Oceanic CO₂ levels are expected to rise during the next 2 centuries to levels not seen for 10–150 million years by the uptake of atmospheric CO₂ in surface waters or potentially through the disposal of waste CO₂ in the deep sea. Changes in ocean chemistry caused by CO₂ influx may have broad impacts on ocean ecosystems. Physiological processes animals use to cope with CO₂-related stress are known, but the range of sensitivities and effects of changes in ocean chemistry on most ocean life remain unclear. We evaluate the effectiveness of various designs for in situ CO₂ release experiments in producing stable perturbations in seawater chemistry over experimental seafloor plots, as is desirable for evaluating the CO₂ sensitivities of deep sea animals. We also discuss results from a subset of these experiments on the impacts of hypercapnia on deep sea meiofauna, in the context of experimental designs. Five experiments off central California show that pH perturbations were greatest for experiments using “point source” CO₂ pools surrounded by experimental plots. CO₂ enclosure experiments with experimental plots positioned within a circular arrangement of CO₂ pools had more moderate pH variation. The concentration of dissolution plumes from CO₂ pools were related to the speed and turbulence of near-bottom currents, which influence CO₂ dissolution and advection. Survival of meiofauna (nematodes, amoebae, euglenoid flagellates) was low after episodic severe hypercapnia but lower and variable where pH changes ranged from 0 to 0.2 pH units below normal.