Atoll | Define Atoll at Dictionary.com atoll [at-awl, -ol, -ohl, uh-tawl, uh-tol, uh-tohl] /ˈæt ɔl, -ɒl, -oʊl, əˈtɔl, əˈtɒl, əˈtoʊl/ Spell noun 1. a ring-shaped coral reef or a string of closely spaced small coral islands, enclosing or nearly enclosing a shallow lagoon. Origin of atoll Expand 1615-1625 1615-25; earlier atollon < French: a word used in early descriptions of the Maldive Island atolls; said to be Divehi (Indo-Aryan language of the Maldives) atoḷu Dictionary.com Unabridged Examples from the Web for atoll Expand Amelia Earhart Breakthrough? Don’t Bet On It Clive Irving June 2, 2012 Plus, Kate and William could hardly have picked a worse time for their luxury vacation to a resort in Noonu atoll.

ANOTHER Vacay for Kate and William Tom Sykes March 5, 2014 Historical Examples But there is a third kind of reef, of very common occurrence in the Pacific and Indian Oceans, which goes by the name of "atoll." British Dictionary definitions for atoll Expand a circular coral reef or string of coral islands surrounding a lagoon Word Origin C17: from atollon, native name in the Maldive Islands Collins English Dictionary - Complete & Unabridged 2012 Digital Edition © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012 Word Origin and History for atoll Expand n. 1620s, atollon, from Malayam atolu "reef," probably from adal "closing, uniting." Popularized in present form by Darwin's writings.

Online Etymology Dictionary, © 2010 Douglas Harper atoll (āt'ôl', ā'tôl') A coral island or series of coral islands forming a ring that nearly or entirely encloses a shallow lagoon. Atolls are surrounded by deep ocean water and range in diameter from about 1 km (0.62 mi) to over 100 km (62 mi). They are especially common in the western and central Pacific Ocean and are believed to form along the fringes of underwater volcanoes. Compare barrier reef , fringing reef. The American Heritage® Science Dictionary Copyright © 2002. Published by Houghton Mifflin. All rights reserved. Show Prices Aldabra is a raised coral atoll on the western side of the archipelago, closer to Africa, and has been basically untouched by humans including the Persians and Arabs who found the island before the Portuguese arrived in 1511. For this reason, it is truly a natural wonder because it has been virtually unchanged for thousands of years. UNESCO World Heritage Site The atoll is made up of four large islands made of coral that surrounds a shallow lagoon.

There is a coral reef that encircles these four islands. The island is not easy to access and for this reason it still has the largest population of giant tortoises in the world. The main importance of this untouched ecosystem is for the study of evolutionary and ecological processes. The extraordinary beauty of this atoll is derived from the diversity of the ocean and land, which join to make a spectacular array of formations and colours. The Geography The four main islands are built form coral limestone. They are separated by narrow passages and encircle a shallow lagoon. Inside this lagoon is a spectacle of natural phenomena and includes many smaller islands. The entire outside of the atoll is surrounded by a fringing reef. The development of the different landforms has created several different marine habitats including coral reefs, sea-grass beds and mangrove mudflats. The oceanic island ecosystem developed without human interruption. The land surface is mostly ancient coral reef that is about 125,000 years old that has been continually raised above sea level. There is great diversity of landforms that support an assortment of island communities with the top of the food chain being the giant tortoise, which is an herbivore. This is unusual.

The tortoises feed on the shrubbery and grasses, which have evolved a growing pattern in response to the tortoises grazing pattern. Aldabra Atoll is a fascinating laboratory with over 400 endemic species and subspecies that include vertebrates, invertebrates and plants. It supports over 100,000 giant tortoises. This is the last place they reside in the Indian Ocean where they once were on many islands. A few hundred were transplanted to Curieuse another island in the Seychelles . The atoll is also an important breeding ground for endangered green turtles and critically endangered hawksbill turtles. Several species of birds are endemic to the island such as the last flightless bird of the Western Indian Ocean. There are several large water bird colonies including one of only two existing oceanic flamingo populations and the second largest colony of frigate bird in the world. The ecosystems are intact and sustain the viable populations of all the key species. Giant Tortoises are not the only animals sustained by the island. The coconut crab, the largest land crab in the world lives there as well as hammerhead sharks, barracuda and manta rays.

