The purpose of the cruise (June–July 2011) was to study the impact of ocean acidification on surface waters of the northwest European shelf, providing a glimpse into what may happen to the ocean as a whole as atmospheric CO₂ concentrations continue to rise. The aim was to examine impacts on marine organisms and ecosystems, on the cycling of carbon and nutrients, and on the production of climatically important gases. These potential impacts were investigated in two main ways. Firstly, observational data were analysed to look at how organisms, ecosystems and processes vary between regions where the chemistry of seawater is naturally more acidic and those where it is more alkaline. This comparison of observational data gives insights into the effects of ocean acidification taking place over centuries, long enough for natural selection and adaptation to play out. The second approach was to conduct short-term CO₂ bioassay perturbation experiments in bottles with natural communities collected in the surface layer and incubated in a controlled laboratory on deck. The natural communities were subjected to various levels of CO₂ that may occur in the future and the responses were determined. This large experiment examined the effects of changing CO₂ levels on real-world samples whilst at sea, as opposed to monocultures in the laboratory.