Coral calcification under environmental change: a direct comparison of the alkalinity anomaly and buoyant weight techniques
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Two primary methods—the buoyant weight (BW) and alkalinity anomaly (AA) techniques—are currently used to quantify net calcification rates (G) in scleractinian corals. However, it remains unclear whether they are directly comparable since the few method comparisons conducted to date have produced inconsistent results. Further, such a comparison has not been made for tropical corals. We directly compared G BW and G AA in four tropical and one temperate coral species cultured under various pCO2, temperature, and nutrient conditions. A range of protocols for conducting alkalinity depletion incubations was assessed. For the tropical corals, open-top incubations with manual stirring produced G AA that were highly correlated with and not significantly different from G BW. Similarly, G AA of the temperate coral was not significantly different from G BW when incubations provided water motion using a pump, but were significantly lower than G BW by 16% when water motion was primarily created by aeration. This shows that the two techniques can produce comparable calcification rates in corals but only when alkalinity depletion incubations are conducted under specific conditions. General recommendations for incubation protocols are made, especially regarding adequate water motion and incubation times. Further, the re-analysis of published data highlights the importance of using appropriate regression statistics when both variables are random and measured with error. Overall, we recommend the AA technique for investigations of community and short-term day versus night calcification, and the BW technique to measure organism calcification rates integrated over longer timescales due to practical limitations of both methods. Our findings will facilitate the direct comparison of studies measuring coral calcification using either method and thus have important implications for the fields of ocean acidification research and coral biology in general.
KeywordsCalcification Buoyant weight Alkalinity anomaly Method comparison Ocean acidification Incubation
We thank all staff at Reef Systems Coral Farm for logistical support. We thank E. Zebrowski, M. Berzelis, M. Ringwald, S. Levas, and S. Blackhurst at The Ohio State University for their assistance in the field and in the laboratory. We thank Jimmie (Luke) Snaric for carrying out the CO2 degassing experiment at Texas A&M University Corpus Christi. This work was supported by funding from the US National Science Foundation (NSF-EF-1041124, 1040940, 1041070 to AGG, MEW, and WJC, respectively). QL and HX acknowledge the partial support of their respective home institutions for their visit of the Cai laboratory at the University of Georgia when part of this work was accomplished. XH was a postdoctoral researcher in WJC’s laboratory at the University of Georgia during the experimental portion of this work, and also acknowledges the startup support provided by the College of Science and Engineering, Texas A&M University—Corpus Christi during the preparation of this manuscript. At WHOI, this work benefited from the assistance of B. Belastock, M. Brosnahan, N. Cantin, A. Cohen, G. Gaetani, S. Gallager, P. Henderson, K. Hoering, F. Keller, D. McCorkle, C. McGraw, J. Ries, T. Rioux, N. Shimizu, R. Schenk, J. Smith, A. Tarrant, K. Thompson, N. Trowbridge, A. Wang, D. Wellwood, M. White, and A. York. This material is based upon work supported under a US National Science Foundation Graduate Research Fellowship and International Society for Reef Studies/Ocean Conservancy Fellowship awarded to MH, and an NSF Grant (Oce-1041106) awarded to Anne Cohen.
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