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The Resilience of a Mediterranean Seagrass Community to Ocean Acidification

Paper Reviewed
Cox, T.E., Nash, M., Gazeau, F., Déniel, M., Legrand, E., Alliouane, S., Mahacek, P., Le Fur, A., Gattuso, J.-P. and Martin, S. 2017. Effects of in situ CO2 enrichment on Posidonia oceanica epiphytic community composition and mineralogy. Marine Biology 164: 103, DOI 10.1007/s00227-017-3136-7.

Seagrass meadows consist of far more marine life than the inherent macroalgae that form them. Taxonomically diverse animal and algal species, known as epiphytes, colonize seagrass leaves and rhizomes. In the Mediterranean Sea, Posidonia oceanica covers nearly a quarter of the shallow water substratum and leaf epiphytes constitute roughly the same percentage of the canopy biomass, being comprised principally of coralline and filamentous algae, polychaetes, foraminiferans and bryozoans. Altogether, these epiphytes can contribute 20 percent of the primary production in these meadows and 60 percent of the nutrient uptake, while providing a key food source for herbivores and grazers. Recognizing these important facts and the contribution of epiphytes to Mediterranean seagrass communities, Cox et al. (2017) decided to investigate the possible response of these marine species to so-called ocean acidification, their hypothesis being that they would respond negatively to the declining seawater pH that is projected to occur by the end of this century.

To accomplish their objective, the team of ten scientists conducted a Free Ocean Carbon Dioxide Enrichment (FOCE) experiment wherein they manipulated the pH of small enclosures deployed in seagrass meadows in the Bay of Villefranche, NW Mediterranean Sea, France (43.68°N, 7.32°E) for a period of four months. Oceanic pH values in the acidified treatment chambers were kept at a mean value of 0.26 unit below that in the ambient or control chambers. At multiple intervals during the experiment the authors collected seagrass leaves from the different treatment chambers and examined them for differences in epiphytic coverage and composition, as well as bulk epiphytic mineralogy.

In reporting their findings, Cox et al. state that (1) the "percent coverage of invertebrate calcifiers and crustose coralline algae (CCA) did not appear to be affected by the lowered pH," (2) "fleshy algae did not proliferate at lowered pH," (3) "bulk epiphytic magnesium carbonate composition was similar between treatments," (4) "CCA did not exhibit any visible skeleton dissolution or mineral alteration at lowered pH and carbon saturation state" and (5) only leaf epiphytic Foraminifera "had a directional change in abundance distinct from the change in abundance on leaves from the control enclosure and reference plot," yet they add that "this taxon is rare (indicated by low coverage, <1%) and coverage between leaves can be highly variable."

Commenting on the significance of these several observations, Cox et al. write that "the present study outcome adds to the growing literature which suggests that calcified communities in their natural settings can be little affected by minimal changes in surrounding carbonate chemistry," i.e., ocean acidification, and they cite several additional studies in support of this statement.

Posted 30 August 2017