Long-Lived, Slow-Growing Corals in Deep Waters of the Gulf of Mexico
In stark contrast to an earlier notion in the 1800s that life should not exist below 600 m in the ocean, deep-sea coral ecosystems are now widely recognized as biodiversity hotspots. Like shallow-water coral reefs, deep-sea coral-reef ecosystems are among the most diverse and productive communities on Earth, providing shelter and feeding grounds for both commercial and noncommercial fish species and their prey, as well as breeding and nursery areas. They also serve as a valuable hunting ground for new medicines; a source of human income and food from such commercially important fishes as rockfish, shrimp, and crab; and a window into past environmental conditions recorded in the chemistry of coral skeletons. Activities that affect the seafloor, such as certain methods of petroleum exploration and commercial fishing, can affect these ecosystems. A better understanding of the complexity and interconnectivity of deep-sea-coral ecosystems will help us better assess natural and human impacts, as well as decipher chemical records of past conditions.
To improve our understanding of deep-sea coral ecosystems, U.S. Geological Survey (USGS) scientists and their partners recently investigated the growth rates and ages of deep-sea black corals in the Gulf of Mexico. Their results were reported in the February 10 issue of Marine Ecology Progress Series.
This study confirms that black corals are the slowest-growing deep-sea corals and are extremely long lived. The authors applied 14C dating methods to black coral samples collected within 40 to 50 km north and northeast of the recent Deepwater Horizon oil spill and discovered that the sampled animals have been growing continuously for at least the past 2 millennia, with growth rates ranging from 8 to 22 micrometers (µm) per year. In comparison, the shallow-water coral Porites lobata, typically found in tropical areas like Hawai‘i, grows about 10 mm per year, or more than 600 times as fast as black coral; and human fingernails grow about 36 mm per year, or more than 2,000 times as fast as black coral.
The investigators detected the presence of bomb-derived 14C (artificially produced by the detonation of nuclear test weapons in the atmosphere in the late 1950s and early 1960s) in the samples, confirming that black corals make their skeletons predominantly from particulate organic matter that sinks from the ocean surface, rather than from dissolved inorganic carbon in their deep-water surroundings. Thus, black corals can capture and record in their skeletons the history of changing concentrations of 14C in surface waters and the atmosphere, despite living at water depths greater than 50 m.
Unlike the skeletons of most shallow-water corals, which consist of calcium carbonate (CaCO3), black coral skeletons are composed mainly of organic matter: successive layers of protein and chitin (a long molecule containing carbon, oxygen, hydrogen, and nitrogen) glued together by a cement layer. These skeletons are very similar to insect cuticles in that they are quite flexible and can thus bend in water currents. The flexibility and shiny luster of black coral have made it a precious commodity in the coral jewelry trade. In fact, black corals have been harvested for centuries to create charms and medicines; the scientific name of the order to which black corals belong, "Antipatharia," comes from Greek roots meaning "against suffering."
Black corals grow in treelike or bushlike forms. Because they get their food from sinking organic matter instead of from symbiotic algae, like their shallow-water counterparts, black corals need skeletons that are flexible but strong enough to withstand currents that transport food to the colonies. In addition to a constant flow of water bringing them food and oxygen, the corals require a stable substrate, such as volcanic or calcareous rock, or even a shipwreck or oil rig that can serve as a platform for the corals to settle on and build their skeletons.
Like trees, black corals exhibit radial growth, with the oldest skeletal material in the center and successively younger material building out toward the edges. Viewed in a horizontal cross section, the black coral's growth bands resemble tree rings. The recent study found a fairly good correlation between ages derived from 14C analysis and visual counts of coral growth bands, indicating that each band represents a year of growth. Annual variations in food supply may be a factor affecting annual skeletal growth; however, the exact mechanism to explain the formation of annual growth bands remains unclear.
Reliably dating the corals, as done in the recent study, is a critical step in using them as natural archives of climate change. The skeletons that these animals secrete continuously over hundreds to thousands of years offer an unprecedented window into past environmental conditions. Dating used in combination with emerging technologies, such as sampling skeletal material onsite with a laser to determine its chemical composition, enables scientists to reconstruct environmental conditions in time slices smaller than a decade over the past 1 to 2 millennia. USGS scientists and their colleagues, for example, are measuring trace metals and stable isotopes in black coral skeletons that are related to nutrient supplies in surface waters, which, in turn, may reflect the amount of runoff from nearby land surfaces. With a proper understanding of how these chemical constituents vary over time, scientists can reconstruct a record of environmental changes, such as changes in land-based sources of nutrients and natural variations in climate. This approach is similar to the use of chemical variations in sediment cores to reconstruct climate histories. However, whereas sedimentary records can be disrupted by low sedimentation rates and bioturbation (mixing by animals living in the sediment), records from coral skeletons are continuous and can be used to determine changes from centuries to millennia at time scales comparable to that of modern observations.
The recent study was part of the USGS Diversity, Systematics, and Connectivity of Vulnerable Reef Ecosystems (DISCOVRE) Expedition, in which USGS scientists are partnering with other Federal agencies, such as the Bureau of Ocean Energy Management, Regulation and Enforcement (BOEMRE) and the National Oceanic and Atmospheric Administration (NOAA), as well as several academic institutions, to study deep-sea coral reefs. (See http://soundwaves.usgs.gov/2009/12/fieldwork2.html and http://fl.biology.usgs.gov/DISCOVRE/.) Experts from various fields among the Federal agencies and academic institutions are working together to try to understand the oceanography, biology, and ecology of these complex ecosystems. In doing so, they hope to decipher the degree to which geographically distinct communities share interconnected food chains and (or) are genetically related. Genetic diversity helps species adjust to changing conditions. If human or natural disturbances reduce the exchange of genetic traits between black coral communities, the older corals become more valuable as "banks" of genetic material for future populations. Overexploitation of black corals without proper management could easily lead to local population extinction. Given the extremely long life spans and very slow growth rates of black corals, it is unlikely that these species are renewable within the context of fishery management (2- to 3-decade time scale) or even within a human lifespan.
The corals reported in this study were collected before the Deepwater Horizon oil spill from a study site later overshadowed by surface slicks; long-term monitoring may be required to assess potential damage from the spill. The USGS DISCOVRE team has conducted several research cruises since the Deepwater Horizon event to investigate its impacts (http://fl.biology.usgs.gov/DISCOVRE/discovre_2010/discovre_10_oil_spill_update.html).
The full citation for the new paper is:
Prouty, N.G., Roark, E.B., Buster, N.A., and Ross, S.W., 2011, Growth rate and age distribution of deep-sea black corals in the Gulf of Mexico: Marine Ecology Progress Series, v. 423, p. 101-115, doi:10.3354/meps08953 [http://dx.doi.org/10.3354/meps08953].
in this issue:
Gulf of Mexico's Deep-Water Coral