg 1997), coccolithophores (Merrett et al. 1993; Israel and Gonzales 1996), and macroalgae (Johnston et al. 1992). Several authors (Muscatine et al. 1989b; Weis et al. 1989; Dubinsky et al. 1990; Lesser et al. 1994) have speculated on DIC limitation in coral. The only study that tested experimentally the effect of adding DIC (Burris et al. 1983) focused only on photosynthesis and was inconclusive.