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Ecosystems: Coral Reefs
Flower Garden Banks National Marine Sanctuary
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| This photograph depicts the landscape of the East and West Flower Garden Banks. It illustrates the high coral coverage (around 50 percent) and competition for space between colonies. (Photo: Frank and Joyce Burck) |
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Location: Northwestern Gulf of Mexico; over 200 km SSW of Galveston, Texas (east bank 27o55' N, 93o36' W; west bank 27o52' N, 93o49' W)
Reef Structure and Physical Features: The Flower Garden Banks are outer shelf structures originating at depths of 100 to 200 m, 25 km apart on the edge of the continental shelf. The banks are surface expressions of uplift caused by the intrusion of salt from Jurassic evaporite deposits approximately 10 km below the sea floor. High salinity brine seepages have been discovered on the East Flower Garden Bank at 45 m and 71 m depth.
Surficial hard substratum at the Flower Gardens is exclusively carbonate rock, constructed primarily by contemporary populations of coralline algae and corals. The surrounding water is clear, with visibility generally greater than 30 m and light penetrating well to 85 m. Above 85 m depth, the biotic communities are dominated by reef-building organisms. Living coral reefs, made up of massive heads produced by 18 species of corals are the primary features between 15 and 46 m depths. The coral debris facies at depths of 25 to 50 m consist of coarse carbonate sand and gravel in basins and valleys between coral heads and in narrow aprons surrounding the reefs. An Algal Nodule Zone (Gypsina-Lithothamnium Facies) extends downward and outward from the coral debris facies to depths of 60 to 75 m.
Coral Species and Cover: A Diploria-Montastrea-Porites Zone is found at depths of 15 to 36 m. Only 18 species of reef building coral are found in this zone. Shallow water octocorals, such as sea fans and sea whips, and branching corals of the genus Acropora are conspicuously absent. Live coral coverage in this zone generally exceeds 50 percent on hard substrate. The Montastrea annularis species complex is dominant, comprising about 30 percent of the coral cover. Coral ridges, dominated by Madracis mirabilis, occur on peripheral parts of the main reefal structure between 28 and 46 m depth. Other common coral species include: Diploria strigosa, Colpophyllia species, Siderastrea siderea, and Stephanocoenia intersepta.
Other Fauna and Flora: The Flower Garden Banks harbor at least 80 species of algae, 196 known macro-invertebrate species, and more than 200 fish species. Loggerhead and green sea turtles, manta rays, spotted eagle rays, and schooling hammerhead sharks also occur in the sanctuary. Spotted dolphins are frequently reported in the area.
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