Effect of elevated CO₂ on the community metabolism of an experimental coral reef

Abstract

[1] 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 matm during the present-day pCO 2 experiment and 658 ± 59 matm during the elevated pCO 2 experiment. Nutrient levels were low and typical of natural reefs waters (NO 3 À 0.5-0.9 mM, NH 4 + 0.4 mM, PO 4 3À 0.07-0.09 mM). 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 À2 d À1 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 À2 during the present-day and 8.7 mol C m À2 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 À2 d À1. 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 À2 d À 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 increase pCO 2 would produce an increase in net production that would counterbalance the effect of decreasing saturation state on calcification is not supported by these data.

Planktonic and whole system metabolism in a nutrient-rich estuary (the Scheldt estuary)

“…In order to convert hourly values to daily rates, we assumed that both CR and NIT rates are constant over 24 h and this might have introduced a bias in our results. Several authors have stated that CR is likely to be enhanced in the presence of light (Grande et al 1989b;Martinez 1992;Langdon et al 2003) although such observations were not always corroborated (Grande et al 1989a,b). NIT has been found to be inhibited by a factor of 40% to 50% in light (Horrigan and Springer 1990;Ward 2005).…”

Marine ecosystem community carbon and nutrient uptake stoichiometry under varying ocean acidification during the PeECE III experiment

“…Gattuso et al, 1998;Langdon et al, 2003;Kleypas et al, 2006;Delille et al, 2005;Langer et al, 2006). The dominant calcifier during the experiment was the prymesiophyte Emiliania Huxleyi which is common to the Norwegian fjords and open waters of the Norwegian and Barents Sea.…”

Spatiotemporal Assessment of CO2–Carbonic Acid System Dynamics in a Pristine Coral Reef Ecosystem, French Frigate Shoals, Northwestern Hawaiian Islands

“…Although numerous experimental and field studies show that increasing CO 2 and decreasing aragonite saturation state (X ar ) lead to decreased coral calcification rates and increased dissolution rates of carbonate substrates, the predicted point in time at which an individual reef ecosystem will shift from net accretion to net dissolution varies across studies and ecosystems (Silverman et al 2007;Ohde and van Woesik 1999;Langdon et al 2000Langdon et al , 2003Andersson et al 2009;Shamberger et al 2011;Shaw et al 2012;Andersson et al 2014). While the global distribution of coral reefs is governed by light availability, temperature, nutrients and X ar (Kleypas et al 1999), at the local scale, the conditions that determine coral reef calcification, distribution and community composition are much more complicated.…”