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NATURAL SETTING

The natural setting in which Flower Garden Banks National Marine Sanctuary is situated is the key to protecting this national treasure. It is crucial that we understand the complexities of the sanctuary's local ecosystem and the interplay of individual plant and animal populations.

It is also vital that we understand the larger context of the surrounding Gulf of Mexico ecosystem and watershed that influence, and are influenced by, the sanctuary. Gaining this understanding is a continuous process.

Included below are some topics of interest. Feel free to click on a link to jump directly to a specific topic, or continue down the page to read them all.

Salt Domes
Coral Reef Formation
Regional Context
Currents
Watershed
Connectivity

Another great tool for learning about the sanctuary and the surrounding region, is the interactive map compiled by NOAA's National Coastal Data Development Center (NCDDC) using sanctuary data. This map will allow you to add different layers of information such as boundaries, bathymetry, buoys, oil and gas infrastructure, ROV tracks, vessel tracks, and more.

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SALT DOMES

Located about 115 miles directly south of the Texas/Louisiana border, East and West Flower Garden Banks are actually salt domes rising above the sea floor. Stetson Bank, located about 30 miles northwest of West Flower Garden Bank, is also a salt dome formation.

What is a salt dome? To understand, we have to go back in history about 190 million years when the Gulf of Mexico was a very shallow sea. The hot dry climate at that time caused a lot of evaporation, depositing a thick layer of salt on the sea floor.

As the Gulf of Mexico deepened and rivers began to flow from the land to the sea, mud, sand, and silt were steadily deposited over the salt layers. Eventually, pressures from these denser overlying sediments became great enough that the salt layers began to push upward. In some places the salt layers broke through completely, while in others they simply forced the seafloor to bulge upward in distinct domes.

 

Video animation of how a salt dome forms, using East
Flower Garden Bank as an example.
For a more detailed explanation of this video, please visit our Video Library.

The shape of the salt deposit, or salt diapir, is most often a roughly cylindrical form about 1/2 to 2 miles across. Movement of this salt "plug" causes faults and cracks in the overlying and surrounding rock.

How close is the salt to the earth's surface (i.e. the sea floor)? That varies by location. At some sites the salt lies only a few feet beneath the sea floor sediments. In others, it may lie over 10,000 feet deep.

Salt domes occur across the continental shelf of the northwestern Gulf of Mexico (see map below). Many of these are also named banks such as McGrail, Bright, Alderdice, Sonnier, etc.

Some salt domes even occur on land in coastal Louisiana and Texas. High Island, TX and Avery Island, LA are two examples.

Map of the northwestern Gulf of Mexico showing the location of various banks off the TX and LA coasts.

Click on the map to view a larger version

For more detail about salt dome formation, please read Geology of Stetson Bank by Dick Zingula (also available as a 12kb pdf).

For further information on the underlying geology of Stetson bank, please consult Geology & Paleontology of Stetson Bank (2008) by Dick Zingula (also available as a 952kb pdf).

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CORAL REEF FORMATION

The coral reef communities of East and West Flower Garden Banks probably began developing on top of the salt domes 10,000 to 15,000 years ago. These communities have thrived to the extent that dense coral reefs hide all traces of the deformed bedrock underneath.

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 Gardens probably originated from Mexican reefs when currents in the western Gulf of Mexico carried the young corals northward. A few of these larvae were lucky enough to settle on the hard substrate (sea floor) of the Flower Gardens salt domes.

Amazingly, this location in the northwestern Gulf of Mexico provided all the comforts of home for hard corals: a hard surface for attachment, clear sunlit water, warm water temperatures (between 68 and 84 degrees Fahrenheit), and a steady food supply.

View of large boulders of coral piled on top of one another forming a reef.

Typical view at East and West Flower Garden Banks

The difference in location between the Flower Garden Banks and Stetson Bank (about 30 miles NW) produces an amazing difference in the habitat. Because of Stetson's more northerly position, winter water temperatures are four degrees Fahrenheit cooler, on average, than the Flower Garden Banks.

That small temperature difference is enough to prevent corals from growing fast enough to pile up into a coral reef at Stetson, as they have at the Flower Garden Banks. Instead, you find individual coral colonies interspersed with a much denser population of sponges. You can even see the siltstone bedrock showing through in many places.

While the predominant coral species at the Flower Garden Banks are large boulder shaped corals such as brain coral and mountainous star coral, the prevalent species at Stetson are smaller encrusting corals, such as fire coral and green cactus coral.

These corals now form the basis for a complex, yet balanced ecosystem, providing a regional reservoir of shallow-water Caribbean reef species.

