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In Dead Water

Ocean Acidification

The oceans act as a natural reservoir for CO2. The dissolved CO2 reacts with the seawater to form hydrogen ions. The uptake of anthropogenic carbon since 1750 has led to the ocean becoming more acidic, with an average decrease in pH of 0.1 units. However, the effects of observed ocean acidification on the marine biosphere are yet mostly undocumented. Progressive acidification of the oceans due to increasing atmospheric carbon dioxide is expected to reduce biocalcification of the shells, bones and skeletons most marine organisms possess. Though the limited number of studies available makes it difficult to assess confidence levels, potentially severe ecological changes would result from ocean acidification, especially for corals both in tropical and cold water, and may influence marine food chains from carbonate-based plankton up to higher trophic levels.

Figure 17:Atmospheric concentration of CO2 is steadily rising, and oceans directly assimilate CO2. As ocean concentration of CO2 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-sensitive relationships in marine ecosystems, are still unknown.

The oceans are naturally alkaline, with an average pH of around 8.2, although this can vary up to 0.3 units depending on location and season. Atmospheric carbon dioxide dissolves naturally in the ocean, forming carbonic acid (H2CO3), a weak acid. The hydrogen ions released from this acid lower the pH. These reactions are part of a natural buffer system, but recent studies have shown that the huge amounts of CO2 created by burning fossil fuels are over-stretching the rate by which the natural process can neutralise this acidity. The pH of the oceans has decreased 0.1 unit compared to pre-industrial levels, which equals an increase of 30 per cent in hydrogen ions. While records show that the pH of the seas can vary slightly over time and in certain areas, the continued increases in atmospheric CO2 are expected to alter ocean pH values within a very short time – an effect greater than any experienced in the past 300 million years (Caldeira et al., 2003).

More parts of the oceans will become undersaturated with calcium 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.

Figure 18:As carbon concentrations in the atmosphere increase, so do concentrations in the ocean, with resultant acidification as a natural chemical process. The skeletons of coldwater coral reefs may dissolve, perhaps already within a few decades. The impacts will be greatest at high latitudes.

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 undersaturated 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 conditions 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  Southern 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 suggested previously (Orr et al., 2005).