Long-term dynamics of coral reefs in St. John, US Virgin Islands

Abstract

In this study, long-term (1987–1998) dynamics are described on a local scale (<20 km) for coral reefs in St. John, US Virgin Islands, which are located in a marine protected area (MPA). The study consists of two sites (Yawzi and Tektite) which were selected in 1987 based on relatively high coral cover (=32%), and six sites that were randomly selected in 1992. Over 12 years, mean coral cover at Yawzi (9 m depth) changed significantly, declining from 45 to 20% cover between 1987 and 1998 (a 56% reduction). Less than 1 km away at Tektite (14 m depth), coral cover also changed significantly, but here it increased 34% (from 32 to 43% cover). Over the same period, macroalgal cover showed a significant upward trend at both sites, increasing from 2 to 26% at Yawzi, and from 6 to 13% at Tektite. The random sites (7–9 m depth) differed from the initial sites in both community structure and dynamics. Mean coral cover at the random sites (~8%) was less than one third of that at Yawzi and Tektite, and varied significantly among sites and years in an idiosyncratic pattern. The percentage cover of macroalgae and the pooled coverage of crustose coralline algae, algal turf, and bare space showed a strong site × time interaction, illustrating that the sites differed in dynamics, but that the differences varied among times. Thus, as has been reported elsewhere in the Caribbean, serious reef degradation has occurred on at least one reef in St. John, but the patterns of change vary markedly on a kilometer-wide scale. In comparison with other long-term studies of Caribbean coral reefs, the degradation of a coral reef in an MPA around St. John is noteworthy since there are few local anthropogenic disturbances that can be held responsible for the decline. The strong possibility that large-scale events such as hurricanes and global warming have played a pivotal role in the decline of at least one reef in St. John emphasizes the need to embrace landscape- and regional-scale phenomena in order to understand and manage local coral reef dynamics. The occurrence of small patches of relatively healthy reef (i.e., at Tektite) appears trivial in comparison to region-wide reef decline, but such anomalies should be studied further because of their potential roles as refugia for corals and reef-associated taxa.