Ocean acidification: How aquatic plants and animals can avoid it

[Australia] Hurd (2015) writes from her home base in Australia that the 0.4-unit reduction in seawater pH — which is typically referred to as ocean acidification (OA), and which is projected by many researchers to develop between now and 2100 — may negatively impact benthic coastal organisms that produce calcium carbonate “skeletons,” and, hence, that there is thus “an urgent need” to discover “refuge habitats that will afford protection to vulnerable species.” And in this regard, she notes that “the susceptibility of calcium carbonate skeletons to dissolution by OA depends on the pH at their surfaces” and that “this is controlled by the interaction between seawater velocity and organismal metabolism.”

In light of these well-established facts, Hurd goes on to further consider “how seawater velocity modifies the responses of calcifying organisms (seaweed, shellfish and tropical corals) to OA through its action on controlling diffusion boundary layer thickness and thereby the pH and calcium carbonate saturation state (Ω) at the organisms’ surfaces.” And what did this search reveal?

The scientist from “down under” says “there is a growing body of evidence to suggest that biogenic habitats created by seaweeds and sea-grasses, via their action of slowing flow and thereby retaining the high pH seawater generated via photosynthesis within the canopy boundary layer, create a habitat that is more conducive to net calcification than habitats without canopy-forming macrophytes,” citing the findings of Cornwall et al. (2015). In addition, she cites the study of Semesi et al. (2009), who “showed that spirorbid worms growing on the surface of seagrass calcify faster than those growing without seagrass, and by Garrard et al. (2014), who found that there were “almost twice as many calcifying invertebrates growing within seagrass beds in an acidified volcanic vent in Italy compared to locations without seagrass,” as well as by Anthony et al. (2013), who found that “on coral reefs too, under certain conditions, seaweeds may be beneficial to coral calcification by their action of raising seawater pH.”

And so it was that Hurd thus concluded that “slow flow habitats, including those created by seaweed or seagrass canopies, are likely to provide a natural refuge for many calcifiers from OA.”

Anthony, K.R.N., Diaz-Pulido, G., Verlinden, N., Tilbrook, B. and Andersson, A.J. 2013. Benthic buffers and boosters of ocean acidification on coral reefs. Biogeosciences 10: 4897-4909.

Cornwall, C.E., Pilditch, C.A., Hepburn, C.D. and Hurd, C.L. 2015. Canopy macroalgae influence understorey corallines’ metabolic control of near-surface pH and oxygen concentration. Marine Ecology Progress Series 552: 81-95.

Garrard, S.L., Gambi, M.C., Scipione, M.B., Patti, F.P., Lorenti, M., Zupo, V., Paterson, D.M. and Buia, M.C. 2014. Indirect effects may buffer negative responses of seagrass invertebrate communities to ocean acidification. Journal of Experimental Marine Biology and Ecology 461: 31-38.

Semesi, I., Beer, S. and Bjork, M. 2009. Seagrass photosynthesis controls rates of calcification and photosynthesis of calcareous macroalgae in a tropical seagrass meadow. Marine Ecology Progress Series 382: 41-47.


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