In Dead Water

More parts of the oceans will become undersaturated with cal- cium carbonate, even most or all surface waters in the polar regions. All marine organisms which need carbonate to build their calcareous skeletons and shells, such as corals, seashells, crabs and crayfish, starfish and sea urchins, could be affected. Even single-celled, planktonic organisms with calcareous shells (e.g. coccolithospores, certain foraminifera etc.), which form the basis of many marine food chains, may be affected. The impacts of ocean acidification are potentially wide- spread and devastating, and may change marine life as we know it. The first effects will be felt in deeper waters and the polar regions. It is expected that by 2100, around 75% of all cold-water corals will live in calcium carbonate under- saturated waters. Any part of their skeleton exposed to these waters will be corroded. Dead coral fragments, important for the settlement of coral larvae e.g. to re-colonise a reef after a bleaching event, will be dissolved. The base of the reefs will be weakened and eventually collapse. Even those organisms which might be able to cope with the undersaturated condi- tions will have to spent more energy in secreting their shells and skeletons, which makes them more vulnerable to other stresses and pressures. Tropical areas will remain saturated, but experience a severe fall from the optimal aragonite (a metastable form of calcium carbonate used by corals) concentrations in pre-industrial times to marginal concentrations predicted for 2100. This will add to the already increasing stresses from rising sea temperatures, over-fishing and pollution. Ocean acidification may have severe impacts on scleractinian cold-water and deep-sea corals (Royal Society 2005; Guinotte et al . 2006; Turley et al ., 2007). Projections suggest that South- ern Ocean surface waters will begin to become undersaturated with respect to aragonite by the year 2050 (Orr et al ., 2005). By 2100, this undersaturation could extend throughout the entire Southern Ocean and into the subarctic Pacific Ocean. Studies have suggested that conditions detrimental to high-latitude ecosystems could develop within decades, not centuries as sug- gested previously (Orr et al ., 2005).

Atmospheric CO 2 concentration (ppm)

380

360

340

320

300

1957

1967

1977

1987

1997 2007

Figure 17. Atmospheric concentration of CO 2

is steadily rising,

. As ocean concentration of

and oceans directly assimilate CO 2

CO 2 increases, the oceans automatically become more acidic. This, in turn, may have severe impacts on coral reefs and other biocalcifying organisms. There is little debate on the effect as this is a straight-forward chemical process, but the implications for marine life, that may be severe due to many very pH-sensi- tive relationships in marine ecosystems, are still unknown.

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