| Summary: | In the near future, the marine environment is predicted to be subject to ocean acidification and warming simultaneously. To study the effects of future ocean pH and temperature levels on meiofauna, replicate intertidal invertebrate communities (obtained via Artificial Substrate Units) from the extreme low intertidal zone, were placed in a mesocosm experiment for a period of 60 days. The eight treatments comprised of four pH levels: 8.0 (ambient control), 7.7 & 7.3 (predicted) and 6.7 (CO2 point source leakage) crossed with two temperature levels: 12C (ambient control) and 16C (predicted). Thirty-six meiofauna assemblages were extracted, subsampled and identified to major taxonomic groups with nematodes identified to species level. Changes in nematode community structure, biomass, and structural & functional diversity were also investigated. Meiofauna and nematode community structure were significantly affected by pH and temperature, but not their interaction. Copepods and copepodites showed the greatest decline in abundance in response to low pH and elevated temperature (copepods initially increased). Nematodes increased in abundance in response to low pH and temperature rise, possibly caused by decreased predation and competition for food owing to declining macrofauna density. Limited information was obtained from the remaining taxa (e.g. adult polychaetes) due to their low abundance. Changes in nematode species composition occurred in all treatments, and were confirmed by the identification of numerous species cluster groups with different abundance patterns in response to low pH and temperature rise. In the nematode community, the estimated species diversity, species evenness, and maturity index, was substantially lower at 16C. Trophic diversity was slightly higher at 16C except at pH 6.7. This study demonstrates the significance of investigating changes in structural and functional diversity measures, which can occur without corresponding changes in abundance. In collaboration with Plymouth Marine Laboratory
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