Impacts of Ocean Acidification on Coral Reefs

Abstract

Research findings of the past decade have led to mounting concern that rising atmospheric carbon dioxide (CO2) concentrations will cause changes in the ocean’s carbonate chemistry system, and that those changes will affect some of the most fundamental biological and geochemical processes of the sea. Thanks to the efforts of large-scale physical and biogeochemical ocean programs such as WOCE, JGOFS, and OACES, ocean-wide changes in the carbonate system are now well documented. Since 1980 ocean uptake of the excess CO2 released by anthropogenic activities is significant; about a third has been stored in the oceans. The rate of atmospheric CO2 increase, however, far exceeds the rate at which natural feedbacks can restore the system to normal conditions. Oceanic uptake of CO2 drives the carbonate system to lower pH and lower saturation states of the carbonate minerals calcite, aragonite, and high-magnesium calcite, the materials used to form supporting skeletal structures in many major groups of marine organisms. A variety of evidence indicates that calcification rates will decrease, and carbonate dissolution rates increase, as CaCO3 saturation state decreases. This evidence comes from principles of thermodynamics, the geologic record, and the evolutionary pathways of CaCO3 secreting organisms. Further evidence, from controlled experiments of biocalcification under increased CO2 conditions, confirms that calcification rates of many organisms decrease with decreasing CaCO3 saturation state. Extrapolation of these results to the real world suggests that calcification rates will decrease up to 60% within the 21st century. We know that such extrapolations are oversimplified and do not fully consider other environmental and biological effects (e.g., rising water temperature, biological adaptation); nor do they address effects on organism fitness, community structure, and ecosystem functioning. Any of these factors could increase or decrease the laboratory-based estimates, but it is certain that net production of CaCO3 will decrease in the future. The St. Petersburg Workshop, sponsored by NSF, NOAA, and the USGS, and held at the USGS Center for Coastal and Watershed Studies on 18–20 April 2005, was designed to take the next step toward understanding the response of marine calcification to increasing atmospheric CO2 concentration. The aims of the workshop were to summarize existing knowledge on the topic, reach a consensus on what the most pressing scientific issues are, and identify future research strategies for addressing these issues. Although workshop participants were drawn from a wide range of scientific disciplines, there was a clear convergence on the major scientific issues that should be pursued over the next 5–10 years.