Why Are Scientists Exploring Deepwater Hard Bottom Habitats in the Gulf of Mexico?
A key purpose of NOAA’s Ocean Exploration Initiative is to investigate the more than 95 percent of Earth’s underwater world that until now has remained virtually unknown and unseen. Such exploration may reveal clues to the origin of life on Earth, cures for human diseases, answers on how to achieve sustainable use of resources, links to our maritime history, and information to protect endangered species.
In recent years, rising costs of energy and a growing desire to reduce the United States’ dependence upon foreign petroleum fuels have led to intensified efforts to find more crude oil and drill more wells in the Gulf of Mexico. This region produces more petroleum than any other area of the United States, even though its proven reserves are less than those in Alaska and Texas. Managing exploration and development of mineral resources on the nation’s outer continental shelf is the responsibility of the U.S. Department of the Interior’s Minerals Management Service (MMS). Besides managing the revenues from mineral resources, an integral part of this mission is to protect unique and sensitive environments where these resources are found.
To locate new sources of hydrocarbon fuels, MMS has conducted a series of seismic surveys to map areas between the edge of the continental shelf and the deepest portions of the Gulf of Mexico. These maps provide information about the depth of the water as well as “seismic amplitude,” which is affected by the type of material that is found on the seafloor. In areas where the seafloor is covered with soft mud, seismic amplitude is low, while hard surfaces produce a high seismic amplitude; and hard surfaces are often found where hydrocarbons are present. Carbonate rocks (such as limestone), in particular, are a part of nearly every site where fluids and gases containing hydrocarbons have been located. This is because when microorganisms consume hydrocarbons under anaerobic conditions, they produce bicarbonate which reacts with calcium and magnesium ions in the water and precipitates as carbonate rock. This rock, in turn, provides a substrate where the larvae of many other deepsea bottom-dwelling organisms may attach, particularly corals. In addition to carbonate rocks associated with hydrocarbon seeps, deepwater corals in the Gulf of Mexico are also found on anthropogenic (human-made) structures, particularly ship wrecks and oil platforms.
Many metal structures in seawater experience galvanic reactions, which produce weak electric currents. Studies have shown that these currents may enhance calcium carbonate precipitation and stimulate the growth of corals and other organisms that produce carbonate structures. Through these processes, ships and oil platforms may enhance the development of deepwater coral communities, as well as provide substrates for larval attachment.
Deepwater coral reefs were discovered in the Gulf of Mexico nearly 50 years ago, but very little is known about the ecology of these communities or the basic biology of the corals that produce them. Recent studies suggest that deepwater reef ecosystems may have a diversity of species comparable to that of corals reefs in shallow waters, and have found deepwater coral species on continental margins worldwide. One of the most conspicuous differences between shallow- and deepwater corals is that most shallow-water species have symbiotic algae (zooxanthellae) living inside the coral tissue, and these algae play an important part in reef-building and biological productivity. Deepwater corals do not contain symbiotic algae (so these corals are termed “azooxanthellate”). Yet, there are just as many species of deepwater corals (slightly more, in fact) as there are species of shallow-water corals.
The major structure-building corals in the deep sea belong to the genus Lophelia, but other organisms contribute to the framework as well, including antipatharians (black corals), gorgonians (sea fans and sea whips), alcyonaceans (soft corals), anemones, and sponges. While these organisms are capable of building substantial reefs, they are also quite fragile, and there is increasing concern that deepwater reefs and their associated resources may be in serious danger. Many investigations have reported large-scale damage due to commercial fishing trawlers, and there is also concern about impacts that might result from exploration and extraction of fossil fuels. These impacts are especially likely in the Gulf of Mexico, since the carbonate foundation for many deepwater reefs is strongly associated with the presence of hydrocarbons. Potential impacts include directly toxic effects of hydrocarbons on reef organisms, as well as effects from particulate materials produced by drilling operations. Since many deepwater reef organisms are filter feeders, increased particulates could clog their filter apparatus and possibly smother bottom-swelling organisms. To protect against these impacts, the Minerals Management Service has developed rules that require the oil and gas industry to avoid any areas where geophysical survey data show that high-density chemosynthetic communities are likely to occur. Similar rules have been adopted to protect archeological sites and historic shipwrecks.
