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Coral reef ecosystems of the U.S. Virgin Islands

Coral reefs

Coral reefs are a major and conspicuous component of the shelf regions of the U.S. Virgin Islands. Four other ecologically distinct marine habitats (seagrass beds, algal plains, mangrove forests and salt ponds) interact in various ways with the coral reefs and are considered to be part of the coral reef ecosystem. Other habitats such as octocoral hardbottom, sand communities, shallow mud, and fine sediment habitats also interact with the coral reefs.

Extensive coral reefs lie in deeper water along the shelf edge in depths from approximately 37 to 61 meters. These deeper reefs are dominated by plating forms of the Agaricia spp. and Montastraea spp. complexes, while corals in shallower water vary from columnar forms of Montastraea spp. to Acropora spp. to gorgonian dominated habitats. Maps of USVI benthic habitats (to 30 meters) show that 61 percent of the 485 km2 area is coral reefs and corals on hard bottom; 33 percent is predominantly seagrass beds, and 4 percent is sediment or rocky bottom.

Fringing and patch reefs, along with spur and groove formations, are typical of St. John and St. Thomas. St Croix, on the other hand, has several large barrier reefs, some of which are associated with well-developed lagoons. Several threatened and endangered species, in addition to elkhorn and staghorn corals, feed, reproduce, nest, rest, or calve in the waters of the USVI. Vertebrates, such as humpback whales, pilot whales, four species of dolphins, several sea birds, and marine turtles all use portions of these waters. The reefs of the USVI also provide habitats for many species of reef fishes, invertebrates, and plants.

Seagrass beds

A coral reef at St. Croix.
A coral reef at St. Croix. The photograph features blue chromis, stoplight parrotfish, elkhorn coral, branching fire coral, sea plumes, sea rods, and sea whips. (Photo: NOAA CCMA Biogeography Team)

The highly productive seagrass beds provide food and shelter for a great variety of marine vertebrates and invertebrates. Seagrasses also act as sediment filters and consequently improve the water clarity over coral reefs. Four major sea grasses occur in the U. S. Virgin Islands: shoal grass, Halodule wrighti), turtle grass (Thalassia testudinum), manatee grass (Syringodium filiformis, and small turtle grass (Halophila baillonis). All four of these species have been recorded in St. Croix (probably because of well-protected lagoons). Shoal grass, turtle grass, and manatee grass are reported from St. John, while only turtle grass and manatee grass are known to occur in St. Thomas. In the U. S. Virgin Islands (as elsewhere) seagrass beds are typically limited to shallow, clear-water areas which have good water circulation.

Algal plains

A turtle grass meadow at St croix
A turtle grass meadow at St Croix (Photo: NOAA CCMA Biogeography Team)

Algal plains occur over coral rubble and coarse sand. They are best developed at depths of approximately 18 meters. Various species of green, brown, and red algae, and the spermatophyte Halophila baillonis (Florida Keys seagrass) are the dominant plants. Associated biota includes sponges, tunicates, bryozoans, mollusks, polychaete worms, and gorgonians. Fifty-two species of algae and 43 species of fishes have been recorded from a typical algal plain off St. Thomas.

Mangrove forests

Mangrove forests help stabilize shorelines and protect low-lying lands by buffering them against severe tropical storms, winds, and waves. Mangrove prop roots and leaf litter provide excellent habitat for a large number of invertebrate species as well as nursery areas for coral reef fishes (Boulon, 1992). They also provide nesting areas for birds. Mangrove root systems trap and cycle nutrients and organic materials. Mangrove forests are poorly developed in the U. S. Virgin Islands, accounting for only three percent of the total land area. Based on a survey undertaken by the U. S. Geological Survey (USGS, 1994) there were 960 acres of mangrove/salt pond habitat on St. Croix, 424 acres on St. John, and 320 acres on St. Thomas. Red mangrove (Rhizopora mangle), black mangrove (Avicennia germinans) and white mangrove (Laguncularia racemosa) are the dominant trees in the mangrove forests.

Salt ponds

Salt ponds are tidal flats or basins which are at least partially separated from the sea by beach berms. They are a dominant feature of the wetlands of the U. S. Virgin Islands (more than eighty have been counted on the three main islands). Salinities vary from 10 to 100 parts per thousand (ppt). Salt ponds also trap sediments before the sediments reach the nearshore reefs.

Protected and managed areas

Both the federal government and USVI environmental agencies share responsibility for protecting the coral reef ecosysems in the U.S. Virgin Islands. These include the Virgin Islands National Park (St. John), The Buck Island National Monument (St. Croix), the Virgin Islands National Monument (St. John), Marine Conservation Districts, the East End Marine Park (St. Croix), and marine sanctuaries.

The monument south of St. John contains predominantly deep algal plains with communities of mostly red and calcareous algae. Scattered areas of raised hard bottom are colonized with hard corals, sponges, gorgonians, and other invertebrates. They provide shelter for spiny lobsters, sea basses, and snappers, as well as spawning sites for some reef fish species. These algal plains and raised hard bottom areas link the shallow water reef, sea grass, and mangrove communities with the deep water shelf and shelf edge communities of fishes and invertebrates.