In ancient times, the dominant land predator was the crocodilian Aldabrachampsus, a horned crocodile of which some remains from the Pleistocene were found on the island. The island is managed by the Seychelles Islands Foundation . They limit tourism and oversee protection of the ecosystems from invasive alien species, oil spills and climate change. A Close Call Aldabra was not always considered ecologically important. In the 1960s, the British government considered putting a Royal Air Force base on the island and asked the United States to help. The US would get a base too if they helped fund the project. Also, the BBC wanted to put transmitters on the island to rebroadcast the BBC Overseas Service to Africa. Fortunately, international ecologists protested, and the military plans were abandoned. The BBC couldn’t do it alone. At this time, the atoll received full protection and was designated a UNESCO World Heritage site in 1982. The only permanent population on the island is the Island Manager, the Research Officer and rangers and staff. How to see Aldabra? People wishing to visit Aldabra must get permission from the Seychelles Islands Foundation at their head office.

Tourism increases the revenue needed to support the conservation and protection of Aldabra, but tourism is strictly limited to nature tourism and education. Permission for tourism is only given to: • Small and medium sized cruise ships • Live-aboard charter boats • Live-aboard dive boats • Educational and Scientific visits on land that are limited to 12 persons at a time Visitors are given limited access according to the zoning policy and the visits are lead in a sustainable way with any impact on the atoll constantly being assessed. Visitors may not collect shells or in any way interfere or damage the plants and animals. No geological or biological specimens can be collected and there are strict sanitary methods that apply to all visitors. A staff member must always be with visitors and they may not wander around the island unaccompanied. Transportation to Aldabra There is a bi-monthly supply boat that belongs to the Island Development Company that will take visitors to the island. However, this needs to be arranged by the visitors once permission is granted. The SIF may help, but it is not their responsibility to arrange transportation to Aldabra. Chartered planes can be taken to Assumption where a chartered boat can be taken to Aldabra.

Visitors can also hire a private yacht once they have received permission to visit the island. It is possible to drift dive into the lagoon depending on the tide and the marine life there is exceptional. Only about 1000 people visit Aldabra every year, so the birds and other animals are not afraid of humans. This means it is possible to have very close observation. Fees There is a daily landing fee for all foreign visitors as well as a fee for professional photographers and journalists. There is a fee for still photographs as well as for video photography. The fees are all applied to the continuing conservation of the island. Aldabra is a natural wonder almost completely untouched by humans for thousands of years. For the few visitors who get to the island, the experience is extraordinary. atoll - National Geographic Society atoll Atolls, like this one near Fiji, can be perilous to ships at sea.

Photograph by Luis Marden Pseudo-Atolls The island nation of Bermuda is sometimes called the world's "northernmost atoll." Bermuda is actually a pseudo-atolla ring of islands that look like an atoll, but have several distinguishing characteristics: high elevations, mostly submerged reefs, and a wide reef-front terrace (the broad, sloping shelf running from the island's highest elevation to the sea). No Atolls for the Soviets The isolated atolls of the Pacific and Indian Oceans were widely used by the American, British, and French militaries to test their nuclear weapons in the 20th century. The Soviet Union, which also possessed a nuclear arsenal, did not test on tropical Pacific atolls. The Soviet Union tested most of its nuclear weapons in the remote Novaya Zemlya archipelago in the Russian Arctic. An atoll is a ring-shaped coral reef , island, or series of islets. An atoll surrounds a body of water called a lagoon . Sometimes, atolls and lagoons protect a central island. Channel s between islets connect a lagoon to the open ocean or sea. Atolls develop with underwater volcano es, called seamount s.

