POTENTIAL IMPACT OF WATERSHED DEVELOPMENT ON THE COMPOSITION AND TEXTURE OF BAY-FLOOR SURFACE SEDIMENTS IN BAYS WITH FRINGING CORAL REEFS, ST. JOHN, US VIRGIN ISLANDS

Degradation of marine systems due to increased watershed erosion from development is a threat to near shore coral reefs. Within the Virgin Islands National Park on the island of St. John, entire watersheds overlying bays with coral reefs are undeveloped, providing potential control sites to examine the impact of watershed development on sedimentation. The composition (organic, terrigenous & CaCO₃ %) and texture of bay-floor surface sediments collected in 2007 from 4 bays (1 below a developed watershed [Fish Bay] and 3 below undeveloped watersheds [Great Lameshur, Hawksnest, and Reef Bays]) were compared between bays and to samples collected in similar locations in 1987. In 2007, sediments were collected along transects extending from locations where ephemeral streams (guts) enter the bays. The texture of bay-floor surface sediments was analyzed using a laser particle sorter. Loss on Ignition was used to determine the % composition of the sediments. Mean grain sizes were primarily sand sized with finer sediments near guts, especially in Fish Bay (187 ± SE 2.1 µm). Contrary to what was expected, of the 4 bays, Reef Bay (undeveloped) sediments contained the highest proportion of terrigenous material overall (17 ± SD 1.3%). Near terrigenous outflows in the developed bay, sediments were significantly finer and contained greater proportions of terrigenous sediment than the undeveloped bays. However, at distances > 200 m from the guts, reef growth & oceanographic flushing may diminish the proportion of terrigenous material. In both Fish and G. Lameshur Bays the relative proportion of terrigenous sediment decreased with increasing distance from the guts, while in Hawksnest and Reef Bays the relative proportion of terrigenous sediment increased. 2007 sediment compositions and textures of the bay-floor surface sediments had no major significant differences compared to Hubbard (1987) for any of the bays despite predicted and modeled increases in watershed erosion in the past 20 years in Fish Bay. These results may indicate that 1) bay-floor surface sediments are not sensitive indicators of increased terrigenous input and/or 2) CaCO₃ production increased with terrigenous input associated with development.