Monitoring the effects of land development on the near-shore reef environment of St. Thomas, USVI

Abstract

This study evaluated the impacts of shoreline development on the coral reef at Caret Bay, St. Thomas USVI. Studies in rates of sedimentation, changes in water quality and changes in the abundance and diversity of corals and other reef organisms were conducted along five permanent transects from July 1997 and March 1999. Results from monthly monitoring before, during and after construction indicated that sedimentation and total suspended solids increased during large rainfall events, and that sediment load onto Caret Bay reef was greatest directly below ravine outlets and in locations where shoreline was sheltered. After buildings, landscaping and road paving were completed peak sedimentation rates decreased relative to average monthly rainfall. Visual assessment of coral condition documented that coral pigment loss was associated with both influx of terrigenous sediments and with natural seasonal phenomena. Bleaching of coral colonies during the 1998 bleaching event showed a strong positive relationship with sedimentation (r 2 = 0.92). Reef sites exposed to sedimentation rates between 10 to 14 mg cm - 2 d -1 showed a 38% increase in the number of coral colonies experiencing pigment loss than reef sites exposed to sedimentation rates between 4 to 8 mg cm -2 d -1 . Coral cover along the entire reef tract declined about 14% (range: -3.92% to -31.34%). This decline in coral cover from pre- to post-construction surveys showed weak negative associations with sedimentation (r 2 = 0.52) and bleaching (r 2 = 0.48). Patterns of abundance of macro algae, sponges and encrusting gorgonians were primarily related to natural seasonal changes rather than to rates of sedimentation. This study provides evidence that stress from sedimentation may lead to decline in living coral through secondary effects such as bleaching. Monthly monitoring was able to detect changes in the reef environment associated with human activity and was able to segregate these changes from natural causes. The results of this study provide coastal managers quantitative measures of allowable sedimentation rates to better evaluate and mitigate future development in sensitive coastal areas.