First, the volcano erupts, piling up lava on the seafloor. As the volcano continues to erupt, the seamount's elevation grows higher, eventually breaking the surface of the water. The top of the volcano becomes an oceanic island . In the next stage, tiny sea animals called corals begin to build a reef around the island. The type of corals that build reefs are called hermatypic corals, or hard coral s. Hermatypic corals create a hard exoskeleton of limestone (calcium carbonate). Billions of these limestone exoskeletons are the reef. This coral reef, called a fringing reef , surrounds the island just below the ocean surface. The thin, shallow strip of water between the fringing reef and the island is the lagoon. Over millions of years, the volcanic island erode s and sinks to the seafloor. This process is called subsidence . The seamount erodes into the sea, its top made flat by the constant pounding of powerful ocean waves. As it subsides, the flat-topped seamount is called a guyot . As the island subsides to become a guyot, its ring-shaped fringing reef turns into a barrier reef .

A barrier reef is further from shore, and has a deeper lagoon. The barrier reef protects the lagoon from the harsh winds and waves of the open ocean. Subsidence brings slight differences in ocean chemistry that change the reef radically. The outer, ocean-facing side of the reef remains a healthy marine ecosystem . Corals on the inner, lagoon-facing side, however, begin to slowly decay . The algae that corals need to survive face much more competition for fewer nutrient resources. The limestone decays, changing the color of the lagoon from deep ocean blue to bright teal . In the final stage of an atoll’s formation, ocean waves break apart pieces of the limestone reef. They pound, break, and erode the coral into tiny grains of sand. This sand and other material deposited by waves or wind pile up on the reef. This material, including organic matter such as plant seeds, form a ring-shaped island or islets. This is an atoll. Hermatypic corals only live in warm water. An island that is located where ocean temperatures are just warm enough to support hermatypic corals is said to be at the “ Darwin point ," named after Charles Darwin.

The famous naturalist was the first to outline how atolls form. Atolls and People The rocky or sandy shores of atolls have been important sites throughout human history. Often, their low-lying elevation has proved perilous . Atolls are often hidden by ocean waves. Thousands of ships, from ancient Polynesian canoes to sophisticated American warships, have been stranded and wrecked on hidden atolls. The Kon-Tiki , probably the most famous raft in history, became one of these atoll casualties. The Kon-Tiki was a large balsa raft built and sailed by explorer Thor Heyerdahl and his crew in 1947. The Kon-Tiki successfully sailed 6,980 kilometers (4,340 miles) from Peru to the South Pacific. The most difficult challenge of the journey was not the waves, current s, or trade wind s of the open ocean. It was the atolls of Polynesia, the final part of their journey. The quick-moving currents around atolls prevented the Kon-Tiki from docking at the first Polynesian island it encountered. It wrecked on the shallow coral of the second, Raroia atoll.

Raroia was uninhabited, but nearby native islanders in canoes eventually rescued the Europeans on the washed-up wreck. The Kon-Tiki was eventually hauled out of Raroia, but atoll wrecks are popular dive sites throughout the Pacific. Shipwrecks from the 18th century through World War II lie at the bottom of atolls such as Kwajalein, part of the Marshall Islands. Atolls are often uninhabited "desert" islands. (Desert does not refer to the islands' climate, but their "deserted" or uninhabited status.) Many are remote and difficult to reach. In the 20th century, this isolation made them attractive as testing sites for nuclear weapon s from the United States, Britain, and France. The first hydrogen bomb , for instance, was tested by the United States at Bikini Atoll, part of the Marshall Islands in the Pacific Ocean. The Pacific Proving Grounds, a series of 2,000 atolls and other islands under U.S. jurisdiction , was home to more than a hundred massive nuclear explosions between 1947 and 1962.

