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 recently has been 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.
Deepwater ecosystems on hard substrates in the Gulf of Mexico are often found in locations where hydrocarbons are seeping through the seafloor (see “More About Deepwater Coral Communities and Hydrocarbon Seeps in the Gulf of Mexico,” below). Hydrocarbon seeps may indicate the presence of undiscovered petroleum deposits, and make these locations potential sites for exploratory drilling and possible development of offshore oil wells. Responsibility for managing exploration and development of mineral resources on the Nation’s outer continental shelf is a central mission 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.
For the past three years, NOAA’s Office of Ocean Exploration and Research (OER) has collaborated with MMS on a series of expeditions to locate and explore deep-sea chemosynthetic communities in the Gulf of Mexico. These communities not only indicate the potential presence of hydrocarbons, but are also unique ecosystems whose importance is presently unknown. To protect these ecosystems from negative impacts associated with exploration and extraction of fossil fuels, MMS 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.
OER-sponsored expeditions in 2006, 2007, and 2008 were focused on discovering sea floor communities near seeping hydrocarbons on hard bottom sites in the deep Gulf of Mexico; detailed sampling and mapping at selected sites; studying relationships between coral communities on artificial and natural substrates; and gaining a better understanding of processes that control the occurrence and distribution of these communities.
The Lophelia II 2009: Deepwater Coral Expedition: Reefs, Rigs, and Wrecks will take place aboard the NOAA Ship Ronald H. Brown. The expedition is directed toward exploring deepwater natural and artificial hard bottom habitats in the northern Gulf of Mexico with emphasis on coral communities, as well as archeological studies of selected shipwrecks in the same region. Expedition scientists will use the Jason II remotely operated vehicle (ROV) to:
• Make collections of Lophelia, other corals, and associated organisms from deepwater reefs;Additional samplers will be deployed directly from the ship, and additional oceanographic data will be collected using conductivity/temperature/depth instrument (CTD; see “Exploration Technology”).
• Collect quantitative digital imagery of characterization of deepwater reef sites and communities;
• Collect near-bottom oceanographic data;
• Deploy cameras, current meters, and samplers for microbes and larvae;
• Collect push cores; and
• Conduct archeological/ biological investigations on deepwater shipwrecks.
Jason II/Medea - During the 2006 Expedition to the Deep Slope, scientists used the submersible Alvin to dive on hydrocarbon seep communities that had never before been visited by humans. Scientists participating in the Lophelia II 2009 Expedition will return to these and other communities - but without leaving the deck of their ship!
The key to this kind of expedition is a remotely operated vehicle (ROV) system named Jason II/Medea. This is a two-part system: Jason II is a mobile platform that carries sonar and video imaging equipment as well as manipulator arms for collecting samples. Jason II gives scientists a ‘virtual presence’ in deep ocean waters at depths up to 6,500 meters. A 35-meter cable connects Jason II to a second ROV named Medea, which is connected to the surface ship by a 10-kilometer fiber optic cable. This arrangement allows Medea to buffer Jason II from movements of the ship, and provides a second platform for scientists to observe Jason II during seafloor operations.
The Jason II/Medea system is operated from a control van that is loaded aboard the host ship along with the ROV. Additional equipment and supplies are carried in tool and rigging vans.
The advantage of the Jason II/Medea system is that it allows much longer observation periods than are possible with manned submersibles; the average Jason dive is 21 hours (compared to Alvin dives which are six to ten hours), though dives as long as 71 hours have been made on some occasions. The system is designed, built, and operated by the Deep Submergence Laboratory of Woods Hole Oceanographic Institution. See http://oceanexplorer.noaa.gov/technology/subs/jason/welcome.html for more information.
CTD - Salinity is the concentration of salt and other inorganic compounds in seawater, and is one of the most basic measurements used by ocean scientists. Often, salinity is measured with an instrument called a CTD (conductivity, temperature, and depth) which measures how conductivity and temperature in the water column changes relative to depth. Conductivity is a measure of how well a solution conducts electricity and is directly related to salinity. CTD data can also be used to determine seawater density which is a primary driving force for major ocean currents. See CTD page for more information.
Multi-beam Sonar - Sonar (short for SOund NAvigation and Ranging) systems are used to determine water depth, as well as to locate and identify underwater objects. Multibeam sonar systems provide detailed bathymetric maps and three-dimensional images. All sonar systems are based on acoustic signals or pulses of sound that are transmitted into the water by a sort of underwater speaker known as a transducer. The transducer may be mounted on the hull of a ship, or may be towed in a container called a towfish. If the seafloor or another object is in the path of the sound pulse, the sound bounces off the object and returns an echo to the sonar transducer. The system measures the strength of the signal and the time elapsed between the emission of the sound pulse and the reception of the echo. This information is used to calculate the distance of the object, and experienced operators or computers can use the strength of the echo to make inferences about some of the object’s characteristics. Hard objects, for example, produce stronger echoes that softer objects. Multibeam sonars send out multiple, simultaneous sonar beams in a fan-shaped pattern that is perpendicular to the ship’s track. This allows the seafloor on either side of the ship to be mapped at the same time as well as the area directly below. See the Multi-beam Sonar page for more information.
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.
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 5 m above the bottom and a second current meter will be attached 100 m 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.