A primary goal of the Deepwater Coral Expedition: Reefs, Rigs, and Wrecks is to develop the ability to recognize areas where deepwater corals are “likely to occur” in the Gulf of Mexico. To achieve this long-term goal, the Expedition will accomplish three objectives:
• Discover and describe new locations in the deep (greater than 300m depth) Gulf of Mexico where there are extensive coral communities;
• Gain a better understanding of the processes that control the occurrence and distribution of deepwater coral communities in the Gulf of Mexico; and
• Study the relationships between coral communities on artificial and natural substrates with respect to species composition and function, genetics, and growth rates of key species.
These objectives include both biological and archeological questions. Key biological questions include:
• Where are potentially significant deepwater communities located?
• What are the major reef-building species at these sites?
• What are other major components of biological communities at these sites?
• What are the genetic similarities and differences among these sites?
• How do structure, species richness and diversity compare between communities associated with Lophelia corals on man-made structures and communities from natural sites in the Gulf of Mexico?
• How do variations in temperature, pH, alkalinity, dissolved oxygen and electrical current affect the growth of Lophelia corals?
• What are the maximum growth rates of key cnidarian species that colonize man-made structures, based on images of the largest colonies from structures of known age?
• How do sites with the most significant communities of coral colony development change in response to variations in temperature, currents, larval seasonal distribution and sediment quality and quantity over a period of one year?
Key archeological questions are directed toward as many as six shipwrecks in the north-central Gulf of Mexico. These questions include:
• What is the identity, type of ship, date of construction, nationality, past and present ownership, use history, cause of loss, mission, and cargo at time of loss of each vessel?
• What is the extent and condition of the artifact assemblage and the presence of diagnostic artifacts on each vessel?
• Are any of these vessels potentially eligible to the National Register of Historic Places?
• How are biofouling communities on these shipwrecks affecting their stability and rates of deterioration?
• What are the major bacterial communities on these sites?
The Deepwater Coral Expedition: Reefs, Rigs, and Wrecks is expected to extend over a four-year period. During 2008, major activities will include:
• Preliminary site surveys of areas that have been identified as possible locations of new hard-bottom communities;
• Preliminary survey and imaging of two to three wrecks, and if appropriate, installing deterioration platforms (small thin bars of various metals used to measure and monitor corrosion);
• Collecting deterioration platforms previously deployed on the Gulfpenn wreck and installation of new platforms;
• Quantitative digital imaging of L. pertusa colonies on the Gulfpenn and very limited collection of L. pertusa samples for genetic analyses;
• Collection of imagery for transect analyses, construction of mosaics, and faunal inventory as time and ROV abilities allow;
• Survey of areas in the vicinity of oil rigs that may have been impacted by anchor chains, including limited collections of live L. pertusa for laboratory studies of live corals; and
• Collections of other fauna as time and abilities of the ROV allow for identification and preliminary genetic work.
Remotely Operated Vehicle - Besides the research vessel itself, the key technological component of the Deepwater Coral Expedition is probably the remotely operated vehicle (ROV) that will be used for much of the survey, imaging, and sampling activities. The Seaeye Falcon DR ROV is rated for dives up to 1000 m, is equipped with a fiber optic data transmission system and is connected to a surface vessel by a 7000’ fiber optic umbilical cable. The Falcon uses four thrusters for horizontal movement and a fifth thruster for vertical movement. Dimensions of the standard ROV are 20”H x 24”W x 39”L, with a weight of 220 lbs. The Falcon carries three color CCD cameras, two 150 W halogen lamps, and four 20W high intensity discharge lamps (equivalent to 240W of halogen lighting). Two scanning sonar systems provide bathymetric and bottom profile information. A three-jaw grabber on an extending arm is equipped with a soft line rope cutter.