France continued nuclear testing on the atoll of Moruroa until 1995. Nations throughout Polynesia, including the " nuclear-free zone " of New Zealand, protested extended nuclear testing. Reefs were being destroyed, and some tests dropped toxic fallout onto nearby inhabited islands. After Castle Bravo, the first hydrogen bomb test, the U.S. evacuated residents of the Rongelap and Rongerik atolls, and later compensated them for medical conditions associated with radiation poisoning . The same elements that make atolls popular for nuclear testing also make them attractive to tourists. Atolls are sparse ly populated, low-lying islands whose white, sandy beaches and placid lagoons are ideally suited to the tourism industry. Island nations made of atolls include Maldives, in the Indian Ocean, and Kiribati, in the Pacific. Tourism is a key factor in both the Maldivian and Kiritbati economies. Many atolls, however, have few tourists and are among the world's underdeveloped countries. Atolls have few natural resource s. Soil quality on atolls is very poor, and erosion is a constant threat. Most native residents on atolls practice subsistence agriculture and fishing.

Almost all food and fuel is imported, often at great cost. Fisheries and support for the shipping industry help support communities on remote atolls. Some atoll communities have taken advantage of their equatorial location and established launch sites for low-orbit satellite s. Others have found other sources of revenue . The nation of Tuvalu, for instance, is a series of isolated atolls in the Pacific. Every year, it receives millions of dollars for use of its ".tv" Internet domain name . Atolls, along with sandbars, are among islands with the lowest elevation. They are constantly, naturally at risk from erosion due to wind and waves. Atolls are also at risk from sea level rise . As the the ocean level rises, atolls—and any infrastructure on them—are flooded and may drown altogether. Island nations such as Maldives and Kiribati are fortify ing their atolls by dredging the seafloor. Sand elevates certain areas and widens others, creating a more stable landmass. Maldives and Kiribati have also taken political measures to protect their citizens from the possibility of atolls sinking beneath the sea. Maldives often leads international conferences on the impacts of global warming , which is associated with sea level rise.

Maldives and Kiribati have also taken steps to outline a permanent evacuation process should sea level rise threaten to drown inhabited atolls. Term (singular: alga) diverse group of aquatic organisms, the largest of which are seaweeds. atoll barrier reef Noun ridge of coral or rock found parallel to the coast of an island or continent, but separated from it by a deep lagoon. channel rocky ocean features made up of millions of coral skeletons. current Darwin point Noun a place in an ocean or sea where the temperature is warm enough for corals to form reefs and atolls. decay series of letters and numbers separated by periods, providing an Internet address. dredge to remove sand, silt, or other material from the bottom of a body of water. erode removal of people, organisms, or objects from an endangered area. exoskeleton the hard external shell or covering of some animals. fallout Noun airborne radioactive particles that eventually fall to the ground, usually the result of a nuclear explosion. fortify a type of reef that extends from a coastline. global warming an underwater mesa, or flat-topped seamount. hard coral type of coral that creates a hard shell or exoskeleton around itself.

hydrogen bomb Noun powerful explosive that uses the energy from the fusion of hydrogen isotopes. Also called an h-bomb, fusion bomb, or thermonuclear bomb. infrastructure structures and facilities necessary for the functioning of a society, such as roads. jurisdiction geographic region associated with a legal authority. Kon-Tiki Noun (1947) raft used by explorer Thor Heyerdahl to sail from South America to the Polynesian islands. lagoon Noun shallow body of water that may have an opening to a larger body of water, but is also protected from it by a sandbar or coral reef. molten rock, or magma, that erupts from volcanoes or fissures in the Earth's surface. limestone Noun type of sedimentary rock mostly made of calcium carbonate from shells and skeletons of marine organisms. marine ecosystem community of living and nonliving things in the ocean. natural resource Noun a material that humans take from the natural environment to survive, to satisfy their needs, or to trade with others. nuclear-free zone area where all nuclear weapons and nuclear-energy generation is banned. nuclear weapon explosive device that draws power from the splitting and combining of atomic nuclei.

