The Effect of Watershed Runoff and Sediment Resuspension on Turbidity and Sediment Deposition in St. John, US Virgin Islands: Implications for Watershed and Marine Development and Restoration in Bays with Coral Reefs
Date of Award
Thesis: Open Access
MS Marine Science
Environmental and Ocean Sciences
In the US Virgin Islands (USVI), land-based (terrigenous) sedimentation has been identified as a major cause of coral stress. Development, such as the building of unpaved roads in steep coastal watersheds, has increased sediment yields and marine terrigenous sedimentation by up to an order of magnitude above background levels. When activated during storm events, ephemeral streams transport sediment from the watershed to coastal waters. Once deposited on the seafloor, resuspension of benthic sediments can further increase turbidity and deposition. However, isolating the relative contributions of runoff and resuspension to turbidity and deposition using conventional sediment trap monitoring has been challenging. Here we describe the spatial and temporal variability of marine sediment dynamics in response to runoff and resuspension events in St. John, USVI. Between late July 2013 and January 2014, nephelometers were deployed at three reef and five shoreline sites next to ephemeral stream outfalls equipped with a water level sensor (10-min resolution) and peak crest gauges (~13-day resolution). At each instrument site, benthic sediment samples were collected every ~26 days. While runoff events resulted in high-magnitude spikes in turbidity and deposition, which were up to 900 and 17,000 times background, respectively, they were short-lived (hours). Resuspension-induced spikes in turbidity and deposition were lower in magnitude but of longer duration (days-weeks), particularly at sites with finer-grained benthic sediments, and were associated with increased wave height during low tides. While the relative contribution of runoff and resuspension to turbidity and deposition were spatially variable between our study sites, resuspension contributed at least 7 times more to turbidity and 3 times more to deposition than runoff during the 2013 rainy season at one of our coastal marine monitoring sites. Though previous studies have measured marine sedimentation over months-centuries in St. John, no studies have monitored turbidity or the relative contributions to turbidity and deposition from runoff and resuspension at the time scale of minutes-days. Understanding the relative contribution of runoff vs. resuspension to marine turbidity and deposition is important to effectively manage land-based sedimentation, marine development, as well as evaluate the effectiveness of watershed restoration programs which aim to reduce marine terrigenous sedimentation.
Digital USD Citation
Campbell, Stephen E., "The Effect of Watershed Runoff and Sediment Resuspension on Turbidity and Sediment Deposition in St. John, US Virgin Islands: Implications for Watershed and Marine Development and Restoration in Bays with Coral Reefs" (2017). Theses. 22.