It is not farfetched to say that in the marine environment, coral reefs will be the first major ecosystem to be functionally extinguished because of climate change. Of course, many entire small areas of global systems have disappeared already for a number of reasons, from industrial pollution or coastal construction, and many areas of soft substrate have been totally obliterated (trawled for example). But a whole ecosystem with a pan-tropical span? Probably not.
Warming which causes, firstly, widespread bleaching of corals and which is then sufficiently severe and persistent to cause subsequent widespread mortality was not really noticed until the 1970s. It began to be increasingly noticed from the 1980s, and now occurs with frequent if erratic occurrence. The years 1998, 2001/2 and 2005 were seminal. Several predictions (calculations are a better word) have been made that severity and frequency of such events will increase so that the sea temperatures which cause widespread mortality will become a near annual occurrence well within the lifetime of most people alive today. Some suggestions are that, on average – there are many local variations – this will be most severe across a tropical belt and will expand outwards. Some extrapolations (Sheppard 2003) showed that the timing of critical dates are nearest close to the equator, becoming later as one moves polewards, in some oceans at least.
Many suggestions have been made, in both learned and unlearned articles, that the most poleward of reefs might even benefit from this. Many ‘marginal’ reef areas, such as Bermuda, South Africa, or southern Florida have generally cooler temperatures and lower coral and fish diversity. Coral diversity and growth tail off markedly in cooler waters, therefore (so that argument goes) a bit of warming will perhaps encourage more corals and greater variety of corals at greater latitudes. Indeed expansion and contraction of the northern boundary of coral in Florida as climate warmed and cooled over geological time has been well studied (Precht and Aronson, 2004 W.F. Precht and R.B. Aronson, Climate flickers and range shifts of reef corals, Front. Ecol. Environ. 2 (2004), pp. 307–314. Full Text via CrossRefPrecht and Aronson, 2004).
But while such effects clearly occur, it is not the solution to the survival of reefs or coral assemblages because it ignores several things. It ignores the point that reef growth has little connection with coral diversity: for example some highly diverse coral assemblages in the ‘coral triangle’ do not develop into reefs at all for reasons that remain unclear, while some very substantial reefs in some areas support a low coral diversity for biogeographic reasons. It overlooks the point that coral growth and reef growth are not at all the same thing either, and indeed the two become ‘de-coupled’ as temperatures cool, something described with some detail for Arabian regions especially where upwelling is strong (Sheppard et al 1992).
But most importantly perhaps, the argument that corals can perhaps move polewards a bit overlooks ocean acidification. The oceans are having to absorb more CO2 than ever before and, to date, half to two thirds of all CO2 generated since the start of the industrial revolution has been absorbed by the surface layers of the sea. It is, in fact, only the smaller portion which has not been absorbed by the ocean which causes our greenhouse effect and which is giving rise to all those conferences about global climate change and warming. That portion which has been absorbed, however, has changed the pH of the surface ocean by 0.1, which is a 30% increase in H+ ions (Royal Society, 2005). As a result, the complexities of the carbonic acid – bicarbonate – carbonate buffering system mean that calcification by marine life is increasingly curtailed. The aragonite saturation state is falling, and this increasingly restricts calcification by several critical forms of marine life (not only corals of course). This is affecting polar regions first, spreading towards the tropics. Within this century calcification rates could decline by 60% (Kleypas et al 2006) and previously existing, optimal levels of aragonite saturation will exist nowhere at all by 2040. On coral reefs, calcification will decline by about 30% by 30–50 years (ISRS, 2008). Further, there is evidence that coralline algae, which are essential to reef construction, will be affected as much or more than the corals themselves, even though some of them are very tolerant of warm temperatures. This problem of course affects other major calcifying organisms too, especially important of which are planktonic forms – coccolithophores, foraminifera, pteropods, etc., – although those that deposit calcite rather than aragonite will be slightly less vulnerable (Royal Society, 2005). Some pteropods (aragonite, like corals) are already being found with pitted tests in areas in the Southern Ocean where the carbonate saturation layer now extends to near the surface.
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Sheppard CRC, in press. A marine nutcracker. Marine Pollution Bulletin. Available online 25 April 2008. Article.
Technorati Tags: biological response, coral, ocean acidification
