Ocean acidification causes ecosystem shifts via altered competitive interactions

Ocean acidification represents a pervasive environmental change that is predicted to affect a wide range of species1,2, yet our understanding of the emergent ecosystem impacts is very limited. Many studies report detrimental effects of acidification on single species in lab studies, especially those with calcareous shells or skeletons3–5. Observational studies using naturally acidified ecosystems have shown profound shifts away from such calcareous species6–8, and there has been an assumption that direct impacts of acidification on sensitive species drive most ecosystem responses. We tested an alternative hypothesis that species interactions attenuate or amplify the direct effects of acidification on individual species9–12. Here, we show that altered competitive dynamics between calcareous species and fleshy seaweeds drive significant ecosystem shifts in acidified conditions. Although calcareous species recruited and grew at similar rates in ambient and low pH conditions during early successional stages, they were rapidly overgrown by fleshy seaweeds later in succession in low pH conditions. The altered competitive dynamics between calcareous species and fleshy seaweeds is probably the combined result of decreased growth rates of calcareous species, increased growth rates of fleshy seaweeds, and/or altered grazing rates13. Phase shifts towards ecosystems dominated by fleshy seaweed are common in many marine ecosystems14–16, and our results suggest that changes in the competitive balance between these groups represent a key leverage point through which the physiological responses of individual species to acidification could indirectly lead to profound ecosystem changes in an acidified ocean. We deployed recruitment substrates in zones of extreme low, low and ambient seawater pH caused by shallow CO2 vents at two replicate sites (Supplementary Fig. S1). The pH zones are caused by spatial variability in CO2 venting6, resulting in decreased pH and carbonate ion concentrations and increased dissolved inorganic carbon at ambient temperature and alkalinity (Supplementary Fig. S1 and Table S1). The carbonate chemistry in the ambient pH zone is comparable to current conditions in the temperate surface ocean in the Mediterranean, whereas the low pH zones are comparable to predictions for the acidification of the near-future surface ocean (in the year 2100; ref. 17). The carbonate chemistry in the extreme low pH zones is not predicted in the near future but provides an endmember scenario for understanding acidification impacts. Previous studies have highlighted a reduction in diversity and the abundance of calcareous species in the low and extreme low pH zones6,7,18, but the processes underlying these patterns have not been investigated. Recruitment tiles were deployed at the beginning of the growing season for algae, and an independent