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A mesocosm experiment was conducted to quantify the effects of short-(2 wk) and longterm (20 wk) exposure to acidified seawater on the structure and diversity of macrofaunal and nematode assemblages in 2 different sediment types. The impact of acidified seawater on sediment nutrient fluxes was also determined. Using carbon dioxide (CO 2 ) gas, seawater was acidified to pH 7.3 (mimicking ocean acidification), 6.5 or 5.6 (mimicking leakage from a sub-seabed CO 2 store site). Control treatments were maintained in natural seawater (pH ≈ 8.0). Exposure to acidified seawater significantly altered community structure and reduced diversity for both macrofaunal and nematode assemblages. However, the impact on nematodes was less severe than that on macrofauna. While the communities in both sediment types were significantly affected by changes in seawater pH, impacts on sandy sediment fauna were greater than those on muddy sediment fauna. Sandy sediments also showed the greatest effects with respect to nutrient fluxes. In sand, the efflux of nitrite, nitrate and silicate decreased in response to increased acidification while the efflux of ammonium increased. In mud, acidification increased the efflux of ammonium but had no effect on the other nutrients. We conclude that both leakage from carbon storage and ocean acidification could cause significant changes in the structure and diversity of coastal sediment communities. Lowered seawater pH could also affect nutrient cycling directly by altering bacterial communities and indirectly through impacts on the abundance and activity of key bioturbators. KEY WORDS: Biodiversity · Macrofauna · Meiofauna · Ocean acidification · Nutrient flux · Carbon storage Resale or republication not permitted without written consent of the publisherMar Ecol Prog Ser 379: [59][60][61][62][63][64][65][66][67][68][69][70][71][72][73][74][75] 2009 upwelling is a natural phenomenon, ocean acidification has increased the size of the area exposed to corrosive waters.Political, social and environmental pressures to reduce CO 2 emissions have led several governments to seek new options for CO 2 mitigation. One such option involves injecting CO 2 into underground porous reservoir rocks (Holloway 2005), which is known as geological storage. This technique has been in use at the Sleipner West gas field in the Norwegian sector of the North Sea since 2000 where around 1 × 10 6 tons of CO 2 are currently being sequestered each year (Holloway 2005). At the Gleneagles Summit in July 2005, the leaders of the world's major economic powers (Group of Eight: Canada, Italy, France, Germany, Japan, Russia, UK, USA) declared they would 'work to accelerate the development and commercialization of CO 2 capture and storage (CCS) technologies'. At ~USD140 million, the total budget for CCS research and development in Europe and North America in 2005 was substantial (Tjernshaugen 2007) and geological storage is considered a practical tool in reducing emissions (Gibbins et al. 2006). While it is assumed that storag...

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