Asnaghi, Valentina; Chiantore, Mariachiara; Mangialajo, Luisa; Gazeau, Frédéric; Francour, Patrice; Alliouane, Samir; Gattuso, Jean-Pierre (2013): Effects of ocean acidification on the growth of calcifying and non-calcifying algae and on jaw/test ratio and test robustness of sea urchins fed the different algae. doi:10.1594/PANGAEA.818626, Supplement to: Asnaghi, V et al. (2013): Cascading Effects of Ocean Acidification in a Rocky Subtidal Community. PLoS ONE, 8(4), e61978, doi:10.1371/journal.pone.0061978.t004
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Temperate marine rocky habitats may be alternatively characterized by well vegetated macroalgal assemblages or barren grounds, as a consequence of direct and indirect human impacts (e.g. overfishing) and grazing pressure by herbivorous organisms. In future scenarios of ocean acidification, calcifying organisms are expected to be less competitive: among these two key elements of the rocky subtidal food web, coralline algae and sea urchins. In order to highlight how the effects of increased pCO2 on individual calcifying species will be exacerbated by interactions with other trophic levels, we performed an experiment simultaneously testing ocean acidification effects on primary producers (calcifying and non-calcifying algae) and their grazers (sea urchins). Artificial communities, composed by juveniles of the sea urchin Paracentrotus lividus and calcifying (Corallina elongata) and non-calcifying (Cystoseira amentacea var stricta, Dictyota dichotoma) macroalgae, were subjected to pCO2 levels of 390, 550, 750 and 1000 µatm in the laboratory. Our study highlighted a direct pCO2 effect on coralline algae and on sea urchin defense from predation (test robustness). There was no direct effect on the non-calcifying macroalgae. More interestingly, we highlighted diet-mediated effects on test robustness and on the Aristotle's lantern size. In a future scenario of ocean acidification a decrease of sea urchins' density is expected, due to lower defense from predation, as a direct consequence of pH decrease, and to a reduced availability of calcifying macroalgae, important component of urchins' diet. The effects of ocean acidification may therefore be contrasting on well vegetated macroalgal assemblages and barren grounds: in the absence of other human impacts, a decrease of biodiversity can be predicted in vegetated macroalgal assemblages, whereas a lower density of sea urchin could help the recovery of shallow subtidal rocky areas affected by overfishing from barren grounds to assemblages dominated by fleshy macroalgae.
Asnaghi-etal_2013 * Location: Mediterranean Sea * Device: Experiment (EXP) * Comment: Juveniles of Paracentrotus lividus, about 4 months old, were provided by a sea urchin hatchery in Camogli (NW Mediterranean Sea, Italy), where they had been reared after in vitro fertilization. A total of 144 juveniles, with a test diameter of 5 to 6 mm, had been randomly selected and moved to the Laboratoire d'Oceanographie de Villefranche, where the experiment was performed, one month before the start of the experiment. Algal specimens were collected in the Bay of Villefranche (NW Mediterranean Sea, France) at a depth of less than 5 m, and transported to Villefranche laboratory.