Click on this link for a visual tour of East and West Flower Garden Banks.

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A REGIONAL CONTEXT

East Flower Garden, West Flower Garden and Stetson Banks are only three among dozens of banks scattered along the continental shelf of the northwestern Gulf of Mexico. All of these banks are part of a regional ecosystem, heavily influenced by current patterns within the Gulf. Inflows from the large watershed that drains two-thirds of the continental United States also play a significant role in the health of this region.

Currents

From the south, the Gulf of Mexico is fed by a current of warm water from the Caribbean, which enters the Gulf between Mexico's Yucatan Peninsula and Cuba. This forms the Gulf Loop Current, which curves east and south along Florida's coast and exits through the Straits of Florida.

Map of the southeastern United States and eastern Mexico surrounding the Gulf of Mexico.  Red arrows in the Gulf show the path of various water currentsClick on the map to view a larger version

The Loop Current is variable, sometimes barely entering the Gulf before turning, while at other times, it travels almost to Louisiana's coast before swinging toward Florida. When that happens, the main current passes directly over the eastern banks along the continental shelf.

Simultaneously, bits of the loop often break away from the main current and form circular eddies that move westward, across the Flower Garden, Stetson and other banks to the west. This influx of water brings with it animal larvae, plant spores and other imports from the south, which probably accounts for the many Caribbean species found in the northern Gulf of Mexico. During its progress, the main current is also picking up the same sorts of 'passengers' from the northern Gulf to deliver along its route back to the Caribbean and Atlantic.

Meanwhile, the shallower parts of the water flowing into the Gulf travel to the northwest following the Louisiana, Texas and Mexico coastlines before turning east. These wind driven currents also cross over the Flower Garden, Stetson and other banks from the opposite direction of the Gulf Loop eddies and add to the Caribbean influence in the region.

For a time-lapsed look at the currents traveling through the Gulf of Mexico, visit the Navy's Intra-Americas Sea web page and click on the Speed/Currents Nowcast or the SST (Sea Surface Temperature) Nowcast for the last 30 days.

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Watershed

From the north, two-thirds of the continental United States and part of Canada drains into the Gulf of Mexico. The Mississippi River Basin accounts for the largest portion of this watershed.

From the west, river systems in the eastern half of Mexico also contribute.

Map of North America showing the watersheds that lead to the Gulf of Mexico.  Bathymetric contour lines are visible in the Gulf.

Click on the map to view a larger version.
Map Credit: EPA

These rivers bring with them all of the runoff accumulated from cities, suburbs, rural areas and wildlands along their routes. Nutrients, sediments and physical debris all have the potential to negatively influence the heatlh of the Gulf and the various habitats it contains.

Before this replenishing source of water reaches the Gulf, it is also partially depleted by extractions for municipal, industrial and agricultural consumption. This reduced freshwater outflow can negatively affect the health of coastal estuaries that filter sediments and pollutants from the water and provide nursery areas for many species. Some of these species later move offshore to the system of banks along the continental shelf.

How people manage the watersheds that lead to the Gulf of Mexico is very important to Gulf health.

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Connectivity

We have long been aware that water flows connect the dozens of banks along the continental shelf of the northwestern Gulf of Mexico. Recent explorations, however, indicate that there may be much more physical connection than previously believed.

Screen capture of map showing bathymetry and relief of East and West Flower Garden Banks

Click on the map to link to an interactive map for the Northwestern Gulf of Mexico

Technological advances have allowed higher resolution mapping efforts that reveal systems of low relief geological features (such as rock outcroppings) between those banks that have been more extensively explored in the last few years. Such areas may allow much more direct interaction between the banks than previously thought.

As we build upon the knowledge base established by the discoveries to date, we may discover that these interactions play a crucial role in maintaining the health of the sanctuary's living marine resources.

Added to this mix of influences are the hundreds of oil and gas production platforms (green dots on map below) that serve as de facto artificial reefs by providing a hard surface to which larvae and spores may attach themselves. Scientists are still assessing the extent to which this system of platforms affects the overall biological productivity of the Gulf.

Map of northwestern Gulf of Mexico with red dots showing the location of various banks and green dots showing the location of oil and gas platforms.

Green dots show the locations of oil and gas platforms in the northwestern Gulf of Mexico. Red dots show the location of various banks.

Click on the map to link to an interactive map where you can zoom in on specific areas

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Small, knobby corals in foreground; boulder of brain coral in background.  Long, fingery branches of purple sponge anchored in knobby corals and standing upright.
   
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