Abstract
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.
Highlights
The partial pressure of CO2 in the atmosphere has increased by about 40% (267 to 384 ppm) since the beginning of the industrial revolution, leading to changes in the Earth’s climate and in terrestrial ecosystems functioning [1]
Parameters of the carbonate chemistry are reported in Table 1. pHT was maintained at an average (6SD) of (1) 8.09 6 0.04, (2) 7.98 6 0.06, (3) 7.84 6 0.04 and (4) 7.70 6 0.03, in the four treatments, respectively
Irradiance and temperature in the experimental aquaria were chosen in order to mimic natural conditions, no growth of algal thalli was observed during the experiment in any of the treatments
Summary
The partial pressure of CO2 (pCO2) in the atmosphere has increased by about 40% (267 to 384 ppm) since the beginning of the industrial revolution, leading to changes in the Earth’s climate and in terrestrial ecosystems functioning [1]. Mean surface ocean pH has decreased by approximately 0.1 unit between pre-industrial time and the 1990s [2,3], and a further decrease of approximately 0.4 units is predicted to occur by the end of the century [3] This ‘‘ocean acidification’’ process may have profound impacts on marine biota, mostly through the direct effects of pH on inter-cellular transport mechanisms that control the physiology and metabolism of marine organisms [4] and the decreased availability of CO322, used by many species to build calcareous shells and skeletons. As coral reefs, are threatened by ocean acidification, since most of their organisms use carbonates to build calcareous structures. Experiments investigating the effects of elevated pCO2 on photosynthesis and/or calcification of calcifying algae show complex and species-specific responses, with variable results depending on the pathway of carbonate deposition and on its relative amount [9]
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