BUCK ISLAND REEF – A CITY OF TWO TALES

“It was the best of times, it was the worst of times”. As in any community, “health” versus “decline” is a function of perspective. Using coral diversity as a benchmark, monitoring studies have documented a rapid change in modern reef-community structure. Quaternary reefs, dominated by the same species, have been used to provide a temporal context of reef change over time spans greater than the life of a researcher or a monitoring study. Citing spatial persistence of coral zonation in Pleistocene outcrops, some workers have suggested that earlier reef communities were more stable than those today and that recent losses of keystone reef species might be “unprecedented” events in recent geologic time.

Core data from Buck Island reef, US Virgin Islands (avg. coral abundance=31%; Acropora palmata=53% of total coral) compare more favorably with survey data prior to the onset of coral disease (1976: avg. cover=52%; A. palmata=53% of live coral) than afterward (1988-93: avg. cover <15%; A. palmata <15% of coral). However, close examination of the cores reveals two gaps in the occurrence of A. palmata within the reef over the past 7,000 years (at ca. 6,000 and 3,000 CalBP). In each case, the absence of this primary Caribbean frame-builder lasted a millennium and corresponded temporally to losses of the species throughout the Caribbean and western Atlantic. These observations illustrate that spatial dominance of a particular coral or zonation pattern in core or outcrop need not reflect temporal persistence. The use of Quaternary reefs as a frame of reference for recent change will require characterizations that extend beyond traditional paleoenvironmental descriptions and utilize detailed chronological measurements capable of resolving change at the scale of the monitoring studies to which they are being compared. Until this is accomplished the usefulness of geologic data in separating natural from anthropogenic change will remain enigmatic.