The pelagic ocean, which encompasses the water column from the sea-surface to the ocean floor, hosts the largest ecosystem on Earth. The planktonic organisms inhabiting this ecosystem are responsible for about half of global primary production and form the base of the marine food web. They are the main drivers of elemental cycling, the exchange of climate relevant gases between ocean and atmosphere, including the sequestration of atmospheric CO₂ in the deep ocean, and for the supply of organic matter to the sediments. Through these processes the pelagic ecosystem generates important feedbacks in the climate system. In turn, changes in environmental conditions due to global change modulate pelagic ecosystems and biogeochemistry, with presently unknown consequences for marine ecosystem health, productivity and global climate.

Reliable prediction of ocean change requires a mechanistic understanding of biological responses to a rapidly changing marine environment. Due to the complexity and non-linear behaviour of marine ecosystems, unravelling those mechanisms is challenging. Experimental perturbation studies simulating on-going and projected environmental changes provide crucial information on biological responses to future ocean conditions. Accompanied by field observations and supported by advanced analytical instrumentation, those studies can provide information and data needed to refine our understanding and assessment of the societal impact of ocean change. A better process understanding is also essential for improving numerical models that can be used for prognostic scenario simulations into the future in order to inform society and policymakers.