Photographic Imagery - A variety of camera systems and techniques will be used to document deep-sea species, growth forms, habitat utilization, and behavioral adaptations. A digital macro-camera system will provide images of fine scale features of the fauna and habitats. An autonomous rotary timelapse camera will collect panoramic images of submersible operations and mobile organisms. In areas where detailed samples are to be collected, a series of separate, overlapping images will be collected at a constant scale so that these images can be merged into a larger continuous mosaic that covers the area of interest. These mosaics will be used to:
• Estimate spatial coverage and abundance of foundation coral species;
• Produce detailed maps for planning sample collection and instrument placement;
• Provide information on density and distribution of visible megafauna; and
• Document baseline data that can be used to detect habitat changes in future years.
Photographic sampling will be used to develop statistically valid estimates of the density and diversity of corals and other sessile (non-moving) invertebrates and the substrates on which they occur. At shipwreck sites, the entire area will be systematically surveyed using a combination of still and video photography. A pair of lasers mounted a known distance apart will be used to provide scale in many images.
Food Web Studies Using Radioactive Isotopes - Isotopes are forms of an element that have different numbers of neutrons. For example, carbon-13 (13C) contains one more neutron than carbon-12 (12C). Both forms occur naturally, but carbon-12 is more common.
Ecologists use a tool called “stable isotope analysis” to study food webs. When an animal eats food that contains both carbon isotopes, carbon-12 is selectively metabolized, so the ratio of carbon-12 to carbon-13 in the tissues of the animal contain is higher than the ratio of these isotopes in the food they consumed. In other words, carbon-13 is “enriched” in the animal’s tissues. If this animal is eaten by another consumer, the enrichment process will be repeated. So the ratio of carbon-13 to carbon-12 increases with each increase in trophic level (i.e., “each step up the food chain”). For additional discussion of stable isotope analysis, see “Who Is Eating Whom?” (http://oceanexplorer.noaa.gov/explorations/07mexico/logs/june15/june15.html).
Quantitative Sampling - To calculate community features such as diversity and the relative abundance of various species, sampling techniques are needed that collect the majority of species present in the community, and that provide samples in which the proportion of species in the sample is the same as the proportion of species in the community. The basic requirements for a quantitative sampling device is that it collects a sample of known volume, and that it has a reasonable chance of collecting everything that’s inside the sample area. One of the best-known sampling devices that meets these requirements, called a “mussel pot,” was originally made from a stainless steel stock pot and used to sample deep-sea chemosynthetic communities dominated by mussels. In addition to the stock pot, the mussel pot collector includes a tough kevlar bag fitted with a drawstring, and a rotating handle with a shaft that penetrates through the bottom of the pot. The kevlar bag is fitted into the pot, and is held open with light strings or threads. The drawstring is tied onto the handle shaft inside the pot. To obtain a sample, the pot is pushed into the area being sampled (usually by the manipulator arm of an ROV), then the handle is rotated to pull the drawstring tight and seal the bag. The Deepwater Coral Expedition will use a modified version of the mussel pot called the “coral pot,” which also releases an aluminum ring when the sample is collected to mark the collection site for photographic documentation.
Long-term Monitoring - Data on sedimentation, currents, and temperature will be collected periodically over an entire year at selected sites to understand how these factors vary with season and in response to short term events such as storms. Sedimentation will be studied with sediment traps that periodically collect a water sample (21 samples total over a one year period) into a polyethylene bottle containing dimethyl sulfoxide, which preserves the sample for later analysis of minerals, chemicals, microbes, and larvae, including genetic material. Current meters will be installed on moored lines. On each mooring, one current meter will be located 5m above the bottom and a second current meter will be attached 100m above the bottom so that the influence of the sea floor can be studied. Temperature loggers will be installed on all moorings used for current meters, sediment traps, and long-term camera installations.
Genetic Analyses - One of the many questions about deepwater coral reefs in the Gulf of Mexico is whether corals and other reef-dwelling organisms in one location are the same species as those in other locations. Historically, corals have been classified primarily on the basis of structural characteristics of their skeletons. On this basis, corals in the genus Lophelia are all classified as a single species. But recent research using techniques of molecular biology to study genetic characteristics has shown that Lophelia corals from the North Atlantic are genetically different from Lophelia corals collected near Brazil, suggesting that these populations are genetically isolated from one another. This finding has important implications for the conservation and protection of deep -sea coral reef resources, because if corals on different reefs are genetically distinct, then many more areas must be protected to maintain biological diversity than would be necessary if corals were genetically identical on all deepwater reefs. The same considerations apply to other deep-reef species as well.
