Why Are Scientists Exploring Deep-Sea Ecosystems 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.
Deepwater ecosystems in the Gulf of Mexico are often associated with rocky substrates or “hardgrounds.” Most of these hard bottom areas are found in locations called cold seeps where hydrocarbons are seeping through the seafloor (see “More About Deep-Sea Ecosystems in the Gulf of Mexico,” below). Microorganisms are the connection between hardgrounds and cold seeps. 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. Two types of ecosystems are typically associated with deepwater hardgrounds in the Gulf of Mexico: chemosynthetic communities and deep-sea coral communities. Hydrocarbon seeps may indicate the presence of undiscovered petroleum deposits, so the presence of these ecosystems may indicate potential sites for exploratory drilling and possible development of offshore oil wells. At the same time, these are unique ecosystems whose importance is presently unknown.
For the past four years, NOAA’s Office of Ocean Exploration and Research (OER) has sponsored expeditions to locate and explore deep-sea chemosynthetic communities in the Gulf of Mexico. On April 20, 2010, a gas explosion occurred on the mobile offshore drilling unit Deepwater Horizon about 40 miles southeast of the Louisiana coast. The explosion killed 11 workers, injured 17 others, ignited an intense fire that burned until the Deepwater Horizon sunk 36 hours later, and resulted in a massive release of crude oil that is now considered the greatest environmental disaster in U.S. history. The total volume of oil released into the Gulf of Mexico is estimated to have been 205 million gallons (4.9 million barrels), dwarfing the 11-million gallon Exxon Valdez spill of 1989. Efforts to prevent the released oil from making landfall included the use of dispersants, some of which were injected at the wellhead to reduce the amount of oil that reached the surface. Extensive media attention has been directed toward the ecological impacts of released oil on beaches, marshes, birds, turtles, and marine mammals. Many scientists, however, are also concerned about how oil and dispersants may affect the unusual and biologically-rich communities of the Gulf of Mexico seafloor.
The Lophelia II 2010: Cold Seeps and Deep Reefs expedition will take place aboard the NOAA Ship Ronald H. Brown, and is directed toward re-visiting selected sites explored in prior years, as well as exploring sites that have not been previously visited. Mindful of concerns about potential impacts of the Deepwater Horizon blowout on deep-sea ecosystems, scientists will be particularly alert for any evidence of such impacts.
Expedition scientists will use the JASON II remotely operated vehicle (ROV) to:
- Make collections of Lophelia, other corals, and associated organisms from deepwater reefs; organisms from deepwater reefs;
- 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; and
- Collect push cores to sample sediments and sediment-dwelling organisms.
Long-term samplers deployed during previous expeditions will be recovered, and additional oceanographic data will be collected using a conductivity/temperature/depth instrument and a small number of multibeam surveys (see “Exploration Technology”).
These investigations are targeted toward broad questions that also guided OER-sponsored expeditions in 2006, 2007, 2008, and 2009:
- Where are cold-seep and deepwater coral communities associated with hard-bottom environments in the deep Gulf of Mexico?
- What organisms are characteristic of cold-seep and deepwater coral communities in the deep Gulf of Mexico?
- What are the relationships between coral communities and artificial and natural substrates?
- What processes control the occurrence and distribution of cold-seep and deepwater coral communities in the Gulf of Mexico?
In addition, the Lophelia II 2010 expedition is pursuing a new question:
- What impacts, if any, have occurred in deep-sea communities as a result of the Deepwater Horizon blowout?
A key technology for the Lophelia II 2010: Cold Seeps and Deep Reefs expedition is a remotely operated vehicle (ROV) system namedJason 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 that allows scientists to observe Jason II during seafloor operations.
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.
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. To make it easier for scientists and the interested public to obtain information from Jason IIoperations, Woods Hole Oceanographic Institution provides an online Virtual Control Van system that automatically captures information in the control van during ROV operations and makes this information immediately accessible and searchable via a Web browser. The Virtual Control Van is available to scientists and the public via the Internet. For more information, visit http://4dgeo.whoi.edu/jason .
CTD stands for conductivity, temperature, and depth, and refers to a package of electronic instruments that measure these properties. Conductivity is a measure of how well a solution conducts electricity and is directly related to salinity, which is the concentration of salt and other inorganic compounds in seawater. Salinity is one of the most basic measurements used by ocean scientists. When combined with temperature data, salinity measurements can be used to determine seawater density which is a primary driving force for major ocean currents. Often, CTDs are attached to a much larger metal frame called a rosette, which may hold water sampling bottles that are used to collect water at different depths, as well as other instruments that can measure additional physical or chemical properties.
