Decline in the diversity and abundance of hard corals in reef environments worldwide is a current focus of substantial research and conservation effort. Understanding the changes requires long-term records of reef environments, but relatively few such records exist. The Mary Creek fringing reef complex (MCRC), in the Virgin Islands National Park on St. John, has been the site of several undergraduate research projects over the last thirty years, including an Amherst College field research seminar in 1968, Williams College senior thesis projects in the late 80s and early 90s, and a Williams field research course in1998. Since creation of the National Park in 1956 the reefs have been subject to increasing tourist visitation and infrastructure construction, as well as hurricanes and climate change, but the net effects are unknown because continuous reef-monitoring programs in the park began only in 1989. The undergraduate projects were not undertaken with the intent of reef monitoring, but the accumulated data track large-scale changes in the MCRC between 1968 and 1998. To the best of our knowledge, the MCRC is the only reef in the area for which a 30-year record exists, making this site an excellent candidate for a continued reef-monitoring study.

Substantial changes in the MCRC over the last thirty years include severe loss of living coral cover, and infilling of the back-reef lagoon. In 1968, the reef crest zone was dominated by dense stands of Acropora palmata, with subordinate but prominent colonies of A. cervicornis. By 1986, however, A. palmata occurred in less than 5% of surveyed quadrats, and the 1998 survey encountered only two living colonies. In 1968, a prominent western reef lobe was populated with A. cervicornis, but in 1986 it was recorded in less than 2% of quadrants surveyed. The 1998 map indicates that the A. cervicornis lobe no longer exists as a resistant structure, and is probably represented by an extended rubble zone at the western edge of the MRCR. In 1968, depths in the lagoon averaged 1.5 m, and in places were in excess of 2m. In 1998, in contrast, the average depth was approximately 0.4 m, and the deepest high-tide water level recorded in the back-reef was 0.6 m. This corresponds to an net sediment accumulation rate of approximately 3.6 cm/year.