The Sensitivity of Marine Calcification to carbonate ion concentration

It is now well established that the rate of calcification of biogenic calcification is a function of the carbonate ion concentration. This relationship has been best established in the case of corals. Data is now available for twelve species. For the purpose of comparison it is convenient to normalize the calcification rates to the rate achieved at the pre-industrial carbonate ion concentration of the surface tropical ocean taken for the purposes of this analysis to be 255 μmol kg-1. If the rates from all the available studies are processed in this way and then regressed against the carbonate ion concentration one obtains that the normalized calcification = -24.5+0.47[CO32-], r#2=0.74. From this relationship one can calculate that at the present time the rate of coral calcification may have declined by 19% relative to the pre-industrial rate and by the end of the century, if pCO2 reaches 700 μatm, it could decline by 54%. This assumes that any rise in sea surface temperature does not have a significant effect on coral calcification. At the present time this is a major source of uncertainty. Several studies show that corals are adapted to the mean annual temperature that they experience and the rate of calcification during the summer is depressed relative to the maximal rates observed during the spring and fall. In this scenario any increase in the mean annual temperature will result in a reduced annual rate of calcification. These studies show that the rate of calcification falls off at the rate of 24±17 % per °C once the temperature exceeds the species thermal optimum. Other studies based on long-lived massive corals widely used in paleo-climate reconstructions exhibit a linear relationship with temperature that shows no sign of tapering off at the highest temperatures for which data are available. At this time we do not know which pattern is more representative of the aggregate response of corals on a typical coral reef. It should not be forgotten that coverage of live coral on coral reefs is also in decline due to mortality associated with bleaching events and outbreaks of disease. Both of these could be related to climate change although no positive link has yet been proven. The presumption is that coral reef carbonate production is trending downwards since the onset of the industrial revolution and more steeply since the occurrence of the bleaching and disease events of the 1970s and 1980s but no studies have yet been undertaken to confirm or deny this assumption. Coral reefs are thought to contribute 10-25% of global carbonate production accumulating on the sea floor. The carbonate production of pelagic calcifiers is also known to be a function of carbonate ion concentration. Clearly the balance between the input and removal of alkalinity in the ocean is shifting rapidly and this will have an impact on the rate at which the oceans take up CO2. There is need for studies that track the rate of carbonate production in neritic and pelagic ecosystems over time and for studies that characterize the response of calcifiers to the combined effects of elevated pCO2 and temperature.