Concentration boundary layers and complex assemblages of macroalgae: Implications for the effects of ocean acidification on understory coralline algae

• Published in: Limnology and Oceanography
• Main Authors: Cornwall, CE, Hepburn, CD, Pilditch, CA, Hurd, CL
• Format: Article in Journal/Newspaper
• Language: English
• Published: Amer Soc Limnology Oceanography 2013
• Subjects: Environmental Sciences; Ecological Applications; Ecological Impacts of Climate Change; Ocean acidification
• Online Access: https://doi.org/10.4319/lo.2013.58.1.0121; http://ecite.utas.edu.au/91461

Metabolic processes have the potential to modulate the effects of ocean acidification (OA) in nearshoremacroalgal beds. We investigated whether natural mixed assemblages of the articulate coralline macroalga Arthrocardia corymbosa and understory crustose coralline algae (CCA) altered pH and O 2 concentrations withinand immediately above their canopies. In a unidirectional flume, we tested the effect of water velocity (00.1 m s -1 ), bulk seawater pH (ambient pH 8.05, and pH 7.65), and irradiance (photosynthetically saturating lightand darkness) on pH and O 2 concentration gradients, and the derived concentration boundary layer (CBL)thickness. At bulk seawater pH 7.65 and slow velocities (0 and 0.015 m s -1 ), pH at the CCA surface increased to7.908.00 in the light. Although these manipulations were short term, this indicates a potential daytime bufferingcapacity that could alleviate the effects of OA. Photosynthetic activity also increased O 2 concentrations at thesurface of the CCA. However, this moderating capacity was flow dependent; the CBL thickness decreased froman average of 26.8 mm from the CCA surface at 0.015 m s -1 to 4.1 mm at 0.04 m s -1 . The reverse trends occurredin the dark, with respiration causing pH and O 2 concentrations to decrease at the CCA surface. At all flowvelocities the CBL thicknesses (up to 68 mm) were much greater than those previously published, indicating thatthe presence of canopies can alter the CBL substantially. In situ, the height of macroalgal canopies can be anorder of magnitude larger than those used here, indicating that the degree of buffering to OA will be contextdependent.