Effects of Global Warming and Ocean Acidification on Carbonate Budgets of Eastern Pacific Coral Reefs

Abstract

Eastern tropical Pacific (ETP) coral reefs provide a real-world example of reef growth, development, structure, and function under the high-pCO2, low aragonite saturation state (Ωarag) conditions expected for the entire tropical surface ocean with a doubling to tripling of atmospheric CO2. This provides a unique opportunity to examine various aspects of calcium carbonate (CaCO3) budgets in low-Ωarag conditions in the present day. Unlike anywhere else in the world, the ETP displays a continuum of thermal stress and CO2 inputs up to levels at which reef building is terminated and reef structures are lost. The response of coral reef CaCO3 budgets to El Nino warming across the ETP shows that reefs can be completely lost after experiencing a 2–3 °C thermal anomaly sustained in excess of two months during the warmest time of the year at Ωarag values expected for the rest of the tropics when atmospheric CO2 doubles. ETP coral reefs have persisted and shown resilience to this level of thermal stress or acidification when acting alone, but the combination of the two corresponded with the complete elimination of reef framework structures in the southern Galapagos Islands over the decade after the 1982–83 El Nino warming event. Reef carbonate degradation is exacerbated also by diverse agents of bioerosion such as sea urchins, boring bivalves, and excavating sponges, with experimental evidence demonstrating that the latter may even increase their activities during ocean warming and pH decline. This chapter reviews the CaCO3 budget of ETP coral reefs and discusses how a high-CO2 world may impact the major biotic and abiotic factors responsible for the cycling of carbonate materials. Coral reefs of the ETP serve as a model for conditions that will occur in other regions within a few decades.