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

Metabolic processes have the potential to modulate the effects of ocean acidification (OA) in nearshore macroalgal beds. We investigated whether natural mixed assemblages of the articulate coralline macroalga Arthrocardia corymbosa and understory crustose coralline algae (CCA) altered pH and O₂ concentrations within and immediately above their canopies. In a unidirectional flume, we tested the effect of water velocity (0– 0.1 m s^-1), bulk seawater pH (ambient pH 8.05, and pH 7.65), and irradiance (photosynthetically saturating light and darkness) on pH and O₂ 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 to 7.90–8.00 in the light. Although these manipulations were short term, this indicates a potential daytime buffering capacity that could alleviate the effects of OA. Photosynthetic activity also increased O₂ concentrations at the surface of the CCA. However, this moderating capacity was flow dependent; the CBL thickness decreased from an 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 occurred in the dark, with respiration causing pH and O₂ concentrations to decrease at the CCA surface. At all flow velocities the CBL thicknesses (up to 68 mm) were much greater than those previously published, indicating that the presence of canopies can alter the CBL substantially. In situ, the height of macroalgal canopies can be an order of magnitude larger than those used here, indicating that the degree of buffering to OA will be context dependent.