Simulated sequestration of anthropogenic carbon dioxide at a deep-sea site: Effects on nematode abundance and biovolume

One proposal for ameliorating global warming is to sequester large amounts of carbon dioxide in the deep ocean, but the environmental consequences of sequestration for sediment-dwelling animals are poorly known. In a previous publication, we reported that ∼80% of benthic copepods were killed in an experimental release of CO2 off northern California at 3262 m. The effects of this release on nematodes are reported here. We examined samples of nematodes taken inside two ‘corrals’ into which CO2 was directly injected (providing an extreme endpoint for CO2 exposure) and taken near to and far from this CO2 source. After 30 days, pore-water pH was unchanged (∼7.8) at the sediment–water interface far (∼40 m) from corrals, but pH profiles were reduced by ∼0.75 near (∼2 m) corrals. Corral pH was highly acidic (5.4 in a measurement from a subsequent experiment). Fifty randomly selected nematodes from each of four vertical layers from the 14 cores were photographed. They were assigned to a tail group (based on morphology), and individual biovolume was estimated from measurements of body length and width. Although nematode abundance (expressed as total nematodes and by tail group) was not affected, length, width, and individual biovolume significantly differed between near and far samples. Median nematode biovolume examined across tail group and core layer increased by ∼48% inside and near corrals. Differences between near and corral samples were always less than differences between near and far samples. However, nematode length:width ratio did not differ between near and far, and the shapes of length, width, and biovolume frequency distributions were similar in all samples. We postulate that the nematode community 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 ‘moderate’ 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).