BATHYMETRIC AND ECOLOGICAL CHANGES IN THE MARY CREEK FRINGING REEF AND LAGOON, VIRGIN ISLANDS NATIONAL PARK

Data spanning 40 years reveal shifting changes in the Mary Creek reef complex (MCRC). Our study included bathymetric and substrate data from 1968, 1998, and 2004,  and GIS analysis of air photos taken in 1965, 1983, 1993, and 1999.  The MCRC consists of three fringing reef lobes: two Acropora palmata-dominated lobes, and a small rubbly remnant of a former A. cervicornis lobe.  The coral framework structures are separated from the shoreline by a back-reef lagoon about 120 m wide by 0.5 m deep.  Bathymetric data show progressive lagoon shallowing.  Both time intervals (1968-1998 and 1998-2004) have the same average sedimentation rate, 3.2 cm/year.  This suggests consistent infilling, probably related to periodic storm transport of sediment over the reef crest.  The reef lobes have changed in size, but not progressively.  Between 1965 and 1983 the A. palmata-dominated eastern and central lobes increased in total area (by 7% and 21% respectively), whereas the A. cervicornis-dominated western lobe decreased by 53% in the same time period.  From 1983 to 1996, all reef lobes experienced size decline:  7% and 5% for the two A. palmata lobes, and a further 26% for the A. cervicornis lobe.  The onset of decline in the A. palmata lobes coincides with disease-related death of the constituent corals in the 1980s, and is interpreted to represent subsequent mechanical erosion of the dead coral framework.  The consistent decrease in size of the A. cervicornis lobe, predating the disease outbreaks of the 1980s, may reflect the marginal nature of the very shallow lagoonal habitat for this deeper-water species. A. palmata colonies  have recently begun to repopulate the MCRC.  There was little regeneration after the 1980s die-off, and in January 1998 only 5 colonies, all less than 0.5 m in diameter and 0.3 m high, were observed on the MCRC.  In January 2004, however, we documented about 90 healthy A. palmata colonies in the 70% of the MCRC surveyed, of which 20 were more than 1 m in diameter, and 50 were more than 0.3 m high.  If recolonisation continues, we anticipate that the size decrease of the A. palmata-dominated lobes will reverse.  In contrast, only 2 A. cervicornis colonies were observed, and neither was healthy.  We believe that the westernmost lobe will therefore continue to decline, and that the MCRC is now essentially a two-lobed rather than a three-lobed reef complex.