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Coral reef ecosystem stressors in the U.S. Virgin Islands

Coral reef ecosystem stressors in the U.S. Virgin Islands

Coral reefs are among the World’s most delicate and vulnerable ecosystems.  A number of events and circumstances can effect them drastically resulting in degradation and even death.  Scientists who study coral reefs have coined the term “stressors” to describe these events and circumstances.  Some of these stressors are thought to be anthropologic in origin (pollution, ship damage, over-fishing, etc), while others are regarded as natural (earthquakes, tropical storms, disease, etc).  The number of stressors effecting coral reefs varies, but the following have been clearly identified in the U.S. Virgin Islands:


Corals are susceptible to a number of different diseases.  Four kinds of coral diseases have so far been recognized in the USVI: black band, yellow-blotch, white plague, and dark spot (Nemeth, et al., 2003).  Very little is known about the causative agents of these diseases, but microbial pathogens have been identified in at least five cases.  As with coral bleaching, the recent increases in severity of coral diseases may be a consequence of global warming.

After the dramatic 2005 bleaching event in the Caribbean, corals also suffered significant losses due to a post-bleaching disease outbreak. There was a greater than 2,000 percent increase in disease lesions and nearly 800% increase in denuded skeleton caused by disease over pre-bleaching levels. Mortality was primarily from white plague and resulted in the loss of 52 percent live coral cover from more than 30 acres of coral reef (Rothenberger et al., 2008).

Coastal development and runoff

Coastal sedimentation resulting from heavy rainfall and excessive runoff has been a natural occurrence in the Virgin Islands since the Holocene sea level rise.  However, the causes of coastal sedimentation have clearly shifted from natural to anthropogenic origin.  In the U.S. Virgin Islands this increase in runoff and the resulting increased sedimentation can be attributed to increased population with the resulting need for more housing, roads, and other development.  This is particularly true in St. John and St. Thomas where steep mountain slopes allow rapid runoff (Brooks, et al., 2007).  Nemeth and Nowlis (2001) have pointed out a direct relationship between shoreline development in St. Thomas and increased sedimentation in Caret Bay during periods of heavy rainfall.

Coastal pollution

As in other Caribbean islands, coastal pollution has long been a major problem in the U.S. Virgin Islands.  Both biological and chemical (industrial) pollution are evident.  Bacterial contamination of coastal waters is a primary problem caused by numerous point and nonpoint sources.  Heavy rainfall sometimes overloads existing sewage systems resulting in severe pollution of coastal waters.  In 2003 there were eighty days in which beaches were closed because of biological contamination.  In St. Croix, a rum factory discharges waste water directly into the sea forming a plume which can be traced for about 10 km from its point of origin (Jeffrey, et al., 2005)


How Pollution Affects Coral Reefs


In the coming years, remote coral reef ecosystems will be impacted primarily by global environmental changes. Reefs in close proximity to human populations, however, will also be faced with local stresses, including poor water quality from land-based sources of pollution.

Land-based Pollution Sources


Runoff from this pipe in the U.S. Virgin Islands spews directly into the ocean only a few hundred yards from reefs. Click image for larger view and image credit.

Pollution from land-based sources is a primary cause of coral reef degradation throughout the world. In the Caribbean, for example, approximately 80 percent of ocean pollution originates from activities on land. As human populations expand in coastal areas, development alters the landscape, increasing runoff from land. Runoff often carries large quantities of sediment from land-clearing, high levels of nutrients from agricultural areas and sewage outflows, and pollutants such as petroleum products and pesticides. These land-based sources of pollution threaten coral reef health.

Excess nutrients result in poor water quality, leading to decreased oxygen and increased nutrients in the water (eutrophication). This can lead to enhanced algal growth on reefs, crowding out corals and significantly degrading the ecosystem. In addition, sediment deposited onto reefs smothers corals and interferes with their ability to feed and reproduce. Finally, pesticides interfere with coral reproduction and growth. Sewage discharge and runoff may also introduce pathogens into coral reef ecosystems. For example, Aspergillus sydowii has been associated with a disease in sea fans, and Serratia marcescens, has been linked to white pox, another coral disease.

How Pollution Affects Coral Reefs


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