Ocean Acidification: How We Have Changed the Chemistry of the World's Oceans and Implications for Calcifying Organisms

The addition of fossil fuel carbon dioxide to the atmosphere is rapidly changing seawater chemistry and the calcium carbonate saturation state of the world's oceans as a result of the acidifying effects of CO₂. This acidification makes it more difficult for marine organisms (e.g., corals, plankton, calcareous algae, and mollusks) to build skeletons, tests, and shells of calcium carbonate. Impacts on these calcifying organisms will lead to cascading effects throughout marine ecosystems. Repeat hydrographic cruises and modeling studies in the Atlantic, Pacific and Indian Oceans show evidence for increased ocean acidification. Dissolved inorganic carbon increases, of about 10¹⁵ µmol kg^-1 in surface and intermediate waters over the past 15 years, are consistent with corresponding pH decreases of approximately 0.025 units in surface waters. These dramatic changes can be attributed, in most part, to anthropogenic CO₂ uptake by the ocean. High frequency and long term variability of inorganic carbon parameters is of particular interest in coral reef ecosystems because changes in these parameters have implications for calcification and dissolution and limited data exist for coral reefs and other coastal systems.

A Coral Reef Instrumented Monitoring and CO₂ Platform (CRIMP-CO₂) mooring has collected surface water temperature, salinity, pCO₂, pO₂ and air pCO₂ and pO₂ data every three hours almost continuously since December 2005 in Kaneohe Bay, Hawaii. All measured values showed significant variability over a range of time scales. Surface water pCO₂ ranged from 230 to 590 μatm but was generally higher than atmospheric due to net calcification and surface water saturation state with respect to aragonite varied from 1.2 to 3.5 with low values occurring during storm fresh water input. Our results indicate that the variability of inorganic carbon parameters in coral reef systems is significantly greater than what is shown by extrapolating open ocean model results to the coastal regime.