| Summary: | 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. This study investigated whether bioturbation by the burrowing urchin, Echinocardium cordatum, influenced the impact of ocean acidification on subtidal sediment pH profiles and nematode community structure. A 2-way crossed design experiment was conducted using subtidal sediment, with urchins present or absent and seawater at either pH 8.0 (ambient) or 7.5. In treatments with urchins there was a smaller decline in sedimental pH between 0-1 cm deep in acidified seawater compared with natural seawater. Bioturbation created sediment profiles which were more consistent and less alkaline than that of the overlying seawater. There was a correlation between the sediment pH profiles and community structure of rare species of nematodes. In one-way analyses of combined factors, all treatments had similar nematode community structure, abundance and diversity, apart from the natural seawater with urchin treatment, where the primary effect was a higher abundance. Hence, in seawater at ambient pH, the presence and absence of urchins may create a heterogeneous pattern of nematode communities in subtidal sediments. The higher nematode abundance may have been due to an enhanced food supply. In the other three treatments, alterations in bacterial nutrient processing rates, greater variability in sediment pH and changes in nutrient fluxes between the sediment and the overlying seawater, may have influenced nematode community structure. Ocean acidification could therefore lead to the loss of diverse subtidal benthic communities created by the presence and absence ofbioturbators. KEY WORDS: C02. Ocean acidification. Nematode Community structure Bioturbation . Sediment pH . Echinocardium corda tum Benthic Faculty of Science, Plymouth Marine Laboratory Prospect Place The Hoe Plymouth Devon PL13DH
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