Monitoring Approaches to Assess the Impact of Watershed Development and Restoration on Land-based Sedimentation in the US Virgin Islands: Lessons Learned

Abstract:

Development in coastal tropical watersheds (especially unpaved roads) has led to increased turbidity and runoff of land-based sediment into coastal bays with coral reefs. Unlike other threats to coral reefs, land-based sedimentation can be managed locally through sound watershed management practices. In order to evaluate the effectiveness of watershed restoration projects or track how future watershed building or changing climate affect land-based sedimentation, it is necessary to adapt monitoring approaches that will effectively capture marine sediment dynamics at appropriate temporal and spatial scales.

Since 2007 we’ve monitored rainy-season sedimentation (tube sediment traps) and turbidity (TSS) (26-day intervals) in eastern St. John, USVI. More recently (2013-14) we added SedPods and a program of terrestrial-marine integrated monitoring including shoreline runoff (stream and crest gauges) and current meters and nephelometers, which measure sediment deposition, turbidity, and hydrographic parameters at minute resolution. Our 15 shoreline and reef sites are below developed and undeveloped “reference” watersheds and ARRA-funded watershed restoration projects completed in 2011. Through examples from our study, we will compare monitoring approaches and share lessons learned.

Our sediment trap data resolved greater mean terrigenous accumulation below developed compared to undeveloped watersheds, increases in terrigenous accumulation in response to storm events, and reductions in terrigenous accumulation following watershed restoration during storms. However, they lacked the temporal resolution to capture the sedimentary response to ephemeral runoff or re-suspension events. They also dampen short-term acute sedimentation events that have the greatest impact on coral condition. Based on our studies, we recommend: a) monitoring over periods which capture the natural variability of the system; b) combining time-averaged approaches (long term [core], gross [sed. traps] and net [SedPods] deposition) that provide sample material, with high temporal-resolution instrumentation (nephelometers and current meters); c) integrated monitoring of the watershed-bay-coral reef by an inter-disciplinary team of experts working together; and d) community participation.