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We conducted a series of experiments in Monterey Submarine Canyon to examine potential ecological impacts of deep-ocean CO2 sequestration. Our focus was on responses of meiofaunal invertebrates (< 1 mm body length) living within the sediment at depths ranging between 3000-3600 m. Our particular emphasis was on harpacticoid copepods and nematodes. In the first phase of our DOE funding, we reported findings that suggest substantial (~80%) mortality to harpacticoid copepods. In the second phase of our funding we published additional findings from phase one and conducted follow-up experiments in the Monterey Canyon and in the laboratory. In one experiment we looked for evidence that meiofauna seek to escape areas where CO2 concentrations are elevated. Ã¢ÂÂEmergence trapsÃ¢ÂÂ near the source of the CO2-rich seawater caught significantly more harpacticoids than those far from it. The harpacticoids apparently attempted to escape from the advancing front of carbon dioxide-rich seawater and therefore presumably found exposure to it to be stressful. Although most were adversely affected, species differed significantly in the degree of their susceptibility. Unexpectedly, six species showed no effect and may be resistant. The hypothesis that harpacticoids could escape the effects of carbon dioxide-rich seawater by moving deeper into the seabed was not supported. Exposure to carbon dioxide-rich seawater created partially defaunated areas, but we found no evidence that disturbance-exploiting harpacticoid species invaded during the recovery of the affected area. Based on a detailed analysis of nematode biovolumes, we postulated that the nematode community in Monterey Canyon throughout the upper 3 cm suffered a high rate of mortality after exposure to CO2, and that nematodes were larger because postmortem expansions in body length and width occurred. Decomposition rates were probably low and corpses did not disintegrate in 30 days. The observable effects of a reduction in pH to about 7.0 after 30 days were as great as an extreme pH reduction (5.4), suggesting that Ã¢ÂÂmoderateÃ¢ÂÂ CO2 exposure, compared to the range of exposures possible following CO2 release, causes high mortality rates in the two most abundant sediment-dwelling metazoans (nematodes and copepods). While we found evidence for negative impacts on deep-sea benthos, we also observed that small-scale experiments with CO2 releases were difficult to replicate in the deep sea. Specifically, in one CO2-release experiment in the Monterey Canyon we did not detect an adverse impacts on benthic meiofauan. In laboratory experiments, we manipulated seawater acidity by addition of HCl and by increasing CO2 concentration and observed that two coastal harpacticoid copepod species were both more sensitive to increased acidity when generated by CO2. Copepods living in environments more prone to hypercapnia, such as mudflats, may be less sensitive to future acidification. Ocean acidification is also expected to alter the toxicity of waterborne metals by influencing their speciation in seawater. CO2 enrichment did not affect the free-ion concentration of Cd but did increase the free-ion concentration of Cu. Antagonistic toxicities were observed between CO2 with Cd, Cu and Cu free-ion. This interaction could be due to a competition for H+ and metals for binding sites

The Influence of deep-sea bed CO<sub>2</sub> sequestration on small metazoan (meiofaunal) community structure and function