Molecular biology techniques provide and extremely sensitive way to identify species and study their relationships with other species. These techniques examine organisms’ DNA, the molecule in every cell that carries the information for constructing the organism. A DNA molecule has two spirally twisted strands, and each strand is a string of smaller molecules, called "nucleotides." The specific order of the nucleotides along the strand is a biological code that corresponds to specific products found in the organism. One sequence of nucleotides may code for a protein that produces skin, while a slight change in the sequence may produce fingernails. Genes are chunks of DNA that code for a specific product. Molecular biology techniques allow scientists to read the sequence of nucleotides along DNA strands, and since every species differs in various ways from other species, their nucleotide sequences are also different.
Even within the same species, the nucleotide sequences are different from individual to individual (which is why everyone you know looks different in some way, even though they are the same species). But the nucleotide sequences of closely related individuals are more similar than the sequence of unrelated individuals. If individuals of the same species have not had a common ancestor for a long time, they are likely to have more differences in their genes than individuals that are siblings. So if corals of the same species from different locations have many differences in their genes compared to differences between individuals from the same location, this would suggest that the two locations are biologically isolated, which means that these locations do not exchange coral larvae. This information may be important to conservation for reasons discussed above.
Laboratory Studies on Live Corals - Special marine aquaria will be used for experiments to determine the effects of temperature, pH, dissolved oxygen, and electrical current on growth and survival of L. pertusa. The results of these experiments will help identify conditions that are favorable for development of deepwater coral communities in the Gulf of Mexico. During the experiments, the number of live polyps will be monitored daily, and the buoyant weight of the coral colonies will be monitored weekly to measure calcification rate.
Temperature is believed to control both the upper and lower depth distribution of L. pertusa in ocean habitats around the world. Changes in pH can affect the ability of corals (and other organisms) to produce body structures made of calcium carbonate. These changes are increasingly significant to deep water corals as rising atmospheric CO2 levels result in oceanic acidification. Dissolved oxygen (DO) concentrations are relevant because other researchers have found that the critical DO concentration for L. pertusa is 3.26 ml/1, and DO measured near L. pertusa habitats in some areas of the Gulf of Mexico was between 2.6 and 3.2 ml/1. Experiments with electrical current are important because studies of tropical corals have shown higher growth rates in the presence of an electrical field due to the increased abundance of dissolved mineral ions (such as calcium) for calcification. Since the metal components of shipwrecks often produce electric currents, L. pertusa growth rates on wrecks and rigs could be higher than natural growth rates.
More About Deep-water Corals
Deepwater corals are usually found on hard-bottom areas where there are strong currents and little suspended sediment. Lophelia pertusa is the best-known deepwater coral species. L. pertusa prefers water temperatures between 4-12 oC, dissolved oxygen concentrations above 3 ml/l, and salinity between 35 and 37 ppt. The influence of other factors, including pH, is not known. Extremely strong currents may interfere with feeding and cause breakage.
L. pertusa has separate sexes and spawns during September and October in the Gulf of Mexico. Gametes are released and fertilized externally, producing planula larvae that settle onto hard substrates and metamorphoses into individual coral polyps. As the polyps grow, new polyps are added by budding to form branches and may eventually produce “thickets” and massive reef structures. Larger thickets of coral usually consist of an outer layer of living coral surrounding a central dead portion of coral skeleton that provides a substrate for settlement by larvae of L. pertusa and other deepwater coral species. The largest known continuous reef structure is roughly oval in shape covering 13 km along its axis and 300 m in diameter and includes coral thickets up to 35 m thick. The age of dead coral skeletons in the middle of L. pertusa thickets in the Gulf of Mexico has been estimated to be greater than 40,000 years.
Distribution of deepwater gorgonians can be limited by the slope of the bottom and low temperatures. Other coral species are expected to have slightly different tolerances for these and other environmental factors.
For More Information
Contact Paula Keener-Chavis, national education coordinator for the NOAA Office of Ocean Exploration, for more information.
Other lesson plans developed for this Web site are available in the Education Section.