In situ enclosure experiment using a benthic chamber system to assess the effect of high concentration of CO2 on deep-sea benthic communities

In order to evaluate the environmental impact associated with sequestration of carbon dioxide in the deep sea, a free fall type field experimental device, the benthic chamber, was developed. In situ experiments to expose deep-sea communities to elevated concentrations of carbon dioxide (average of 20,000 ppm, 5,000 ppm and control) were carried out using this device 3 times, viz., in the winter of 2002 and in the spring and the summer of 2003, in the Kumano Trough at a depth of 2,000 m. In the long-term experiments (about two weeks in winter of 2002 and summer of 2003), the abundance of meiobenthos declined whereas that of bacteria increased under the condition of 20,000 ppm carbon dioxide compared with the control. Among meiofauna, the abundance of foraminifers at the same concentration of carbon dioxide became less than the control even in the short,term (3 days in spring of 2003) experiment, suggesting that organisms with a calcium carbonate exoskeleton are more sensitive to the raised concentration of carbon dioxide. The respiration rate of the benthic community exposed to 20,000 ppm was lower in the early stage of the experiment than in the latter half, whereas it was opposite under the condition of 5,000 ppm. The increase of biological activity in the 20,000 ppm exposure group is probably due to an increase of bacteria adapted to high carbon dioxide concentrations. The present results suggest that the influence of carbon dioxide on the deep-sea benthic ecosystem does not follow a simple, linear relationship with concentration.