nutrient land formed from the eruption of a volcano on the ocean floor. open ocean area of the ocean that does not border land. perilous radiation poisoning Noun set of illnesses, including burns, cancers, and organ damage, that results from exposure to radioactive material. revenue income, or money earned before production costs are subtracted. satellite Noun object that orbits around something else. Satellites can be natural, like moons, or made by people. sea level rise Noun increase in the average reach of the ocean. The current sea level rise is 1.8 millimeters (.07 inch) per year. seamount transportation of goods, usually by large boat. soil top layer of the Earth's surface where plants can grow. sparse scattered and few in number. subsidence sinking or lowering of the Earth's surface, either by natural or man-made processes. subsistence agriculture Noun type of agriculture in which farmers grow crops or raise livestock for personal consumption, not sale. teal the industry (including food, hotels, and entertainment) of traveling for pleasure. toxic trade wind Noun winds that blow toward the Equator, from northeast to southwest in the Northern Hemisphere and from southeast to northwest in the Southern Hemisphere.

underdeveloped country country that has fallen behind on goals of industrialization, infrastructure, and income. volcano Noun an opening in the Earth's crust, through which lava, ash, and gases erupt, and also the cone built by eruptions. Types of Coral Reefs: Atolls, Barrier Reefs, Fringing Reefs Types of Coral Reefs Types of Coral Reefs Most reef scientists generally recognize three MAJOR types of coral reefs: Fringing Reefs, Barrier Reefs, and Atolls. The traditional and most widely recognized basis for differentiating these reef types is large-scale reef morphology; the size and shape of a reef, and its relation to nearby land (if any). This is usually (but not always) sufficient to clearly distinguish one type from the others. Nonetheless, there is often a great deal of overlap among the major reef types (within a given biogeographic region) in terms of the dominant groups of animals and plants , as well as their ecological interactions. Fringing Reefs (Shore Reefs) Fringing reefs are reefs that grow directly from a shore.

While there may be areas of shallow intertidal or sub-tidal sand bottom lying between the beach and the inshore edge of coral growth, there is no lagoon between the reef and shore. Fringing reefs surrounding Pacific islands The fringing reef is by far the most common of the three major types of coral reefs, with numerous examples in all major regions of coral reef development . Without an intervening lagoon to effectively buffer freshwater runoff, pollution, and sedimentation, fringing reefs tend to particularly sensitive to these forms of human impact. In is no surprise then that increasing human populations in coastal areas - and the accompanying increases in coastal development and intensive agriculture - have resulted in the decimation of fringing reefs throughout the world in recent years. Barrier Reefs Barrier reefs are extensive linear reef complexes that parallel a shore, and are separated from it by lagoon. WHAT IS A "LAGOON"? - A lagoon - as used in the context of coral reef typology - refers to a comparatively wide band of water that lies between the shore and the main area of reef development, and contains at least some deep portions. Barrier reefs are far less common than fringing reefs or atolls, although examples can be found in the tropical Atlantic as well as the Pacific.

Classic barrier reef morphology The 1200-mile long Great Barrier Reef off the NE coast of Australia is the world's largest example of this reef type. The GBR is not actually a single reef as the name implies, but rather a very large complex consisting of many reefs. The second largest IndoPacific barrier reef lies off New Caledonia's NE coast - it is some 400 miles long with a lagoon 1-8 miles wide. Another large barrier reef extends for nearly 170 miles to the north of Fiji and Vanua Levu. This reef type is rare in the Caribbean region, where only 2 true barrier reefs are found. The largest of these runs off the coast of Belize, and the other off the north coast of the island of Providencia (east of Nicaragua). Atolls An atoll is a roughly circular (annular) oceanic reef system surrounding a large (and often deep) central lagoon. IndoPacific atolls In the South Pacific, most atolls occur in mid-ocean. Examples of this reef type are common in French Polynesia, the Caroline and Marshall Islands, Micronesia, and the Cook Islands. The Indian Ocean also contains numerous atoll formations.

Examples are found in the Maldive and Chagos island groups, the Seychelles, and in the Cocos Island group. In contrast, atolls are relatively rare in the Caribbean. Published counts range from 10-27, depending upon who is doing the classification. The far greater number of atolls in the IndoPacific region of coral reef development - as opposed the Greater Caribbean region - can be mainly attributed to the far greater size of the former region along with its unique geomorphology, which is far more conducive to volcanic island formation and subsequent subsidence (see below). A Caveat on Reef Typology Readers should be aware that identifying major reef types may not always be quite as simple as the above discussion may suggest. Some reefs seem to be intermediate versions that defy any simple "either or" classification scheme. This is especially true in the Caribbean region, where some of the fringing reefs are separated from shore by open waters that may reach 6-8m in depth and extend a kilometer or more from shore. Are these fringing reefs, barrier reefs, or something else? The answers to such questions depend on whom you ask, but don't let that bother you.

