Why is ocean acidification a problem?

Why should we be concerned about ocean acidification?  Well, one of the main threats is that ocean acidification will cause wide spread extinctions of ocean fauna.  This will then have consequences that will work up the food chain and could potentially be catastrophic.  The rate of change of ocean pH is particularly problematic as it means that many organisms (especially those which calcify) will not have enough time to adapt (Guinotte and Fabry, 2008).  The increase of carbon dioxide into the oceans causes an increase in H⁺ and a decrease in CO^2-₃ which in turn causes a fall in the calcium carbonate saturation state.  Fewer CO^2-₃ will make it harder for calcifying organisms to make calcium carbonate.  Biogenic calcium carbonate is required by a huge amount of marine organisms- most notably the majority of coral species, foraminifera and coccolithopores.  Calcite and Aragonite are also used by calcifying organisms and the abundance of both is influenced by CO₂ levels.  The positions of the calcite and aragonite saturation zones are crucial as CaCO₃ precipitation can only occur above this horizon and below it dissolution will occur (Guinotte and Fabry, 2008).  As oceans acidify the horizons are moving towards the surface, with those of the North Pacific shoaling at a rate of 1-2m/year (Feely, 2007).   Orr et al. (2005) predict that the surface waters of the southern ocean will become undersaturated in Aragonite by 2050 and by 2100 this could include the subarctic pacific as well as the whole of the southern ocean.  Calcifying organisms appear to be incapable of maintaining their shells and skeletons when the oceans are undersaturated in aragonite.  Once the organisms exist below the aragonite/calcite saturation horizon dissolution occurs and so, they become deformed (Fabry et al. 2008).

This cartoon shows the deformation of a shell in an acidic ocean over a period of 45 days (NAOO).

A reduction in calcification and increase in dissolution is not the only detrimental influence that ocean acidification will have on these creatures; ion capacity, buffering capability; acid base regulation and metabolism are all also affected (See Fabry et al. 2008).  In my next post I will take a closer look at how the earth’s coral reefs are responding to these changes in calcite, aragonite and biogenic CaCO₃ levels.

References:

Fabry, V. Seibel, B. Feely, R. Orr, J, (2008). ‘Impacts of ocean acidification on marine fauna and ecosystem processes’, ICES journal of marine science, 65, 414-432.

Guinotte, J. Fabry, V. (2008). ‘Ocean acidification and its potential effects on marine ecosystems’, Annals of the new york academy of sciences, 1134, 320-342.

Orr, J. Fabry, V. Aumont, O. Bopp, L. Doney, S. Feely, R. et al. (2005). ‘anthropogenic ocean acidification over the twenty first century and its impact on calcifying organisms’, Nature, 437, 681-686.