This chapter reviews some limitations on studies of the effects of ocean acidification on ecosystems and their biodiversity. It recalls that warming magnitude may be as pronounced as 4-5ºC in worst-case scenarios by 2100. The chapter examines long-term changes in the spatial distribution of various calcifying planktonic species or taxa recorded by the Continuous Plankton Recorder survey. Research on the impacts of ocean acidification on marine organisms focuses on calcifying taxa as they are likely to be the most vulnerable to pH modifications and their effects on calcification and other physiological processes. Plankton is a major contributor to carbon cycling and can generate important flux of calcium carbonate from the surface to the bottom. Open-ocean ecosystems might also undertake rapid changes as a result of ocean acidification. Although cyanobacteria might be stimulated by PCO2 increase, pteropods, coccolithophores and foraminifers might be affected by ocean acidification.