More About Deepwater Coral Communities and Hydrocarbon Seeps in the Gulf of Mexico
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. These corals are usually found on hard-bottom areas where there are strong currents and little suspended sediment (but extremely strong currents may interfere with feeding and cause breakage). Lophelia pertusa, the best-known deepwater coral species, prefers water temperatures between 4-12 °C, dissolved oxygen concentrations above 3 ml/l, and salinity between 35 and 37 psu. The influence of other factors, including pH, is not known.
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 metamorphose 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. It is 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.
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 deepwater structure-building corals 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-dwelling organisms.
Why are deepwater coral reefs in the Gulf of Mexico so often associated with hydrocarbon seeps? One reason is that 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 larvae of many other bottom-dwelling organisms may attach, particularly larvae of corals. It has also been suggested that microorganisms that feed on hydrocarbons could also provide a food source for corals, many of which obtain their nutrition through filter-feeding. Recent research, however, has shown that the skeletons of corals from seep communities do not have a chemical composition that supports this hypothesis (Becker, et al., 2009).
Deepwater corals in the Gulf of Mexico are also found on anthropogenic (human-made) structures, particularly ship wrecks and oil platforms. In addition to providing substrates for larval attachment, these structures may actually enhance the development of deepwater coral communities through weak electric currents produced by galvanic reactions that take place between dissimiliar metals in seawater. Studies have shown that these currents can increase calcium carbonate precipitation and stimulate the growth of corals and other organisms that produce carbonate structures.
More About Chemosynthetic Communities
The first chemosynthetic communities were discovered in 1977 near the Galapagos Islands in the vicinity of underwater volcanic hot springs called hydrothermal vents, which usually occur along ridges separating the Earth’s tectonic plates. Hydrogen sulfide is abundant in the water erupting from hydrothermal vents, and is used by chemosynthetic bacteria that are the base of the vent community food chain. These bacteria obtain energy by oxidizing hydrogen sulfide to sulfur:
CO2 + 4 H2S + O2 > CH2O + 4S +3H2O
(carbon dioxide plus sulfur dioxide plus oxygen yields organic matter, sulfur, and water).
Visit www.pmel.noaa.gov/vents for more information and activities on hydrothermal vent communities.
Chemosynthetic communities in the Gulf of Mexico were found by accident in 1984. These communities are similar in that they are based upon energy produced by chemosynthesis; but while energy for the Galapagos communities is derived from underwater hot springs, deep sea chemosynthetic communities in the Gulf of Mexico are found in areas where gases (such as methane and hydrogen sulfide) and oil seep out of sediments. These areas, known as cold seeps, are commonly found along continental margins, and (like hydrothermal vents) are home to many species of organisms that have not been found anywhere else on Earth. Typical features of communities that have been studied so far include mounds of frozen crystals of methane and water called methane hydrate ice, that are home to polychaete worms. Brine pools, containing water four times saltier than normal seawater, have also been found. Researchers often find dead fish floating in the brine pool, apparently killed by the high salinity.
Methane hydrate is a type of clathrate, a chemical substance in which the molecules of one material (water, in this case) form an open lattice that encloses molecules of another material (methane) without actually forming chemical bonds between the two materials. Methane is produced in many environments by a group of Archaea known as the methanogenic Archaeobacteria. These Archaeobacteria obtain energy by anaerobic metabolism through which they break down the organic material contained in once-living plants and animals. When this process takes place in deep ocean sediments, methane molecules are surrounded by water molecules, and conditions of low temperature and high pressure allow stable ice-like methane hydrates to form. These deposits are significant for several reasons:
• The U. S. Geological Survey has estimated that on a global scale, methane hydrates may contain roughly twice the carbon contained in all reserves of coal, oil, and conventional natural gas combined.
• Methane hydrates can decompose to release large amounts of methane which is a greenhouse gas that could have (and may already have had) major consequences to the Earth’s climate.
• Sudden release of pressurized methane gas may cause submarine landslides which in turn can trigger catastrophic tsunamis.
• Methane hydrates are associated with unusual and possibly unique biological communities containing previously unknown species that may be sources of beneficial pharmaceutical materials.
Where hydrogen sulfide is present, large tubeworms known as vestimentiferans are often found, sometimes growing in clusters of millions of individuals. At present, vestimentiferans are generally considered to be part of the phylum Annelida, but they are sometimes grouped as a separate phylum (Pogonophora). These unusual animals do not have a mouth, stomach, or gut. Instead, they have a large organ called a trophosome that contains chemosynthetic bacteria. Vestimentiferans have tentacles that extend into the water. The tentacles are bright red due to the presence of hemoglobin which can absorb hydrogen sulfide and oxygen which are transported to the bacteria in the trophosome. The bacteria produce organic molecules that provide nutrition to the tubeworm. A similar symbiotic relationship is found in clams and mussels that have chemosynthetic bacteria living in their gills. Bacteria are also found living independently from other organisms in large bacterial mats. A variety of other organisms are also found in cold seep communities, and probably use tubeworms, mussels, and bacterial mats as sources of food. These include snails, eels, starfish, crabs, lobsters, isopods, sea cucumbers, and fishes. Specific relationships between these organisms have not been well-studied.
Deepwater chemosynthetic communities are fundamentally different from other biological systems, and there are many unanswered questions about individual species and their interactions in these communities. These species include some of the most primitive living organisms (Archaea) that some scientists believe may have been the first life forms on Earth. Many deepwater species are new to science, and may prove to be important sources of unique drugs for the treatment of human diseases. Because their potential importance is not yet known, it is critical to protect these systems from adverse impacts caused by human activities.
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.