Such issues are common in science when our human classification schemes simply fail to account for the seemingly infinite variability found in nature. Development Of Major Reef Types The basic coral reef classification scheme described above was first proposed by Charles Darwin , and is still widely used today. The evolution of main types of coral reefs, as first proposed by Charles Darwin Darwin spent most of his coral reef explorations in the IndoPacific region , and viewed the three types of coral reefs he described as simply different stages in the geological 'evolution" of Pacific oceanic islands. Darwin theorized that fringing reefs began to grow near the shorelines of new islands as ecological conditions became ideal for hard coral growth. Then, as the island began to gradually subside into the sea, the coral was able to keep pace in terms of growth and remained in place at the sea surface, but farther from shore; it was now a barrier reef. Eventually, the island disappeared below the sea surface, leaving only the ring of coral encircling the central lagoon; an atoll had formed (see right).

Darwin's general "reef evolution" theory was finally verified for IndoPacific reefs in the early 1950s after analyses of the results of deep core drilling at Bikini and Eniwetok Atolls. However, it has also now become apparent that each of the three major types of coral reefs (described above) is often also formed by quite different geomorphic processes as well. The atoll-like Bahama Banks are a prime example of such alternate forms of reef development. A Note On Patch Reefs The term "patch reef" is commonly used to refer to comparatively small, isolated outcrops of coral surrounded by sand and/or seagrass (see photo, below). Numerous patch reefs form part of the fringing reef system bordering a tropical island shore. © Fotolia While patch reefs have sometimes been described as a fourth "coral reef type" , such comparisons are clearly not appropriate. Rather, patch reefs are more properly considered regular micro-scale reef features of all three of the macro-scale reef types first described by Darwin - fringing reefs, atolls and barrier reefs.

In the sense that Darwin described coral reefs - the same reef classification system widely in use today - patch reefs are not remotely comparable to the major coral reef types, and should not be confused with them. Updated January 4, 2015 lagoon - National Geographic Society lagoon The tropical lagoons of Bora Bora are protected from harsh ocean waves by atolls and barrier islands. Photograph by George F. Mobley Hapua Hapua ecosystems are lagoons located near river mouths. As rivers carve deep channels parallel to the coastline, they create a unique type of coastal lagoon. Hapua are primarily freshwater ecosystems, but interact strongly with ocean tides. Hapua are identified almost entirely with river systems in New Zealand. Blue Lagoon The world's most famous lagoon, the Blue Lagoon in Iceland, is not a lagoon at all. It is a manmade feature where water from a local geothermal power plant is pumped over a lava bed rich in silica and sulfur. These elements react with the warm water to create a bright blue lake used as a spa.

A lagoon is a shallow body of water protected from a larger body of water (usually the ocean) by sandbar s, barrier island s, or coral reef s. Lagoons are often called estuaries, sounds, bays, or even lakes. Coastal Lagoons Lagoons sheltered by sandbars or barrier islands are called coastal lagoon s. Coastal lagoons form along coastal plain s—flat or gently sloping landscapes. They form in areas with small tidal range s. Coastal lagoons are created as a shallow basin near the shore gradually erode s, and the ocean seep s in between the sandbars or barrier islands. The size and depth of coastal lagoons often depend on sea level. When the sea level is low, coastal lagoons are swampy wetland s. When the sea level is high, they can look like coastal lakes or bays. The Outer Banks are barrier islands along the coast of the U.S. states of North Carolina and Virginia. The Outer Banks create a series of lagoons known as sound s: Currituck Sound, Albemarle Sound, and Pamlico Sound.

