salinity relative to a baseline slope in a defined oceanic region could indicate a decrease in net calcification. Thus, an increase in the slope of A T vs. Upward divergence from the oceanic conservation of A T relative to salinity in surface waters indicates net dissolution of CaCO 3 or independent production of A T, while downward divergence of A T relative to salinity in surface waters indicates net CaCO 3 precipitation or independent uptake of A T. In the oceans, A T is typically conservative with seawater salinity, which itself changes mostly due to evaporation and/or precipitation. Furthermore, it has been proposed that this method can be applied to ocean basins or oceanic regions with a high prevalence of coral reefs, where changes in A T, in conjunction with strontium and calcium concentrations that diverge from conservation with salinity can be used to determine the relative contributions of corals and calcareous plankton to their CaCO 3 budget 24. A different approach to assess the state of whole coral reef ecosystems is to measure changes in the water chemistry induced by biological activity, where precipitation of CaCO 3 by reef organisms (mainly hermatypic corals) induces changes in seawater total alkalinity (A T) 22, 23. This method provides a wealth of information regarding the state of corals reef communities and allows the exploration of the processes that influence the state of the communities, yet the spatial coverage of these surveys is limited and they are very labour intensive. Assessment of the state of coral reefs has traditionally relied mostly on annual visual community structure surveys 21. This global decline in the state of coral reefs warrants careful monitoring of these important ecosystems. The alarming global decline in the state of coral reefs is largely due to periods of prolonged thermal stress that are increasing in frequency and duration, resulting in massive coral bleaching and mortality 18, in addition to local stress factors, such as coral mining and eutrophication 19, 20. In contrast, the same type of data suggests that coral calcification rates were stable in the decades preceding 19 in the Florida Keys and the Red Sea, respectively 14, 17. In addition, numerous coral growth records derived from coral cores taken from live corals in the Great Barrier Reef, Red Sea and reefs in southeast Asia indicated that growth rates have been continuously declining since the 1990's, on the order of ~6–10% per decade 14, 15, 16. Studies of community metabolism in the Great Barrier Reef compared rates of calcification measured in the past few years with similar measurements conducted 3–4 decades ago, which together suggested an alarming decline in net calcification rates, and confirmed the predicted decline according to the Eilat rate equation 12, 13. Using this equation, together with modelled values of sea surface temperature and aragonite saturation for different future levels of atmosphere CO 2, it has been predicted that many tropical coral reefs might not be able to maintain their calcareous frameworks by the middle of the 21st century, when atmospheric CO 2 is expected to double relative to its pre-industrial level 4. For example, measurements of community calcification made over two annual cycles in the Eilat Nature Reserve Reef, northern Red Sea, have been used to develop a gross coral reef calcification rate equation, which is a function of live coral coverage, reef-water temperature and aragonite saturation 11. These findings have motivated numerous studies of coral reef community metabolism, which established a baseline for our understanding of temporal variability in photosynthesis, respiration, calcification and CaCO 3 dissolution rates of whole reef communities 8, 9, 10. The adverse effects of ocean acidification on hermatypic coral calcification, as well as whole coral reef community calcification has been demonstrated experimentally in numerous laboratory experiments 5 and two controlled field experiments 6, 7. The effect of ocean acidification may be particularly hard for coral reefs as corals, which form the foundation of their carbonate framework will not be able to produce CaCO 3 at a rate that will equal or offset the sum of mechanically and biologically mediated erosive processes 4. The anticipated decrease in CaCO 3 production with increasing atmospheric CO 2 and resulting ocean acidification will significantly impact many aspects of the marine carbon cycle and lead to the deterioration of shallow-water-carbonate platform habitats, such as coral reefs 3, 4. Absorption of excess anthropogenic CO 2 from the atmosphere into the oceans is reducing seawater pH and thus the carbonate ion concentration, making it increasingly more difficult for calcareous organisms to build their skeletons 1, 2.
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