Background
The authors state that anthropogenically-modulated reductions in pH, termed ocean acidification or OA, "could pose a major threat to the physiological performance, stocks and biodiversity of calcifiers and may devalue their ecosystem services." However, they note that "recent studies suggest that OA could be less detrimental to calcifying species that photosynthesize, such as coralline algae, growing in slow-flow compared to fast-flow habitats ... because the diffusion boundary layer (hereafter 'DBL') provides a thin but significant biologically-controlled buffer between the calcifying organism's external structure and the outer bulk seawater, potentially reducing rates of dissolution (Hurd et al., 2011; Cornwall et al., 2013.)."

What was done
To further explore this possibility, Cornwall et al. grew the coralline macroalga Arthrocardia corymbosa in a multi-factorial experiment with two mean pH levels (8.05 'ambient' and 7.65 a worst case 'ocean acidification' scenario projected for 2100), each with two rates of seawater flow: either fast or slow, producing either thin or thick DBLs, respectively.

What was learned
The eight researchers report that "growth under ambient seawater pH (8.05) was not significantly different in thin and thick DBL treatments," but they found that "coralline algae grown under slow flows with thick DBLs maintained net growth and calcification at pH 7.65, whereas those in higher flows with thin DBLs had net dissolution."

What it means
In light of what they learned, Cornwall et al. conclude that low-flow conditions that promote the formation of thick DBLs may enhance the subsistence of various calcifiers by creating localized hydrodynamic conditions "where metabolic activity ameliorates the negative impacts of ocean acidification." And they thus propose the possibility of "maintaining seaweed beds characterized by localized biological modification of pH and seawater flow, as potential refugia from OA."

References
Cornwall, C.E., Hepburn, C.D., Pilditch, C.A. and Hurd, C.L. 2013. Concentration boundary layers around complex assemblages of macroalgae: Implications for the effects of ocean acidification on understory coralline algae. Limnology and Oceanography 58: 121-130.

Hurd, C.L., Cornwall, C.E., Currie, K.I., Hepburn, C.D., McGraw, C.M., Hunter, K. and Boyd, P. 2011. Metabolically-induced pH fluctuations by some coastal calcifiers exceed projected 22nd century ocean acidification: a mechanism for differential susceptibility? Global Change Biology 17: 3254-3262.