Abstract. The concentration of CO2 in the atmosphere is projected to reach twice the preindustrial level by the middle of the 21 st century. This increase will reduce the 2 concentration of CO3-of the surface ocean by 30% relative to the preindustrial level and will reduce the calcium carbonate saturation state of the surface ocean by an equal percentage.Using the large 2650 m 3 coral reef mesocosm at the BIOSPHERE-2 facility near Tucson, Arizona, we investigated the effect of the projected changes in seawater carbonate chemistry on the calcificafion of coral reef organisms at the commtmity scale. Our experimental design was to obtain a long (3.8 years) time'series of the net calcificafion of the complete system and all relevant physical and chemical variables (tem•rature, salinity, light, nutrients, Ca 2+, pCO2, TCO2, and total alkalinity). Periodic additions of NaHCOz, Na2CO•, and/or CaC12 were made to change the calcium carbonate saturation state of the water. We found that there were consistent and reproducible changes in the rate of calcificafion in response to our manipulations of the saturation state. We show that the net community calcificafion rate suggests that saturation state or a closely related quantity is a primary environmental factor that influences calcffication on coral reefs at the ecosystem level. We compare the sensitivity of cal½ification to short-term (days) and long-term (months to years) changes in saturation state and found that the response was not significantly different. This indicates that coral reef organisms do not seem to be able to acclimate to changing saturation state. The predicted decrease in coral reef calcification •een the years 1880 and 2065 A.D. based on our longterm results is 40%. Previous small-scale, short-term organismal studies predicted a calcification reduction of 14-30%. This much longer, community-scale study suggests that the impact on coral reefs may be greater than previously suspected. in the next century coral reefs will be less able to cope with rising sea level and other anthropogenic stresses.

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The decrease in the saturation state of seawater, X, following seawater acidification, is believed to be the main factor leading to a decrease in the calcification of marine organisms. To provide a physiological explanation for this phenomenon, the effect of seawater acidification was studied on the calcification and photosynthesis of the scleractinian tropical coral Stylophora pistillata. Coral nubbins were incubated for 8 days at three different pH (7.6, 8.0, and 8.2). To differentiate between the effects of the various components of the carbonate chemistry (pH, CO 3 2-, HCO 3-, CO 2 , X), tanks were also maintained under similar pH, but with 2-mM HCO 3 added to the seawater. The addition of 2-mM bicarbonate significantly increased the photosynthesis in S. pistillata, suggesting carbon-limited conditions. Conversely, photosynthesis was insensitive to changes in pH and pCO 2. Seawater acidification decreased coral calcification by ca. 0.1mg CaCO 3 g-1 d-1 for a decrease of 0.1 pH units. This correlation suggested that seawater acidification affected coral calcification by decreasing the availability of the CO 3 2substrate for calcification. However, the decrease in coral calcification could also be attributed either to a decrease in extra-or intracellular pH or to a change in the buffering capacity of the medium, impairing supply of CO 3 2from HCO 3- .

) during 3 different chemical states, mimicking the conditions of the Last Glacial Maximum (LGM), the present day and the year 2100 (Y2100). Calcification rate changed with light following the typical hyperbolic tangent function. Calcification rate was positively correlated with saturation state, which ranged from 5 (LGM) to 2.4 (Y2100). From the glacial experimental conditions (Ω = 5.05, pCO 2 = 186) to those of the future (Ω = 2.25, pCO 2 = 641), calcification dropped by 30%; from present day conditions (Ω = 3.64, pCO 2 = 336) to those of the future, calcification dropped by 11%. This decrease in calcification rate occurred at all light levels, indicating that rising CO 2 will impact corals living at all depths.

Biogenic calcification is influenced by the concentration of available carbonate ions. The recent confirmation of this for hermatypic corals has raised concern over the future of coral reefs because [CO 2 2 3 ] is a decreasing function of increasing pCO 2 in the atmosphere. As one of the overriding features of coral reefs is their diversity, understanding the degree of variability between species in their ability to cope with a change in [CO 2 2 3 ] is a priority. We cultured four phylogenetically and physiologically different species of hermatypic coral (Acropora verweyi, Galaxea fascicularis, Pavona cactus and Turbinaria reniformis) under 'normal' (280 m mol kg 2 1 ) and 'low' (140 m mol kg 21 ) carbonate-ion concentrations. The effect on skeletogenesis was investigated quantitatively (by calcification rate) and qualitatively (by microstructural appearance of growing crystalline fibres using scanning electron microscopy (SEM)). The 'low carbonate' treatment resulted in a significant suppression of calcification rate and a tendency for weaker crystallization at the distal tips of fibres. However, while the calcification rate was affected uniformly across species (13-18% reduction), the magnitude of the microstructural response was highly species specific: crystallization was most markedly affected in A. verweyi and least in T. reniformis. These results are discussed in relation to past records and future predictions of carbonate variability in the oceans.

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 μM) 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.

Both CO2 chemistry and nutrient concentrations of seawater affect coral calcification. The relative effects of these factors on growth of corals were studied using coral tips or 'nubbins' of the hermatypic coral Porites compressa. Coral nubbins were grown over 5 wk in different combinations of p C 0 2 (760 and 3980 patm), HCO3-(1670 and 1520 FM), CO3'-(110 and 20 FM), and NO,-(0.42 to 5.66 PM). The pC02 was increased and CO3'-decreased by adding HCI to normal seawater; NO3-was increased by adding KN03 to ambient seawater. Corals growing in seawater at a reduced pH of 7.2 calcified at half the rate of control corals at pH 8.0, indicating that coral growth is strongly dependent on the concentration of CO3'-ions in seawater. Reduction of calcification from lowered pH and C032' was greater than reduction from nitrate addtions. Corals in low pH treatments recovered their initial calcification rates within 2 d of re-introduction to ambient seawater, indicating the effects of CO2 chemistry are immediate and reversible. Changes in calcification from increases in atmospheric CO?, and hence decreases in CO~'-, may be larger than local effects from elevated nutrients.