Ocean Acidification Effects

Ocean plays a substantial role in the storage of carbon dioxide emissions through the uptake of roughly half of the fraction released by human activities up to 1994 (Sabine et al., 2004), and about 30% of recent emissions (Feely et al., 2004). Nevertheless, this regulating effect does not come without a price - continuous CO2 uptake is estimated to create pH reduction of 0.3-0.5 units over the next 100 years in the ocean surface (Caldeira and Wickett, 2003). This magnitude of acidification is higher than any other pH fluctuations inferred from the fossil record over the past 200-300 million years (Caldeira and Wickett, 2003). With a rate of change in pH that is 100 times greater than at any

Calcification and CO2

Atmospheric CO2 equilibrates rapidly with the surface layer of the ocean, where most additional CO2 combines with carbonate ions (Gattuso and Buddemeier, 2000):

This leads to a decrease in the concentration of CO32-, one of the building blocks of calcium carbonate, and in the saturationstateof calcium carbonate,Q (Q = [Ca2+] x [CO32-]/Ksp, where Ksp is the equilibrium constant of CaCO3). Q seems to be the controlling factor of calcification (Marubini and Thake, 1999).

time over that period, marine organisms' tolerance and ability to adapt to it is challenged and considerable impacts on the ecology of marine ecosystems are bound to happen (Guinotte and Fabry, 2008). However, impacts of these chemical changes in the ocean are still poorly understood, especially at the community to ecosystem levels (Riebesell, 2008).

It has been shown that marine plants (except seagrasses) are carbon-saturated (Gattuso and Buddemeier, 2000), and hence, are not expected to increase growth rates due to elevated CO2 concentrations. Therefore, dissolved CO2 concentrations rise may lead, in some localities, to macroalgae replacement by seagrasses due to carbon-limitation variations stemming from different evolvement eras of these two functional groups (Harley et al., 2006).

Furthermore, pH reduction associated with increased CO2 levels in seawater bears profound physiological consequences in subcellular processes such as protein synthesis and ion exchange, with a disproportional extent of effects among taxa (Portner et al., 2005). Ocean acidification can also have longer-term physiological, mechanical, and structural effects, especially on organisms that build carbonate structures. For example, pH reduction manipulations have demonstrated lower metabolic rates and growth in mussels (Michaelidis et al., 2005), which involved increased hemolymph bicarbonate levels (mainly from dissolution of shell CaCO3) in order to limit hemolymph acidosis, a drop in oxygen consumption rate, and an increase in nitrogen excretion (indicating net protein degradation) correlated with a slowing of growth. Another study of pH manipulation demonstrated reduced growth and survivorship in gastropods and sea-urchins (Shirayama and Thornton, 2005).

Calcification rates themselves decreased in response to increased CO2 in coccolithophorids, coralline algae, reef-building scleractinian corals, and pter-opod mollusks (Kleypas et al., 1999; Riebesell et al., 2000; Feely et al., 2004). Using laboratory and mesocosm experiments on open ocean plankton, it was shown that a decrease in the carbonate saturation state represses biogenic calcification of dominant marine calcifying organisms such as foraminifera and coccolithophorids (Riebesell et al., 2000; Riebesell, 2004). On the ecosystem level, these responses influence phytoplankton species composition and succession, favoring algal species that predominantly rely on CO2 utilization. In benthic communities, it was predicted that calcification rates in corals and coralline red algae are very likely to drop by 10-40% with a climatically realistic doubling of the pre-industrial partial pressure of CO2 (Feely et al., 2004). Moreover, changes in ocean chemistry may cause weakening of the existing coral skeletons and reduce the accretion of reefs (Hughes et al., 2003). Recent work actually demonstrated a 14.2% decline in coral calcification of the massive reef-building coral Porites along the Great Barrier Reef since 1990 (De'ath et al., 2009). The authors suggest that this decline is attributed to the increase in temperature stress and decline in saturation state of seawater aragonite. Can some coral species cope to some degree with such effect or are they doomed? Recent work on the nonreef-building hard coral Ocullina patagónica demon strated the existence of physiological refugia response mechanism, allowing corals to alternate between nonfossilizing soft-body ecophenotypes and fossilizing skeletal forms in response to changes in ocean chemistry (Fine and Tchernov, 2007).

Remarkably, some of the predictions regarding high latitude regions, where planktonic shelled pteropod gastropods constitute a prominent trophic component, suggest undersaturation with respect to aragonite even within the next 50 years that may cause the collapse of their populations (Orr et al., 2005). The collapse of populations of such major components in the polar food-web may alter the structure and biodiversity of polar ecosystems.

The potential ecosystem-scale effects of change in CO2 and pH levels was recently demonstrated in Italy at shallow coastal sites near volcanic CO2 vents (Hall-Spencer et al., 2008). Rocky shore sites near the vents with pH levels lower by 0.5 units than the mean ocean pH (ocean acidification levels predicted by 2100 by the IPCC) exhibited remarkable community-level effects. Along pH gradient ranging from 8.1-8.2 to 7.4-7.5, communities with abundant calcareous organisms shifted to communities lacking scleractinian corals and with significant reduction in abundance of sea urchin and coralline algae. The low pH communities exhibited peaking seagrass production with no indication of adaptation or replacement of sensitive species by others capable of filling the same ecological niche (Riebesell, 2008). Another study, this time from the Pacific Northwest shores of North America, suggests that reduced pH levels in nearshore seawater over the last decade was expressed in community-level effects in the rocky intertidal (Wootton et al., 2008). There, calcareous species generally preformed more poorly than noncalcareous species in years with low pH and thus have caused change in community structure.

Ocean chemistry changes and primarily ocean acidification is a poorly understood, yet potentially crucial, factor in climate change effects on marine environments at population, community, and ecosystem scales. Scientists predict that pH reduction through the twenty-first century will exceed any other documented pH fluctuations over the last 200-300 million years and thus would have profound consequences to organisms' physiology, growth, and survivorship, along with species distribution, abundance, and biogeography. Because acidification imposes a genuine threat on organisms' tolerance and ability to adapt to it, it should be recognized as an essential research target for conservation purposes in the following years.

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