MEPS 525:81-95 (2015)  -  DOI: https://doi.org/10.3354/meps11190

Canopy macroalgae influence understorey corallines’ metabolic control of near-surface pH and oxygen concentration

Christopher E. Cornwall1,5,*, Conrad A. Pilditch2, Christopher D. Hepburn3, Catriona L. Hurd1,4

1Department of Botany, University of Otago, PO Box 56, Dunedin 9054, New Zealand
2Department of Biological Sciences, University of Waikato, Private Bag 3105, Hamilton 3240, New Zealand
3Department of Marine Sciences, University of Otago, PO Box 56, Dunedin 9054, New Zealand
4Institute for Marine and Antarctic Studies, University of Tasmania, Private Bag 129, Hobart, Tasmania 7001, Australia
5Present address: School of Earth and Environment & ARC Centre of Excellence for Coral Reef Studies, University of Western Australia, 35 Stirling Highway, Crawley 6009, Western Australia, Australia
*Corresponding author:

ABSTRACT: Understorey macroalgae can alter pH at their surface via metabolic activity within the concentration boundary layer (CBL), but it is unknown to what degree the presence of larger macroalgal canopies can modify the pH micro-environment of understorey species. We examined whether flow reduction by a canopy-forming macroalga could alter the thickness of the CBL at the surface of understorey crustose coralline macroalgae (CCA). This could lead to a greater metabolic influence of macroalgae on the pH and oxygen environment at the coralline’s surface. Three experimental treatments were examined in a re-circulating flume: (1) a full canopy (consisting of Carpophyllum maschalocarpum) and understorey (Corallina officinalis and CCA), (2) a mimic (plastic/silk) canopy plus understorey, and (3) an understorey only. Profiles of seawater velocity and pH/O2 concentration gradients were measured at 3 bulk seawater velocities (2, 4 and 8 cm s-1) above the CCA in both the light and dark. Canopy macroalgae altered the pH and O2 environment encountered by understorey coralline algae via their physical presence rather than by directly altering bulk seawater chemistry through their metabolism. Reduced seawater velocities beneath Carpophyllum and mimic canopies resulted in increased CBL thicknesses, higher pH (up to 8.9) and O2 concentrations in the light, and lower pH (down to 7.74) and O2 concentrations in the dark. The ability of canopies to facilitate greater metabolic changes in pH at the surface of understorey species highlights a previously unrecorded species interaction that could play an important role in influencing the physiology and ecology of understorey assemblages.


KEY WORDS: Hydrodynamics · Understorey-canopy interactions · Seawater pH · Coralline algae · Boundary layers · Ocean acidification


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Cite this article as: Cornwall CE, Pilditch CA, Hepburn CD, Hurd CL (2015) Canopy macroalgae influence understorey corallines’ metabolic control of near-surface pH and oxygen concentration. Mar Ecol Prog Ser 525:81-95. https://doi.org/10.3354/meps11190

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