Sarah Gray

Effects of Watershed Erosion Control on Land-Based Sources of Pollution to Coral Reefs in RCP Priority Sites

2014

NOAA, NOS, Office of Coastal Management

Tech Report

"This project extended our previous ARRA and NOAA CRC funded sediment monitoring work in St. John, USVI over another field season (7/11-3/12) and included assessments of trace metal transport from ridge to reef and water quality. Our aim was to make spatial comparisons of sedimentation, geochemistry, and water quality between sites drained by undeveloped (control) watersheds and mitigated sites to determine the source of variation in sedimentation. In addition, we compared our 2011-12 postmitigation data to data collected prior to the ARRA mitigation (2008-11). We also analyzed the geology and geochemistry of watershed bedrock, soil and streambed sediments as well as representative marine sediments to determine: 1) what metals or metal compounds traced terrigenous sediment input; 2) how the mineralogy/lithology and concentration of metals and metal compounds varied from ridge to reef and differed between developed vs. undeveloped areas. For this grant we conducted sediment and water-quality (TSS) monitoring at 14 stations (below developed and undeveloped watersheds) in 4 bays in eastern St. John, US Virgin Islands 10 times from July 2011-March 2012. We found that the mean proportion of terrigenous sediment (percent terrigenous) and the total and terrigenous sediment accumulation rates (mg/cm2 /day) decreased significantly from mangrove to shore to reef. Terrigenous sediment accumulation below developed watersheds were 5-40 times, 6-55 times, and up to 60 times higher in the shore, reef and mangrove environments, respectively below developed watersheds than at equivalent environments below undeveloped watersheds. Though relatively low sediment accumulation rates (less than 10 mg/cm2 /day) persisted for most of the time series, major storm events resulted in total sediment accumulation rates greater than 50 mg/cm2/day on the coral reefs. However, these integrated (26-day) mean accumulation rates may underestimate the acute sedimentation rates that occur during the few days that a storm passes over."

FY11; CRCP Project ID: 20412; Project Title: FY11 State and Territory Coral Reef Management Grants; Principal Investigator: Jenny Waddell; Grant Number: NA11NOS48200014