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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:

Climate change and coral bleaching

Coral bleaching in St Croix (Photo: NOAA)
Coral bleaching in St Croix (Photo: NOAA)

Over the last few decades there has been a conspicuous increase in average air temperature throughout the World, which, in great measure is probably attributable to the accumulation of “greenhouse” gases resulting from human activities.  As a consequence of these changes, sea water surface temperatures (SSTs) are also rising. Coral reefs in the USVI and other parts of the Caribbean experienced extensive and widespread bleaching during 2005, with more than 90 percent of coral cover bleached in some areas. On average, water temperatures surrounding the reefs were much higher than any time during the previous 14 years. Corals are symbiotically associated with various species of autotrophic dinoflagellate algae called zooxanthellae, which live in the coral tissues and provide the corals with pigmentation, and through photosynthesis, energy-rich carbon compounds needed for the corals’ metabolic processes. As water temperatures rise, the zooxanthellae, which provide most of the corals’ energy requirements, disintegrate or are expelled from the corals. Consequently, the white limestone skeleton of the unpigmented coral is visible, resulting in a bleached (whitened) appearance.  Death of the coral may or may not occur, depending on the duration and severity of the temperature increase, any subsequent diseases, and the corals’ resiliency.  Major coral reef framework building species, for example, elkhorn coral, have nearly disappeared from some sites. Climate change may also be involved with some of the other stressors discussed below.

Diseases

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).

Tropical storms

Severe storms can directly affect the coral reef environment by reducing living stands of live coral to coral rubble and by causing major shifts in the topography of reef areas (for example, moving the location of reef crests).  The effects of storm-caused degradation on reefs in the Virgin Islands was clearly demonstrated following Hurricane Hugo in 1989. Fifteen years after the storm, reefs in Lameshur Bay, St. John, had shown no significant increase in live coral cover (Jeffrey, et al., 2005)

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)

Tourism and recreation

The transition from an agricultural/fisheries economy to an economy based primarily on tourism has been strikingly evident during the last few decades and has resulted in an ever increasing demand for land-based accommodations (houses, resorts, recreational facilities, new roads).  As forests and top soils are removed, rain water runoff increases, resulting in increased siltation of the coral reefs.  Direct recreational use of coral reefs can be especially damaging.  For example, with an annual visitor population of about 170,000, a “snorkeling trail” in the Virgin Islands exhibited major deterioration over the first twenty-five year period following its establishment in the 1960s (Bruckner, et al., 2005).

Fishing

As in most other Caribbean Islands, overfishing, habitat degradation, more efficient fishing gear and techniques, and less than rigorous enforcement of fishing regulations have all contributed to major losses in fishery resources in the USVI.  Many species of reef fishes have declined remarkably in the last several decades.  This is especially true of a number of species of snappers and groupers.  Both the lobster and conch fisheries are under ever increasing pressure regardless of the numerous regulations aimed at protecting them (Jeffrey, et al., 2005).

Live trade and Souvenir collecting

In spite of strong regulations prohibiting the taking of both living and dead biological material from the coral reefs of the Virgin Islands, U. S. Customs Agents confiscate large amounts of such material at all of the major airports in the Virgin Islands as well as at points of entry in the continental United States.  Fortunately there is no aquarium trade in the Virgin Islands, and the few permits which have been issued for collecting coral reef plants and animals have been issued only for the purposes of research and education.

Vessels and related problems

Vessel impacts such as groundings, anchor damage and waste discharges affect coral reef ecosystems in the USVI.  Accidental groundings can cause serious structural damage to the coral reef environment.  The anchoring of boats and the dragging of anchor chains over coral reefs and sea grass beds are also major boat-related threats.  Recreational vessels, the numbers of which are increasing rapidly in the USVI, can also cause environmental degradation through the release of sewage and fuel-related hydrocarbons.

Marine debris

Plastic bags, glass and plastic bottles, plastic lids, cans, plates, utensils and discarded fishing line are among an array of items which are washed directly into the sea from beach activities and from several landfills in the Virgin Islands.  During the 2006 Ocean Conservancy’s annual International Coastal Cleanup, volunteers removed nearly 20,000 pounds of trash and debris from about 50 miles of shoreline. Underwater cleanups removed about 500 pounds of marine debris. Over 90 percent of the marine debris had its origin in the USVI; only a very small percentage washed in from offshore sources.

Introduced and invasive species

Invasive species can rapidly and seriously degrade endemic or established island populations by altering natural processes and reducing biodiversity. Native to South America, the red imported fire ant (Solenopsis invicta) has been introduced into parts of some Caribbean islands, notably Puerto Rico and the Virgin Islands. The small Asian mongoose was deliberately introduced into many Caribbean Islands, including the Virgin Islands, as a biological control agent against rats in sugar cane fields. However, the mongoose became a serious pest that wreaked havoc with local biodiversity. Non-native rats and feral pigs also interfere with natural ecosystem dynamics. Buffelgrass (PennisTum ciliare var ciliare ) is a non-native pasture grass that can form thickets that displace native species. However, aquatic invasive species have not been recognized as a major threat in the USVI.

Saharan dust

The Sahara is the major source on Earth of mineral dust (60-200 millions of tons per year), which are eroded mineral soils from the Sahara Desert and the transition zone (sahel) between the Sahara Desert and tropical forests to the south. This mineral or Saharan dust is lifted by convection to high altitudes where is transported by trade winds across the Atlantic Ocean to the Americas and the Caribbean. Besides depositing  nutrients utilized by Atlantic phytoplankton and other organisms, the dust also contains harmful contaminants which may play a role in the degradation of the Caribbean coral reefs. Found in the dust are viable pathogenic microorganisms, metals, heavy metals, pesticides, polychlorinated biphenyls (PCBs), and polycyclic aromatic hydrocarbons. A known coral pathogen, the fungus Aspergillus sydowii, which produces the disease aspergillosis in sea fans, is also transported by Saharan dust.

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