Impacts of ocean acidification and burrowing urchins on within-sediment pH profiles and subtidal nematode communities

Rising carbon dioxide levels in the atmosphere will affect the ocean carbonate system, resulting in a predicted future decrease in the pH of seawater by 0.3-0.5 units. To investigate whether the presence of a burrowing urchin, Echinocardium cordatum, might influence the impact of ocean acidification on subtidal sediment pH profiles and nematode community structure an experiment was conducted using subtidal sediment, with urchins present or absent and seawater at either pH 8.0 (ambient) or 7.5. The presence of urchins, and a reduction in pH, both had significant effects on within-sediment pH profiles. Where urchins were present sediment profiles were more consistent and sediment pH was lower than that of the overlying seawater. There were significant differences in nematode abundance between treatments. The primary effect was a higher abundance of nematodes in replicates with urchins in natural seawater. All treatments had similar nematode community structure and diversity. Ocean acidification could therefore lead to changes in nematode communities in subtidal sediments affected by burrowing urchins.