Coral reef health response to chronic and acute changes in water quality in St. Thomas, United States Virgin Islands

• Water quality is chronically impaired in nearshore zones.
• Coral diversity and reef health decline in conjunction with water quality.
• Rain events were linked to more severe coral bleaching.
• We suggest nearshore development is having a negative impact on coral reefs.
• Greater attention to watershed management could alleviate negative impacts.

It is suspected that land cover alteration on the southern coast of St. Thomas, USVI has increased runoff, degrading nearshore water quality and coral reef health. Chronic and acute changes in water quality, sediment deposition, and coral health metrics were assessed in three zones based upon perceived degree of human influence. Chlorophyll (p < 0.0001) and turbidity (p = 0.0113) were significantly higher in nearshore zones and in the high impact zone during heavy precipitation. Net sediment deposition and terrigenous content increased in nearshore zones during periods of greater precipitation and port activity. Macroalgae overgrowth significantly increased along a gradient of decreasing water quality (p < 0.0001). Coral bleaching in all zones peaked in November with a regional thermal stress event (p < 0.0001). However, mean bleaching prevalence was significantly greater in the most impacted zone compared to the offshore zone (p = 0.0396), suggesting a link between declining water quality and bleaching severity.