Ocean explorers often use CTD measurements to detect evidence of volcanoes, hydrothermal vents, and other deep sea features that cause changes to the physical and chemical properties of seawater. Masses of changed seawater are called plumes, and are usually found within a few hundred meters of the ocean floor. Since underwater volcanoes and hydrothermal vents may be several thousand meters deep, ocean explorers usually raise and lower a CTD rosette through several hundred meters near the ocean floor as the ship slowly cruises over the area being surveyed. This repeated up-and-down motion of the towed CTD may resemble the movement of a yo-yo; a resemblance that has led to the nickname “tow-yo” for this type of CTD sampling. See: http://oceanexplorer.noaa.gov/technology/tools/sonde_ctd/sondectd.html and http://www.pmel.noaa.gov/ for more information.
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 can 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.
More About Deep-Sea Ecosystems in the Gulf of Mexico
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 web. 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 the individual species and interactions between species found 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 species are new to science, and may prove to be important sources of unique drugs for the treatment of human diseases. Organisms from hydrothermal vent communities have proven to be useful in a variety of ways, including treatment of bone injuries and cardiovascular disease, copying DNA for scientific studies and crime scene investigations, and making sweeteners for food additives.
Because their potential importance is not yet known, it is critical to protect deepwater chemosynthetic ecosystems from adverse impacts caused by human activities.
Deepwater Coral Reefs — 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 ppt. The influence of other factors, including pH, is not known.
Recent studies suggest that deepwater reef ecosystems may have a diversity of species comparable to that of coral 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. Sulak (2008) provides extensive information on deepwater hard-bottom coral communities at Viosca Knoll in the Northern Gulf of Mexico, including illustrations of fishes, benthic invertebrates, and typical biotopes associated with these communities.
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 the carbonate rock resulting from microbes feeding on hydrocarbons 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).
More About Oil and Dispersants Associated with the Deepwater Horizon Blowout
The Deepwater Horizon and its associated blowout site are located at 28°44.20’ N latitude, 88°23.23’ W longitude. This site is on a steep slope in an area known as Mississippi Canyon Block 252, where the continental shelf drops sharply to depths that exceed 2,000 m. The blowout site is about 1,500 m (5,000 ft) deep.
Oil released from the Deepwater Horizon blowout is classified as “light crude,” but the released material is a mixture of pressurized oil and gas. As the more volatile components evaporate, oil becomes thicker and tar-like. As the oil moves through the water, turbulence breaks the oil mass into smaller particles that become tar balls. Oil and water may become mixed into thick emulsions called mousse. Small droplets of oil rise more slowly than larger oil masses, and very small droplets (less than about 100 µm in diameter), rise so slowly that they may remain in the water column for several months.
Since deep-sea ecosystems in the Gulf of Mexico are often found in close proximity to hydrocarbon seeps, it might be supposed that organisms in these systems may have some tolerance for small droplets of oil from the Deepwater Horizon blowout. In 2003, the National Research Council estimated that the amount of oil seeping into the entire Gulf of Mexico ranges from 80,000 to 200,000 tonnes per year (a tonne is equal to 1,000 kg or about 2,205 lb, and is approximately the mass of one cubic meter of water at 4°C). For comparison, the amount of oil released by the Deepwater Horizon blowout is estimated to have been about 4.9 million barrels (205 million gallons) or about 780,000 cubic meters of crude oil. Since the density of crude oil is about 80% to 90% the density of water, 780,000 cubic meters of crude oil is approximately 624,000 – 702,000 tonnes.
Oil masses may also be broken up by chemical dispersants. The idea is to disperse the oil into a much larger volume of water in hopes that dilution will reduce toxic effects, and to remove large masses of oil from the water surface where they are harmful to birds and other organisms. Preventing landfall of oil from the Deepwater Horizon blowout was a major objective of response efforts, and at least 1.8 million gallons of dispersant were applied as part of these efforts. About 720,000 gallons of this total were injected below the surface near the wellhead. At the end of May, 2010, a group of engineers, scientists, and spill responders concluded that “…up to this point, use of dispersants and the effects of dispersing oil into the water column has generally been less environmentally harmful than allowing the oil to migrate on the surface into the sensitive wetlands and near shore coastal habitats.” (Coastal Response Research Center, 2010).
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NOAA Office of Ocean Exploration and Research
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