Diurnal fluctuations in seawater pH influence the response of a calcifying macroalga to ocean acidification

• Published in: Proceedings of the Royal Society B: Biological Sciences
• Main Authors: Cornwall, CE, Hepburn, CD, McGraw, CM, Currie, KI, Pilditch, CA, Hunter, KA, Boyd, PW, Hurd, CL
• Format: Article in Journal/Newspaper
• Language: English
• Published: The Royal Society Publishing 2013
• Subjects: Environmental Sciences; Ecological Applications; Ecological Impacts of Climate Change; Ocean acidification
• Online Access: https://doi.org/10.1098/rspb.2013.2201; http://www.ncbi.nlm.nih.gov/pubmed/24107535; http://ecite.utas.edu.au/89038

Coastal ecosystems that are characterized by kelp forests encounter daily pHfluctuations, driven by photosynthesis and respiration, which are larger than pH changes owing to ocean acidification (OA) projected for surface ocean waters by 2100. We investigated whether mimicry of biologically mediated diurnal shifts in pHbased for the first time on pH time-series measurements within a kelp forestwould offset or amplify the negative effects of OA on calcifiers. In a 40-day laboratory experiment, the calcifying coralline macro- alga, Arthrocardia corymbosa, was exposed to two mean pH treatments (8.05 or 7.65). For each mean, two experimental pH manipulations were applied. In one treatment, pH was held constant. In the second treatment, pH was manipulated around the mean (as a step-function), 0.4 pH units higher during daylight and 0.4 units lower during darkness to approximate diurnal fluctuations in a kelp forest. In all cases, growth rates were lower at a reduced mean pH, and fluctuations in pH acted additively to further reduce growth. Photosynthesis, recruitment and elemental composition did not change withpH, but d13C increased at lower mean pH. Including environmental hetero-geneity in experimental design will assist with a more accurate assessmentof the responses of calcifiers to OA.