Sedimentation and Reef Development in Hawksnest, Fish and Reef Bays, St John, US Virgin Islands

Hubbard, Dennis; Stump, James; Carter, Brian

In recent years, the potential impacts of sedimentation on reef development have been increasingly recognized. With ever-increasing development stress being placed on the upland areas of St. John, both the National Park and local residents have become concerned with the impacts of development on natural resources, and specifically on nearshore reefs. This study was conducted as a first attempt to evaluate the responses of nearshore reefs in three bays over the past two hundred years. Specifically, the impacts of land use on nearshore sedimentation levels were of special interest. A theoretical study of runoff under natural conditions intimated that the present distribution of reefs around the island are primarily controlled by watershed size, bay geometry and exposure, with recent development exerting a secondary impact. It was established, however, that more frequent (10-25 yr) storms play a very important role in controlling the distribution of reefs around the island. Therefore, it is not necessary to invoke the catastrophic event (e.g. 100-yr storms) to explain the occurrence of modern reefs. From this, it can be argued that development can have significant impact under less than heavy runoff conditions. Studies of the present distributions of sediment types and reefs in Hawksnest, Fish and Reef Bays point to controls identical to those identified by the more generalized study. Of critical importance in reef development was distance from sources of terrigenous runoff (the guts) and degree of exposure. On a local level, these both outweighed development at its present level. Over the long term, there appears to be a gradual decline in the reefs over the entire period of record, based on X-rayed cores through large coral heads. This may be related to a long-term compromise of the soil retaining capabilities of the upland watersheds following intensive cane farming in the 1700's and 1800's. With the exception of short-term degradation of the nearshore reefs in Hawksnest Bay, however, reef degradation could not be directly linked to any specific development activity. Based on the findings of this study, specific management strategies are proposed. These include limiting development in main water courses, and establishing reasonable buffers to runoff related to upland construction. Recommendations for future research objectives are also offered.