A mesocosm experiment was conducted to quantify the effects of medium term (5 wk) exposure to acidified seawater on the structure of Nereis virens (Polychaeta) burrows and sediment nutrient fluxes. Worms were exposed to seawater acidified to a pH of 7.3, 6.5 or 5.6 using carbon dioxide (CO 2 ) gas. These treatments mimicked the effects of either ocean acidification (pH 7.3) or leakage from a sub-seabed CO 2 storage site (pH 6.5 and 5.6). Results from these treatments were compared to those from worms maintained in natural seawater with a pH ≈ 7.9. The experiment showed that the presence and structure of N. virens burrows significantly increased the sediment uptake of nitrate and the release of ammonium, nitrite and silicate. Phosphate flux was unaffected by the presence of burrows. Nutrient flux rates were also significantly affected by changes in seawater acidity. A reduction in seawater pH caused an increase in nitrate uptake and increase in ammonium release, a decrease in nitrite release and a decrease in phosphate uptake. The flux of silicate was unaffected by changes in seawater pH. As changes in acidity had no impact on the size and structure of worm burrows, it was concluded that the impact of seawater pH on nutrient flux was probably due to changes in the microbial communities responsible for nutrient transformations. Whilst this paper demonstrates that leakage from sub-seabed storage would have significant and immediate effects on nutrient cycling, impacts of ocean acidification through atmospheric absorption are less obvious. This paper concludes that ocean acidification could have a significant impact on sediment nutrient flux in coastal and shelf seas as a result of potential changes in the structure and function of bioturbating communities. KEY WORDS: Nereis virens · Ocean acidification · Carbon capture and storage · Ecosystem function · BioturbationResale or republication not permitted without written consent of the publisher Mar Ecol Prog Ser 341: 111-122, 2007 decade (Haugan & Drange 1996). As further emissions of CO 2 are inevitable it is not unreasonable to assume that concentrations of atmospheric CO 2 will continue to rise. In the long term we face the prospect of atmospheric CO 2 levels exceeding 1500 ppm sometime between the years 2100 and 2200 (Pörtner et al. 2004), whilst the Intergovernmental Panel on Climate Change (IPCC) has predicted that levels could reach 800 ppm by 2100 (Feely et al. 2004). As a result of these increases it is possible that the pH of surface water could fall by up to 0.4 U before 2100 and a reduction of 0.7 U could occur by 2250 (Caldeira & Wickett 2003).Political, social and environmental pressures to reduce CO 2 emissions have led governments to seek new options for CO 2 mitigation. One such option is that of geological CO 2 sequestration. This method of storage involves injecting CO 2 into underground porous reservoir rocks (Holloway 2005). The techniques required to do this are already well developed and have been in use at the Sleipner West gas fi...
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...
Multivariate analysis of species assemblage data often begins with transformation of abundances, to downweight contributions from dominant taxa to Bray-Curtis dissimilarities computed among samples. Although usually effective, global transformation is a blunt tool: it ignores differences in the variance structure of counts of individual species. Species which are highly spatially clustered should logically be given less weight than those with similar mean abundance but whose replicate counts have the lower variance associated with Poisson distributions (for which individual organisms arrive randomly and independently into the sample). Where replicates are available within sample groups specified a priori, this differential downweighting is achieved by dividing the counts for each species by their index of dispersion D, the variance to mean ratio, a clustering ('clumping') measure calculated from replicates within a group, and then averaged across groups. The procedure is justified by assuming a generalised Poisson model for counts, allowing different species to have arbitrarily differing degrees of clustering. Downweighting is applied only where a species shows significant evidence of clumping, this being tested by a powerful, exact permutation test that replaces the standard (large-sample) χ 2 test for D = 1, which is often invalid because of low 'expected' frequencies. The resulting dispersion-weighted data matrix has a common (Poisson-like) variance structure across species, but unchanged relative responses of a species in different groups. Transformation may still be needed but now only to downweight consistently abundant species relative to equally consistent but less numerous species, rather than also dealing with erratic counts. Dispersion weighting is shown to be effective in 3 studies that examine: soft-sediment copepods in the metal-polluted Fal estuary, UK; benthic macrofauna of mangrove forests in Bicentennial Park, New South Wales, Australia; and sediment nematodes within and outside seagrass beds in the Yealm estuary, UK. A fourth data set, on macrobenthos from Loch Creran, UK, is added to a comparison of the differing cross-species distributions of the dispersion index.
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