THE IMPACT OF WATERSHED DEVELOPMENT AND RESTORATION ON MARINE SEDIMENTATION ON CORAL REEFS IN ST. JOHN, US VIRGIN ISLAND

In the US Virgin Islands, land-based sedimentation is one of the primary causes of coral reef degradation. Previous research on St. John has shown that the extension of a steep and mostly unpaved road network has increased watershed sediment yields and long-term terrignous sedimentation rates in adjacent bays by up to ten times natural rates. To mitigate these impacts, in 2011, erosion control programs were implemented in the 10.7 km² Coral Bay (CB) and 6.0 km² Fish Bay (FB) watersheds. Between 2008-12, our research team monitored watershed erosion and marine sedimentation within these developed (CB, FB) areas, as well an undeveloped Lameshur Bay (LB) area for comparison. Our objectives were to determine: 1) how development and the erosion control program affected marine sedimentation; and 2) whether differences in marine sedimentation between developed and undeveloped areas were consistent with differences in modeled sediment yields. Sediment accumulation rates were measured in tube traps deployed for 26 days at 22 sites in shore and reef areas and sediment composition was determined by LOI. Prior to restoration, terrigenous sediment accumulation rates in shore and reef sites below one developed watershed (CB) were 24 and 6 times greater, respectively than below the undeveloped watershed. Modeled sediment yields were 13 times higher (517 vs. 41 Mg yr^-1) in the developed (CB) than the undeveloped watershed. Rates of marine terrigenous sediment accumulation up to 100 times the 5-year means occurred following major storm events, such as Hurricane Otto in 2010. At the developed (but not the undeveloped) coral reef sites, acute (storm event) and at one site chronic (year-round) terrigenous sediment accumulation rates were consistent with the potential for stress to corals (>50 mg/cm²/day). Post-restoration sediment yields were 73% at CB and just 97% at FB of pre-restoration estimates, and after one season observable drops in marine terrigenous sediment accumulation rates were noted at some shore locations. Ongoing monitoring and further analysis will evaluate whether the observed post-restoration reduction in marine sediment accumulation is related to the implementation of erosion control. These results will inform watershed management strategies for similar islands in the Caribbean.