RAPID RANGE EXPANSION OF REEF CORALS IN RESPONSE TO CLIMATIC WARMING

Acroporid corals appear to be expanding their geographic ranges northward in the western Atlantic: (1) reef-coral assemblages dominated by living thickets of the staghorn coral Acropora cervicornis were discovered off Fort Lauderdale, Florida in 1998, where they had not been reported previously; (2) 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; and (3) acroporid corals have recently been observed in the northernmost Bahamas growing directly on a relict, Holocene-age barrier reef tract. The recent northward expansion of cold-sensitive acroporids is associated with decadal-scale increases in annual sea-surface temperature. Significantly, the present range extension of acroporids has a paleoecological counterpart that can be used to model the future response of these coral assemblages.

During the early to middle Holocene (9-5 ka), oceanic conditions off the eastern Florida Peninsula favored the growth of an Acropora-dominated barrier-reef tract up to 10 m thick, ranging as far north as Palm Beach County. Such thick accumulations of Acropora spp. were deposited as far as 150 km north of Miami which until 1998, was the northern limit of these species. The high-amplitude, millennial-scale climate flicker during the Holocene also correlates with the northward expansion of coral reefs in the Pacific and the northward expansion of a number of other taxa. As temperatures cooled after the mid-Holocene, the northern limit of reef growth regressed southward in both the Atlantic and Pacific.

Climate change is an important determinant of the distribution of high-latitude reef systems. Under current scenarios of global warming, the continued, rapid northward expansion of Acropora-dominated reefs in the western Atlantic is a strong possibility. We predict that acroporid corals will appear in Bermuda in the not-too-distant future.