First published in IGBP's Global Change Newsletter Issue 73, April 2009

Reef development in a high-CO2 world: Coral reefs of the eastern tropical Pacific

Features |
The Ocean in a High-CO2 World: Science highlights from the symposium
Derek P. Manzello
Cooperative Institute of Marine and Atmospheric Studies
Rosenstiel School, Marine Biology and Fisheries
University of Miami,
Miami, Florida, USA

Joan A. Kleypas
Institute for the Study of  Society and Environment National Center for Atmospheric Research
PO Box 3000
Boulder, CO 80307-3000,

Many coral reefs in the eastern tropical Pacific develop at slow – or even marginal – rates: they grow slowly and erode quickly.  This has been blamed on the upwelling of cold, nutrient-rich waters to the surface that can depress calcification and stimulate bioerosion. Our study suggests that the increased carbon dioxide content of the upwelled water, which lowers the pH and depresses the aragonite (CaCO3) saturation state, is also an important factor in the poor reef development.  We verified the low saturation state of waters from the eastern Pacific (Galápagos, Gulf of Chiriquí, and Gulf of Panamá), and then compared coral samples from nearby reefs with samples from the Bahama Islands, a region with high aragonite saturation state. The Bahama samples contained abundant inorganic aragonite cements that tend to fill the pore spaces within corals and the reef, while the eastern Pacific samples contained few to none. The lack of cements are thought to reduce the resistance of the corals and reefs to bioerosion; in fact, bioerosion rates in the eastern Pacific are ten times those of other reef regions. This study shows that increasing atmospheric carbon dioxide, which lowers the aragonite saturation state of seawater, threatens not only the coral calcification rates, but also the reef-structures that support high biodiversity and protect shorelines [1].Content
Figure 1: Carbonate cements within the pore spaces of corals from areas with naturally different CO2 levels.  The Galapagos sample (where seawater is similar to what is expected for the rest of the tropics with a tripling of atmospheric CO2) contains no cement.  The absence of cement is evidenced by a clearly defined boundary between the inner skeletal wall and open pore space.  This is contrary to the sample from the Bahamas (where seawater has very low CO2 levels) — the boundary between the skeletal wall and pore space is blurred by the abundance of cements.  Minor amounts of cement are present from the intermediate CO2 environment from Pacific Panama, but these are still trivial relative to the Bahamas.
1. Manzello DP, Kleypas JA, Budd DA, Eakin CM, Glynn PW, Langdon C (2008) Poorly cemented coral reefs of the eastern tropical Pacific: possible insights into reef development in a high-CO2 world. Proceedings of the National Academy of Sciences 105 (30):10450-10455