CLIMATE FLICKERS AND RANGE SHIFTS OF CORALS

The fingerprint of global climate change can be mapped via the response of species to changes in their physiographic environmental settings. We report the first known example of poleward expansion of a Caribbean coral genus in response to recent patterns of climatic warming. Reef-coral assemblages dominated by the staghorn coral Acropora cervicornis were discovered off Fort Lauderdale, Florida in 1998, where they had not been observed previously. In addition, the elkhorn coral A. palmata was observed for the first time in 2002 on reefs of the Flower Garden Banks in the northern Gulf of Mexico. The appearance of acroporid corals north of their previously known range is associated with a decadal-scale increase in annual sea-surface temperature. Significantly, the present northward expansion of acroporids has a historical precedent that can be used to model the future response of these corals.

During the early to middle Holocene (9-4 ka), oceanic conditions favored the growth and accumulation of Acropora spp. along the shelf margin of the Florida reef tract. Acropora-dominated reefs up to 10 m thick ranged as far north as Palm Beach County. These thick deposits as far as 150 km north of their present limit implies that Acropora spp. are capable of responding rapidly to climate change through range shifts. In the early to middle Holocene, lower sea level placed the active shelf-margin reef system in closer proximity to the Gulf Stream than it is today. Stable isotopic records indicate that the extratropical North Atlantic was 2-4^oC warmer than at present. This millennial-scale, high-amplitude climate flicker during the mid-Holocene caused many organisms to increase their ranges northward. The climate flicker also correlates with the northernmost expansion of coral reefs in the Pacific. As temperatures cooled after the mid-Holocene, the northern limit of acroporid reef growth in the Atlantic regressed south to the northern Florida Keys.

The northward expansion and then southward contraction of framework-building corals implicates climate change as an important forcing mechanism in the distribution of reef systems. Under current scenarios of global warming, the continued northward expansion of Acropora-dominated reefs is a strong possibility, mimicking the geographic distribution of reefs in the mid-Holocene.