Flower Garden Banks National Marine Sanctuary: The Natural Setting
The Flower Garden Banks National Marine Sanctuary encompasses a variety of habitat types, including the northernmost coral reefs in the continental United States. Located about 115 miles directly south of the Texas/Louisiana border, the East and West Flower Garden Banks are perched atop two salt domes (underwater hills) rising above the sea floor.
The nearest tropical reefs to the Flower Gardens are 400 miles away, off the coast of Tampico, Mexico. Scientists believe that corals at the Flower Garden Banks probably originated from Mexican reefs when currents in the western Gulf of Mexico carried the young corals, other animal larvae, and plant spores northward.
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| A spotted moray eel peeks out from under a small
coral formation while a small cleaner goby swims alongside, perhaps
waiting to get to work. (Photo: George Schmahl)
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Amazingly, this location in the northwestern Gulf provided all the comforts of home for reef building corals: a hard surface for attachment, clear sunlit water, warm water temperatures (between 68 and 85 degrees Fahrenheit), and a steady food supply. The corals now form the basis for a complex, yet balanced ecosystem, providing a regional reservoir of shallow-water Caribbean reef species.
It was this wonderful biological diversity and breathtaking beauty that prompted researchers and recreational divers to seek protection for the Flower Garden Banks. They launched what would become a 20-year effort, culminating in 1992, to designate the Flower Garden Banks National Marine Sanctuary.
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| Two divers swim past one of the Madracis covered pinnacles at Stetson Bank.(Photo: George Schmahl)
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In 1996, a third bank was added to the sanctuary. Stetson Bank is located about 30 miles northwest of the West Flower Garden Bank. That small difference in location produces an amazing difference in the habitat. Because of its more northerly position, winter water temperatures average about four degrees (Fahrenheit) cooler than at the Flower Garden Banks. This small temperature difference is enough to prevent corals from growing fast enough to pile up into a coral reef, as they have at the Flower Garden Banks. Instead, you find smaller individual coral colonies interspersed with a much denser population of sponges and algae. You can even see the siltstone bedrock showing through in many places. Divers describe the effect as an underwater moonscape.
In the open water column above each of the banks, you'll find species that survive by cruising from place to place in search of a meal or a mate. You may encounter such charismatic creatures as manta and spotted eagle rays, hammerhead and silky sharks, chub, loggerhead sea turtles, jack crevalle, amberjack, and if you are really lucky, a whale shark.
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| A small Spanish hogfish swimming near a large orange sponge at West Flower Garden Bank. Juvenile Spanish hogfish are important cleaner fish on the reef. (Photo: George Schmahl)
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Just below 50 feet on the Flower Garden Banks, you encounter the reef cap, which continues to depths of about 120-140 feet. Over 300 acres of marvelous high relief reefs include about 23 species of corals, over 250 reef invertebrate species, 175+ fish species, and 80+ types of marine algae. Until recently, it was thought that the Flower Garden Banks had no large branching coral species, such as elkhorn or staghorn coral. Nor were the banks thought to be home to more than a sprinkling of "soft corals" such as sea whips or sea fans. Explorations in recent years, however, have revealed at least two small colonies of elkhorn coral on the reef cap and many additional species of "soft corals," such as gorgonians, in the deeper areas of the sanctuary (140-400+ feet).
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| Large colonies of boulder star coral (Monastraea annularis) layered across the reef at West Flower Garden Bank with a few colonies of brain coral in between. (Photo: George Schmahl)
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The most obvious organisms found on the reef cap are the massive boulder-shaped coral colonies. The dominant coral species are the star corals and the brain corals. Many have been sculpted into interesting shapes by a process called bioerosion, in which other organisms gradually wear away the colony around its base. The coral colonies form an infrastructure that provides food and shelter for other reef inhabitants. Algae, sponges, and other attaching organisms quickly colonize available space, created by bioerosion and breakage.
Below 90 feet on the reef cap, nestled among the larger corals, you will also see ridges or knolls with high concentrations of the small branching finger coral Madracis mirabilis. These unusual thickets also feature finger sponges, encrusting sponges and algae. The Madracis ridges are also scattered around the deeper reef habitat.
To examine these habitats, you must venture below 120 feet into water as deep as 170 feet, not advisable for the average recreational diver. At these depths, corals grow in a flattened manner to maximize their exposure to light, a critical element of life to the symbiotic algae living in the corals' tissues. Habitat relief is much lower than on the shallow reefs. Fewer hard corals live in this zone, primarily because most species need more light. The dominant species are a star coral (Stephanocoenia michilini) and fire coral (Millepora alcicornis).
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| Creolefish cruise above a patches of ten-ray star coral (Madracis decactis) at Stetson Bank while a couple of rock hinds perch on the reef. (Photo: George Schmahl)
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At about 150 feet, you will begin seeing algal-sponge habitat, which extends to around 270 to 290 feet deep. Dominated by coralline algae, this habitat covers several square miles, a much larger area than is inhabited by corals. Algal nodules, up to fist size, cover 50 to 80 percent of the bottom in places. Because of the area covered and the amount of carbonate deposits produced, the algae may be more important to reef formation, overall, than corals are on the banks. Although less is known of the biota in this habitat, some believe that the species diversity may be comparable to that on the reef cap.
The lower two-thirds of the algal-sponge habitat begins a transition zone between organisms that exhibit distinct shallow-water traits and those adapted to deep water. A distinctive transition species is the white, bedspring-shaped, antipatharian called a sea whip.
Scattered throughout the deeper portions of the banks, where coralline algae do not thrive and reef-building corals are totally absent, are remnants of ancient reefs. These drowned reefs are the remains of reefs that probably thrived during periods of lower sea levels when they were closer to the surface. Now, depth and comparatively turbid conditions limit the species diversity.
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| A queen parrotfish uses its sharp, beak-like teeth
to graze on algae. In the process, the parrotfish also crunches up
coral skeletons, which creates sand around the reef. (Photo: George Schmahl)
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Until recently, it was believed that the areas surrounding the bases of the banks were quite flat and mostly un-vegetated sand or mud bottom. Improved mapping technology has revealed that these areas are much more complex than previously thought. They may actually form a sort of "fish highway", providing protection from predators and sources of food for animals moving between the banks. Even the muddy, silty areas are more diverse than many people imagine. They support healthy communities of organisms living on and within the sediments. Microalgae, a variety of worm species, crabs and sea stars are examples of the organisms inhabiting these areas.
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