Direct effect of elevated atmospheric CO2 on the health of coral reefs
Chris Langdon
Lamont-Doherty Earth Obs. Of Columbia Uni.
Palisades, NY 10964, U.S.A.
langdon@ldeo.columbia.edu
The concentration of CO2 in the atmosphere is projected to reach twice the preindustrial level by the middle of the 21st century. This increase will reduce the concentration of CO32- 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. There is steadily increasing experimental evidence that the build up CO2 in the atmosphere will have a direct effect on the functioning of marine ecosystems that is apart from the effect of global warming. Using the large 2650 m3 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 calcification of coral reef organisms at the community scale. Our experimental design was to obtain a long (3.8 years) time series of the net calcification of the complete system and all relevant physical and chemical variables (temperature, salinity, light, nutrients, Ca2+, pCO2, TCO2, and total alkalinity). Periodic additions of NaHCO3, Na2CO3, and/or CaCl2 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 calcification in response to our manipulations of the saturation state. We show that the net community calcification rate responds to manipulations in the concentrations of both Ca2+ and CO32- and that the rate is well described as a linear function of the ion concentration product, [Ca2+]0.69 [CO32-]. This suggests that saturation state or a closely related quantity is a primary environmental factor that influences calcification on coral reefs at the ecosystem level. We compare the sensitivity of calcification 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 between the years 1880 and 2065 A.D. directly due to the rise in atmospheric CO2 is 40% based on our long-term results. 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 accretion of calcium carbonate on some coral reefs may not be able to keep up with rising sea level and erosion due to storms. This change in the health of coral reefs can only be detected by careful monitoring of the in situ calcification rates at selected reefs.
