Utility of deep sea CO 2 release experiments in understanding the biology of a high‐CO 2 ocean: Effects of hypercapnia on deep sea meiofauna

Oceanic CO 2 levels are expected to rise during the next 2 centuries to levels not seen for 10–150 million years by the uptake of atmospheric CO 2 in surface waters or potentially through the disposal of waste CO 2 in the deep sea. Changes in ocean chemistry caused by CO 2 influx may have broad impacts on ocean ecosystems. Physiological processes animals use to cope with CO 2 ‐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 CO 2 release experiments in producing stable perturbations in seawater chemistry over experimental seafloor plots, as is desirable for evaluating the CO 2 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” CO 2 pools surrounded by experimental plots. CO 2 enclosure experiments with experimental plots positioned within a circular arrangement of CO 2 pools had more moderate pH variation. The concentration of dissolution plumes from CO 2 pools were related to the speed and turbulence of near‐bottom currents, which influence CO 2 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.