The Global 200 : a representation Approach to Conserving the Earth’s Distinctive Ecoregions
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- Pelagic Trades
- Pelagic Westerlies
- Abyssal
- Hadal
MarineRealmAlthough the marine realm includes a total of 10 MHTs, four of these were not assessed for the Global 200: Pelagic Trades, Pelagic Westerlies Abyssal, and Hadal (see the main text above for a discussion of this issue). We provide brief descriptions of these areas below. Pelagic TradesThe Pelagic Trades comprise roughly 45 percent of the total area of the open ocean, extending from the equator polewards to approximately 30 degrees north and south. Here the globe encircling Trade Winds meet the Westerlies, a convergence zone of global wind patterns. In contrast to the Pelagic Westerlies and many other marine ecosystems, the layer of mixed water at the surface is maintained continuously and water temperatures are relatively consistent throughout the year. This ecosystem is one of the most taxonomically diverse of the pelagic types. It is in these waters that pelagic fish reach their greatest development. A great variety of shoaling clupeids, loosely schooling tuna, and solitary sharks inhabit the tropical pelagos and comprise multiple and complex food chains. Other distinguishing features of the Pelagic Trades ecosystems include extensive vertical migrations and the existence of cyanobacterium mats. Vertical migrations occur at all seasons and in all areas of the Pelagic Trades. A substantial proportion of organisms, especially copepods, shrimp, myctophid fish, and squids rise to the surface at dusk to feed in the safety of darkness and descend to depths of 200-500 m at dawn. During windless periods of the year, it is also common to see the vast colored mats of the cyanobacterium Trichodesmium (four species). These loose colonial mats offer a substrate for a wide range of organisms. Pelagic WesterliesThe Pelagic Westerlies are the temperate open oceans, defined by the globe encircling Westerly winds. Prevailing wind systems define the boundaries of Pelagic Westerlies in both the Northern and Southern Hemispheres. Poleward, this region is bordered by the Polar front, and toward the equator it is defined by the tropical Trade Winds. The temperate waters within this region represent a biological transition between the polar and trade wind biomes; taxonomic composition at all trophic levels shows characteristics of each region. Surface waters are differentially heated in the summer and winter months, resulting in a change in the depth of the thermocline, or the level in the water column at which the temperature drops drastically and creates a barrier to vertical mixing. In the spring (and less reliably in the autumn), upwelling of nutrient rich bottom waters typically occur, resulting in phytoplankton blooms. The seasonal nature of productivity in these waters supports a number of migratory species that are able to exploit the patchy food resources. Migrants also take advantage of favorable water temperatures. Migrations from north to south and from the pelagic zone to the coastal and freshwater zones are common, particularly in fish. For example, the tunas move poleward as the water warms and towards the equator as it cools. Migratory herring and other clupeids return to the coastal zone to spawn, and the largely pelagic North Atlantic and North Pacific salmon migrate to freshwater to spawn. Numerous pelagic carnivores in the Pelagic Westerlies ecosystems are fish, though diversity is not as high as in warmer seas. AbyssalBeyond the coastal margins and continental shelves, at depths of 5,000 meters or more, lie a large proportion of the world’s ocean habitats. These deep ocean floor habitats are characterized by topographical variation that includes areas of flat plains, extensive hills, eroded plateaus, and seamounts. These underwater mountains are formed by volcanic activity and may rise over a kilometer above the sea floor. Water temperatures are quite cold, generally ranging from 2-4º C, although temperatures tend to vary little. In fact, variation declines with increasing depth. Water pressure—or hydrostatic pressure—is extreme and is measured in tonnes per square centimeter. Light levels are greatly reduced at these depths. Consequently, many nutrients arrive in the form of organic matter that rains down from the pelagic zone above or is washed down from continental shelves. Bottom communities are dominated by numerous species of worms, nematodes, and other mud-dwelling invertebrates. Among the more interesting habitats found in these areas are thermal vents, or areas of the ocean floor where steam is released from the Earth’s crust. Unique communities in these systems are dependent on bacteria that use hydrogen sulfide instead of sunlight for primary production. The giant clam, Calyptogena magnifica; the giant mussel, Bathymodiolus thermophilus; and a number of gastropods that feed on abundant bacteria comprise a portion of the fauna. Typical inhabitants found in the abyssal regions of the Ocean are several species of squid, including Histioteuthis spp. Other organisms; such as the sea pig (Phylum Echinoderms) and other sea cucumbers (Paelopatides spp. among others), tripod fishes (Bathypterois spp.), numerous isopods, giant tube worms (Riftia spp.), and suspension feeding cup sponges (Hyalonema spp.), represent but a portion of the benthic biodiversity found at these depths. The deep sea medusa (Periphylla periphylla) and the swimming cucumber (another Echinoderm) are two of the organisms which freely move through the water column. Migrations of organisms, including fish, from shallower depths appear to be uncommon. Anglerfish (Meanocetus johsoni) are just one of the bizarre looking fishes adapted to these abyssal environments. HadalDeep-sea trenches comprise the deepest parts of the world’s oceans. At depths greater than 6,000 meters these areas are formed by the subduction of the earth’s crustal plates. Among the best known of these areas is the Marianas Trench as well as trenches associated with numerous volcanic islands in the Pacific: Tonga, the Philippines, the Aleutians, and the South Sandwich islands Faunal composition in the hadal regions of the oceans is marked by reduced numbers of fishes, molluscs, sponges, and bryozoans. Other groups common in marine environments, such as decapods, brachiopods, and turbellarians, have not been found here. Overall, the fauna is characterized by high levels of trench endemism and certain morphological similarities: blindness, large size compared to related species found in shallower waters, and decreased pigmentation or color. Species that tolerate these extreme environments represent highly specialized and adapted faunas. Among the better studied taxa are molluscan communities, including herbivorous species such as cocculiniform limpets and xilophagaid clams. Given the proximity of these deep-water regions to volcanic islands, these filter feeders depend in part on dead plant material washing into the trenches from nearby terrestrial systems, as well as ocean shelves. Amazingly, these mollusks may feed on everything from mangrove seedlings to the fruits and berries of rain forest trees. 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