These areas are sheltered from storm surge s and other waves that often pound the shore during the Atlantic Ocean's hurricane season . The Outer Banks are actually enormous sandbars. They are not anchored to the earth, and suffer from coastal erosion during storms. The protection they offer the shores and lagoons is vital to the environment and economy of the region. Engineer s continually monitor and maintain the Outer Banks by dredging sand from the seafloor to fortify the islands. The lagoons of the Outer Banks have mostly brackish water , a mix of saltwater from the Atlantic Ocean and freshwater from many river mouth s in the area. The area is rich in biodiversity : waterfowl and fish from flounder to bass thrive in the region. The tourism industry also thrives in the coastal lagoons of the Outer Banks. Besides fishing, visitors to the sounds enjoy boating and recreational activities such as water skiing and parasailing. Lagoons with more protection from the open ocean have a more freshwater habitat. Lake Nokoue, Benin, is a lagoon whose narrow mouth to the Atlantic Ocean is almost entirely protected by sandbars. Its salinity varies with the seasons.

During the rainy season, when rivers flood the lake with their outflow, Lake Nokoue is almost entirely freshwater. During the dry season, when river slow to a trickle and seawater seeps in, Lake Nokoue has a more brackish ecosystem . Fish indigenous to Lake Nokoue, such as tilapia, have adapted to survive in both brackish and freshwater. Coastal lagoons, which offer protection from harsh ocean waves, are often used as harbor s. Lake Piso, for example, is the largest lake in the African country of Liberia. It is a lagoon protected from the Atlantic Ocean by big barrier islands. Lake Piso was used as a harbor for U.S. seaplanes during World War II. Lake Nokoue offered a different type of protection during the 16th and 17th centuries. Slave-trading tribes were forbidden from entering the waters of the lagoon, so local communities constructed an entire town, Ganvie, directly in the water. Homes and businesses were built on sturdy stilts, and transportation was limited to boats and bridges. Inhabitants were protected from capture and enslavement.

Venice The city of Venice, Italy, is built on barrier islands and a coastal lagoon of the Adriatic Sea. In fact, Venice's nickname is "Queen of the Adriatic." The Venetian Lagoon is the largest wetland in the Mediterranean. It consists mostly of saltwater marsh es and mudflat s. Two large rivers (the Sile and the Brenta) empty into the lagoon. Its thin barrier islands have three narrow openings to the Adriatic. Venice, however, has been one of the largest cities in Italy since the rise of Ancient Rome. Human activity has radically alter ed the ecosystem of the Venetian Lagoon. Today, Venice sits on 118 islands. Not all of these islands are natural features of the landscape. For more than 500 years, engineers and city leaders have dredge d the lagoon to create a series of islands and canal s. Wetland areas have also been drained to create land for housing and industry. The growth of Venice has also drained the aquifer beneath the lagoon and surrounding coast. As the aquifer shrank, the land above it subside d—Venice sank.

Venice's lower elevation made it increasingly vulnerable to strong seasonal tides from the Adriatic. Artesian well s were banned in the 1960s, and engineers have developed a sophisticated tide barrier project to reduce subsidence and protect the city from flooding. The Venetian Lagoon has recovered. Subsidence has slowed, although the famous aqua alta (high water) tide still floods the city in as much as 1.5 meters (5 feet) of water every winter. Atoll Lagoons Atoll lagoon s are similar to coastal lagoons. Instead of being sheltered by sandbars or barrier islands, however, atoll lagoons are protected by coral reefs. Atoll lagoons are very common in the tropical waters of the South Pacific and Indian Oceans. Atoll lagoons form as coral reefs form around volcanic island s. Over millions of years, the island subsides into the ocean. The ring of coral reefs, however, remain. The reefs become the atoll , protecting an enclosed lagoon where the volcano used to be. Atoll lagoons are marine ecosystems. The organisms found in atoll lagoons are usually the same ones found outside it.

