Assessing the short- and long-term effects of land development on watershed erosion and sediment delivery to marine ecosystems of the U.S. Virgin Islands

Abstract:

Throughout history, significant portions of the native vegetation of many Caribbean islands were replaced by cropland. Even though most islands eventually underwent reforestation, sediment yields and deposition rates appear to be higher now than throughout the past millennia, and this suggests that coral reef systems are experiencing an unprecedented level of sediment-related stress. Given the present-day emphasis on erosion control projects to restore coral reefs of the US Caribbean, it is of utmost importance to develop a quantitative understanding of the effects of both land development and watershed restoration activities on sediment delivery at various spatio-temporal scales.

Efforts to measure contemporary erosion, sediment delivery and deposition rates have been conducted on the island of St. John-USVI since 2009. Sediment yields under natural conditions from the small (<10 km²) watersheds in this dry sub-tropical setting are between 1 and 10 Mg km^-2 yr^-1. Current sediment yields are 2 - 50 times higher than background depending on unpaved road network abundance and characteristics. Our efforts indicate that a watershed restoration program implemented in 2010-2011 within the 13-km² Coral Bay watershed resulted in the reduction of annual sediment delivery rates from 445 Mg yr^-1 to 327 Mg yr^-1.

Marine sedimentation rates of terrigenous materials based on sediment trap data were 6 - 24 times greater below developed watersheds relative to undeveloped catchments and were consistent with spatial comparisons of modeled sediment yields. At sites located within reef systems, total and silt deposition rates during sampling periods with major storms exceeded rates shown to harm corals more frequently in developed areas. Terrigenous sedimentation rates during periods with equivalent storms were reduced following watershed restoration. These results suggest that targeted watershed restoration may be effective in reducing sedimentation where land development and sediments are considered a major threat to coral reefs.