Coral Reefs

, Volume 37, Issue 4, pp 1169–1180 | Cite as

Impacts of coral bleaching on pH and oxygen gradients across the coral concentration boundary layer: a microsensor study

  • Verena SchoepfEmail author
  • Christopher E. Cornwall
  • Svenja M. Pfeifer
  • Steven A. Carrion
  • Cinzia Alessi
  • Steeve Comeau
  • Malcolm T. McCulloch


Reef-building corals are surrounded by complex microenvironments (i.e. concentration boundary layers) that partially isolate them from the ambient seawater. Although the presence of such concentration boundary layers (CBLs) could potentially play a role in mitigating the negative impacts of climate change stressors, their role is poorly understood. Furthermore, it is largely unknown how heat stress-induced bleaching affects O2 and pH dynamics across the CBLs of coral, particularly in branching species. We experimentally exposed the common coral species Acropora aspera to heat stress for 13 d and conducted a range of physiological and daytime microsensor measurements to determine the effects of bleaching on O2 and pH gradients across the CBL. Heat stress equivalent to 24 degree heating days (3.4 degree heating weeks) resulted in visible bleaching and significant declines in photochemical efficiency, photosynthesis rates and photosynthesis to respiration (P/R) ratios, whereas dark respiration and calcification rates were not impacted. As a consequence, bleached A. aspera had significantly lower (− 13%) surface O2 concentrations during the day than healthy corals, with concentrations being lower than that of the ambient seawater, thus resulting in O2 uptake from the seawater. Furthermore, we show here that Acropora, and potentially branching corals in general, have among the lowest surface pH elevation of all corals studied to date (0.041 units), which could contribute to their higher sensitivity to ocean acidification. Additionally, bleached A. aspera no longer elevated their surface pH above ambient seawater values and, therefore, had essentially no [H+] CBL. These findings demonstrate that heat stress-induced bleaching has negative effects on pH elevation and [H+] CBL thickness, which may increase the overall susceptibility of coral to the combined impacts of ocean acidification and warming.


Acropora aspera Metabolism Calcification Diffusive oxygen flux Concentration gradients Heat stress 



Funding for this study was provided by the Australian Research Council (ARC) Centre of Excellence for Coral Reef Studies, the Western Australian Marine Science Institution (WAMSI), an ARC Laureate Fellowship awarded to MM and an ARC DECRA Award (DE160100668) awarded to SC.

Compliance with ethical standards

Conflict of interest

On behalf of all authors, the corresponding author states that there is no conflict of interest.


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Copyright information

© Springer-Verlag GmbH Germany, part of Springer Nature 2018

Authors and Affiliations

  1. 1.Oceans Graduate School and UWA Oceans InstituteThe University of Western AustraliaPerthAustralia
  2. 2.ARC Centre of Excellence for Coral Reef StudiesThe University of Western AustraliaPerthAustralia
  3. 3.Department of BiologyHeinrich-Heine-Universität DüsseldorfDüsseldorfGermany
  4. 4.School of GeosciencesUniversity of EdinburghEdinburghUK
  5. 5.Department of Earth and Marine Science (DiSTeM)University of PalermoPalermoItaly
  6. 6.School of Biological SciencesVictoria University of WellingtonWellingtonNew Zealand
  7. 7.Laboratoire d’Océanographie de Villefranche, CNRS-INSUSorbonne UniversitéVillefranche-sur-merFrance

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