Effect of elevated CO 2 on the community metabolism of an experimental coral reef

• Published in: Global biogeochemical cycles 2003-03, Vol.17 (1)
• Main Authors: Langdon, Chris, Broecker, Wallace S., Hammond, Douglas E., Glenn, Edward, Fitzsimmons, Kevin, Nelson, Steven G., Peng, Tsung‐Hung, Hajdas, Irka, Bonani, Georges
• Format: Article
• Language: English
• ISSN: 0886-6236; 1944-9224
• DOI: 10.1029/2002GB001941

The effect of elevated pCO 2 on the metabolism of a coral reef community dominated by macroalgae has been investigated utilizing the large 2650 m 3 coral reef mesocosm at the Biosphere‐2 facility near Tucson, Arizona. The carbonate chemistry of the water was manipulated to simulate present‐day and a doubled CO 2 future condition. Each experiment consisted of a 1–2 month preconditioning period followed by a 7–9 day observational period. The pCO 2 was 404 ± 63 μatm during the present‐day pCO 2 experiment and 658 ± 59 μatm during the elevated pCO 2 experiment. Nutrient levels were low and typical of natural reefs waters (NO 3 − 0.5–0.9 μM, NH 4 + 0.4 μM, PO 4 3− 0.07–0.09 μM). The temperature and salinity of the water were held constant at 26.5 ± 0.2°C and 34.4 ± 0.2 ppt. Photosynthetically available irradiance was 10 ± 2 during the present‐day experiment and 7.4 ± 0.5 mol photons m −2 d −1 during the elevated pCO 2 experiment. The primary producer biomass in the mesocosm was dominated by four species of macroalgae; Haptilon cubense, Amphiroa fragillisima, Gelidiopsis intricata and Chondria dasyphylla . Algal biomass was 10.4 mol C m −2 during the present‐day and 8.7 mol C m −2 and during the elevated pCO 2 experiments. As previously observed, the increase in pCO 2 resulted in a decrease in calcification from 0.041 ± 0.007 to 0.006 ± 0.003 mol CaCO 3 m −2 d −1 . Net community production (NCP) and dark respiration did not change in response to elevated pCO 2 . Light respiration measured by a new radiocarbon isotope dilution method exceeded dark respiration by a factor of 1.2 ± 0.3 to 2.1 ± 0.4 on a daily basis and by 2.2 ± 0.6 to 3.9 ± 0.8 on an hourly basis. The 1.8‐fold increase with increasing pCO 2 indicates that the enhanced respiration in the light was not due to photorespiration. Gross production (GPP) computed as the sum of NCP plus daily respiration (light + dark) increased significantly (0.24 ± 0.03 vs. 0.32 ± 0.04 mol C m −2 d −1 ). However, the conventional calculation of GPP based on the assumption that respiration in the light proceeds at the same rate as the dark underestimated the true rate of GPP by 41–100% and completely missed the increased rate of carbon cycling due to elevated pCO 2 . We conclude that under natural, undisturbed, nutrient‐limited conditions elevated CO 2 depresses calcification, stimulates the rate of turnover of organic carbon, particularly in the light, but has no effect on net organic production. The hypothesis that an i