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.
Two types of deepwater ecosystems are typically associated with rocky substrates or “hardgrounds” in the Gulf of Mexico: chemosynthetic communities and deep-sea coral communities. Most of these hard bottom areas are found in locations called cold seeps where hydrocarbons are seeping through the seafloor. Petroleum deposits are abundant in the Gulf of Mexico: in 2009, oil production from the Gulf accounted for 30 percent of U.S. domestic production and 11 percent of natural gas production. Because deepwater ecosystems are associated with hydrocarbon seeps, 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. Since 2002, NOAA’s Office of Ocean Exploration and Research (OER) has sponsored nine expeditions to locate and explore deep-sea ecosystems in the Gulf of Mexico.Deepwater coral reefs were discovered in the Gulf 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. Although deepwater coral reefs are normally associated with hardgrounds, the corals that form them can also grow on artificial surfaces, including shipwrecks and petroleum production platforms. In 2008, there were more than 4,000 active platforms in the Gulf of Mexico, and thousands of others that are no longer active. The focus of the Lophelia II 2012: Deepwater Platform Corals expedition is to investigate the biology and ecology of deepwater corals and associated organisms growing on oil production platforms.
The Lophelia II 2012: Deepwater Platform Corals expedition will take place aboard the R/V Brooks McCall, and its primary objective is to document the occurrence, depth range, and growth rates of Lophelia pertusa corals on oil rig structures. Other objectives include:
- Collect samples for genetic analyses and other live animal studies;
- Document other corals and associated fauna that also occur on the oil rigs;
- Obtain cores to investigate the effects of rigs on fauna that live in sediments;
- Collect samples for coral aging investigations; and
- Collect samples for studies of the black coral Leiopathes sp.
These investigations are targeted toward broad questions that also guided previous OER-sponsored expeditions:
- What deepwater communities are associated with oil rig structures in the Gulf of Mexico?
- What organisms are characteristic of these communities?
- What are the relationships between coral communities on artificial and natural substrates?
- What processes control the occurrence and distribution of deepwater coral communities in the Gulf of Mexico?
- What is the life history of Lophelia pertusa corals in the Gulf of Mexico?
Remotely Operated Vehicle
A key technology for the Lophelia II 2012: Deepwater Platform Corals expedition is the Kraken 2 remotely operated vehicle (ROV), a tethered robot capable of diving and sampling to depths of 1,000 meters. The Kraken 2 ROV was developed at the University of Connecticut’s Northeast Underwater Research Technology and Education Center to provide a broad range of sampling, sensing and imaging capabilities. When deployed, an electro-optic tether (a cable containing fiber optic and wire conductors) connects the ROV to a control van aboard the support ship. Imaging equipment includes digital still, and HD cameras, a strobe light for still photography, and high intensity LED and HMI (hydrargyrum medium-arc iodide) lights for video. Sampling equipment includes a suction sampler, biobox (a container that holds living specimens in water to protect them from thermal shock), and a manipulator arm.
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.
More about Deepwater Coral Reef Ecosystems in the Gulf of Mexico
Deepwater coral reefs in the Gulf of Mexico 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.
In the Gulf of Mexico, deepwater corals are also found on anthropogenic (human-made) structures, particularly shipwrecks 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 dissimilar 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.
Corals are members of the phylum Cnidaria whose members are characterized by having stinging cells (nematocysts) that are used for feeding and defense. In addition to hard and soft corals, this phylum also includes sea fans, sea anemones, jellyfish, and hydroids. L. pertusa and other hard corals belong to the Scleractinia, within the class Anthozoa. Individual coral animals are called polyps, each of which has an internal skeleton made of limestone (calcium carbonate). In many corals species, including those that build reefs, the polyps form colonies composed of many individuals whose skeletons are fused together. In other species, the polyps live as solitary individuals. Each polyp has a ring of flexible tentacles surrounding an opening to the digestive cavity. The tentacles contain nematocysts that usually contain toxins used to capture prey or discourage predators. Corals are sessile (they stay in one spot) are depend upon currents to bring food within the reach of their tentacles. L. pertusa feeds on a variety of phytoplankton and zooplankton species, as well as dead materials.
The skeletons of individual corals are basically cup-shaped and provide protection as well as support for soft tissues. The fused skeletons of colonial corals may form boulders, plate-like structures, or complex branches. Large coral reefs develop over hundreds of years; some L. pertusa reefs are estimated to be more than 8,000 years old. As the corals reproduce, the skeletons of new corals grow on top of the skeletons of corals that have died (the lifespan of a single polyp is estimated as 10 – 15 years). L. pertusa grows at a rate that has been estimated to range between 4-25 mm per year, and produces complex branches and bushy colonies. This growth form aids feeding by reducing fast currents that could otherwise deform the soft polyps, and also produces strong and complex physical structures. Occasionally, highly branched colonies may partially collapse, producing rubble that traps sediments that add to reefs’ stability. Over time, repeated cycles of coral growth, collapse, and sediment entrapment can produce large reefs and mounds that provide habitats for many other species.
Although some corals are hermaphroditic (single individuals are male and female at some point in their life cycle), L. pertusa is gonochoric (individuals are only one sex during their life cycle). In fact, all of the polyps in a L. pertusa colony are the same sex. Corals may reproduce asexually by budding from the body of an adult polyp, by releasing larvae into the surrounding water, or from pieces of coral broken off from a colony. Corals also may reproduce sexually by forming eggs and sperm that produce a larva when a sperm fertilizes an egg. Anthozoan larvae are called planulae, and are pear-shaped with a fringe of cilia that provides limited swimming ability. One study of L. pertusa in the North Sea found that sexual reproduction occurred seasonally, and coincided with high rates of phytoplankton production in July. L. pertusa in the Gulf of Mexico appear to spawn in September or October.
L. pertusa is found throughout Earth’s ocean except in polar areas. Even with such a wide distribution and no obvious physical barriers, L. pertusa corals from different ocean regions have genetic differences that suggest these corals belong to different populations (a population is a group of organisms that belong to the same species, live in the same geographic area, and are more likely to breed with each other than with organisms from other areas). These differences can be detected and studied using techniques of molecular biology. Studies of this kind provide information about how L. pertusa larvae are dispersed, and are essential to understanding how L. pertusa reef ecosystems can be protected and restored. The extent to which individuals are exchanged between different populations is called connectivity. In the case of corals, which do not move once they have settled, connectivity depends upon larval dispersal.
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. L. pertusa reefs typically include four major habitats: living coral, dead coral, the water column immediately above the reef, and fringing habitats near the edge of the reef that include coral rubble and loose sediments. Organisms found in these habitats include antipatharians (black corals), gorgonians (sea fans and sea whips), alcyonaceans (soft corals), sea anemones, sponges, polychaete worms, crustaceans, gastropods, cephalopods, bivalves, bryozoans, and numerous fish species.
L. pertusa reefs are 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).
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