Water Quality and Coral Reefs

Land-Based Sources of Pollution (LBSP) research highlights from TCRMP

• Land-Based Sources of Pollution increase dramatically from offshore to nearshore waters of the USVI
• Fine particles in terrestrial sediments have strong negative effects on economically important stony corals. However, there is high variability in sediment effects within and between bays
• Cleaner bays offer guidance for best management practices

Study of Nutrient Analysis and Distribution and Sedimentation Rate

Objective: To determine baselines of water quality and coral condition, and their response to acute terrestrial run-off events, in coral communities that were perceived to be among the most impacted in the United States Virgin Islands (USVI).

Survey areas in Charlotte Amalie, St. Thomas (above) and Christiansted, St. Croix (below).

Summary

Poor land-use management and development in the coastal zone can lead to an increase in land-based pollution to the nearshore marine environment. Excess sediment and nutrients contained in land-based pollution can have detrimental impacts on nearshore water quality by increasing water column turbidity or leading to eutrophication. Increased land-based pollution may also be detrimental to coral reef communities by increasing coral bleaching, disease, and mortality or creating a phase shift from coral to macroalgae dominance. Additionally, seasonal storm activity can create large, short-term influxes of sediment and nutrients in association with heavy rainfall. The impacts of acute runoff events on coral reef health have been less frequently studied in the field, but it has been suggested that they may be equal to or more detrimental to reef health than chronic, low-level pollution. The Charlotte Amalie Harbor, St. Thomas and Christiansted Harbor, St. Croix contain areas of coral reef habitat and are suspected to have impaired water quality due to extensive alteration of the coastal land cover. Therefore, these areas were targeted for an assessment of the impacts of land-based pollution on coral reef health at varying levels of perceived human influence in the adjacent watershed.

In St. Thomas, water quality was found to be impaired in the nearshore highly impacted Zone I, especially during periods of heavy rainfall, but improved with both distance from shore and perceived human impact. Coral reef health showed a similar pattern to water quality in that prevalence of macroalgae overgrowth of corals, sediment deposition onto corals, and coral bleaching were greater in the nearshore highly impacted areas and decreased moving offshore. Additionally, coral diversity was lowest in the nearshore highly impacted Zone I but increased as perceived human impact lessened. These patterns were less clear in St.Croix, as there was no significant difference detected for water quality parameters or coral reef health metrics.

Increased sediment to the nearshore environment is visible in the Charlotte Amalie Harbor (left) and Crown Bay (right), St. Thomas a few hours after a heavy rainfall event.

Relevant References

Sabine AM, Smith TB, Williams DE, Brandt ME (in press) Environmental conditions influence tissue regeneration rates in scleractinian corals. Marine Pollution Bulletin. doi:10.1016/j.marpolbul.2015.04.006

Smith TB, Ennis R, Brown K, Wright V (2014) United States Virgin Islands Department of Environmental Protection. Section 106 Research Program. Study of Nutrient Analysis and Distribution and Sedimentation Rate. Phase II. Final Project Report (GC063PNR13). University of the Virgin Islands 76

Smith TB, Brown K, Ennis R, Honisch B, Martens J, Wright V (2013) United States Virgin Islands Department of Environmental Protection. Section 106 Research Program. Study of Nutrient Analysis and Distribution and Sedimentation Rate. Final Project Report (GC085DNR11). University of the Virgin Islands 79

Rothenberger J, et al. (2008) The State of Coral Reef Ecosystems of the U.S. Virgin Islands. In: Waddell JE, Clarke AM (eds) The State of Coral Reef Ecosystems of the United States and Pacific Freely Associated States: 2008. NOAA Center for Coastal Monitoring and Assessment’s Biogeography Team, Silver Spring, MD, pp 567

Smith TB, Nemeth RS, Blondeau J, Calnan J, Kadison E, Herzlieb S (2007) Assessing coral reef health across onshore to offshore stress gradients in the US Virgin Islands. Marine Pollution Bulletin. 56:1983-1991