Two main environmental drivers affect coral assemblages in the tropical eastern pacific (TEP): upwelling and the El Niño-Southern Oscillation (ENSO). Upwelling occurs on a seasonal scale and brings low water temperatures and reduced pH levels, causing a decrease in coral growth. Warm waters associated with multiannual El Niño events are linked to coral bleaching and sudden coral death. Together, these two drivers were predicted to be the reason for poor coral reef framework development in the tropical eastern pacific during the Holocene. Toth et al. (2012) examined the history of reef framework construction (both tempo and mode of framework) in relation to seasonal upwelling and ENSO. They found that coral reefs in TEP collapsed for 2500 years—roughly 40% of their existence—beginning about 4000 years ago. The pacific-wide phenomenon corresponds to a period of increased variability in ENSO activity, suggesting that ENSO was the main driving force behind the collapse.—Kelsey Waite
Toth, L.T., Aronson, R.B., Vollmer, S.V., Hobbs, J.W., Urrego, D.H., Cheng, H., Enochs, I.C., Combosch, D.J., van Woesik, R., Macintyre, I.G., 2012. ENSO Drove 2500-Year Collapse of Eastern Pacific Coral Reefs. Science 337, 81–84.
Toth and colleagues removed 14 push-cores from subtidal reef-slope habitats on three reefs across Pacific Panamá. Each reef had a distinct upwelling pattern—Isla Contadora experiences intense seasonal upwelling while Isla Iguana has intermediate upwelling and Isla Canales de Tierra has no upwelling. Cores were sectioned and sorted by species and taphonomic condition to identify different modes of development. Layers were also dated by C14 analysis, accelerator mass spectrometry, and U/Th analysis by inductively coupled plasma mass spectrometry.
Toth et al. determined reef growth had begun 6900 calibrated calendar years before the present (cal yr B.P). They found a narrow interval within the coral’s history in which the reef was dominated by Psammocora stellata, which does not branch and build framework. The interval also contained degraded Pocillopora rubble, indicating that the reef was degrading. Core dating suggests the reefs stopped growing by 4332 cal yr B.P. and recovered by 2384 cal yr B.P. The hiatus observed in coral framework cores corresponds to a period of increased climatic variability. Beginning 4500 to 4000 cal yr B.P., ENSO frequency and intensity increased. This was the same time of the onset of the hiatus demonstrated in the cores. Reconstructions show that El Niño events between 4000 and 2000 cal yr B.P. were among the strongest in the Holocene due to the coupled influence of ENSO and the Intertropical Convergence Zone.
Toth and colleagues propose that shifts in the frequency and intensity of ENSO were the main cause of the hiatus shown in Panamá and possibly the rest of the Pacific—the intensity of seasonal upwelling acted only as a second-order process. Recently, ENSO activity has devastated coral reefs. At the current rate of warming, reefs in the tropical eastern pacific may be at risk of another collapse.