Sediment flux and stony coral health in the nearshore US Virgin Islands.

Human activities in tropical watersheds are increasing rates of terrestrial silt and clay deposition to nearshore environments and negatively impacting coral reef ecosystems. We sampled sediment flux across a range of coral reef environments in the US Virgin Islands, Caribbean Sea using standard sediment traps and evaluated the relationship of sediment particle size and quality (organic, carbonate, terrigenous) on coral health. We found high variability in rates of sediment deposition that corresponded temporally to storm and swell events. However, this was driven spatially by distinctions between nearshore and offshore habitats and orientation to Atlantic swell regimes, suggesting that sediment impacts also have a strong spatial component on Caribbean reefs. We found that increasing fluxes of silt-clay and terrestrial sediments in nearshore environments were linearly related to a higher prevalence of coral bleaching, old partial mortality and overall health impairment at a site. In contrast to expectations, total sediment and organic fluxes in nearshore environments were not related to any measure of coral health. The result from total sediment flux suggests that sediment quality is as more important to coral health than overall quantity and should be incorporated into coral reef sediment monitoring studies.