Because of the ringing atoll, many lagoons have few indigenous species at all. Organisms, such as fish and jellies, surf in as waves from the ocean crash over the atoll and dump them in the lagoon. Many species of jellies thrive in this protected environment, but larger predators have few food resources. The water of atoll lagoons are often a striking light blue due to their shallow depth and their interaction with limestone . Coral reefs and coral sand are made of limestone, the remains of billions of tiny coral exoskeleton s. As limestone leaches into the lagoon, it turns the water bright blue. The billion-dollar tourism industry of the South Pacific relies on pristine beaches and bright blue lagoons. These atoll lagoons are also the site of some of the most intense debates about climate change and sea level rise . Lagoons and atolls are low-lying ecosystems vulnerable to even the slightest change in sea level. Sea level rise could drown the lagoons, and even their ringing atolls. Island nations such as Maldives could lose not only their primary industry (tourism), but the land itself.

Maldivian leaders have worked to combat sea level rise and coastal erosion by pursuing international agreements to limit human contributions to global warming , erecting buildings on stilts, and even considering evacuating the entire population. Term type of confined aquifer that flows to the Earth's surface without the need for pumping. atoll shallow, circular body of water between the ocean and a ring-shaped atoll. barrier island Noun long, narrow strip of sandy land built up by waves and tides that protects the mainland shore from erosion. basin salty water, usually a mixture of seawater and freshwater. canal wearing away of earth or sand on the beach by natural or man-made methods. coastal lagoon shallow body of water between the coast and a series of sandbars or barrier islands. coastal plain rocky ocean features made up of millions of coral skeletons. dredge to remove sand, silt, or other material from the bottom of a body of water. ecosystem engineer Noun person who plans the building of things, such as structures (construction engineer) or substances (chemical engineer). erode the hard external shell or covering of some animals. fortify hurricane season Noun time of year when the risk of hurricanes is greatest.

Atlantic hurricane season runs from June 1 to November 30. lagoon Noun shallow body of water that may have an opening to a larger body of water, but is also protected from it by a sandbar or coral reef. to separate materials by running water or another liquid through them. limestone Noun type of sedimentary rock mostly made of calcium carbonate from shells and skeletons of marine organisms. marsh mouth Noun place where a river empties its water. Usually rivers enter another body of water at their mouths. coastal wetland formed as rivers or tides deposit sediment. Outer Banks barrier islands off the coast of the U.S. state of North Carolina. pristine underwater or low-lying mound of sand formed by tides, waves, or currents. sea level rise Noun increase in the average reach of the ocean. The current sea level rise is 1.8 millimeters (.07 inch) per year. seep to slowly flow through a border. sound body of water, larger than a bay, partially surrounded by land. storm surge Noun abnormal rise in sea level accompanying a hurricane or other intense storm. Also called a storm tide. to return to a lower level.

tidal range the difference in height between an area's high tide and low tide. tourism the industry (including food, hotels, and entertainment) of traveling for pleasure. volcanic island land formed by a volcano rising from the ocean floor. wetland The Farquar Islands The Farquhar Group of Islands belong to the Outer Islands of Seychelles . They are 700 miles south west of Victoria, the capital city on Mahe Island. The Farquhar Group consists of one separate island and two atolls as well as one submerged reef. The group is made up of Farquhar Atoll that has two larger islands and eight small islets, Providence Atoll with two islets, which are Providence Island and Cerf Island , St. Pierre Island and Wizard Reef, which is submerged. St. Pierre is surrounded with fossil coral cliffs that have been undercut and have many blowholes. Many caverns have been cut by the sea, so there is no fresh water on the atoll. It is almost impossible to access the island by sea, but there is a derelict jetty at a ruined settlement on the northwest coast that can only be accessed by boat in very calm weather .

The Amirantes Islands The Amirantes Islands are part of the Outer Island Group of Seychelles. They are atolls and coral islands that were discovered by Vasco da Gama in 1502. They were most likely also known by early Indian and Arab traders. The Amirantes Islands consist of 11 main islands as well as numerous shoals, sand cays and atolls. The total population of the islands is about 100, and the main settlement is on the island of Desroches with a population of 50. The Islands Lady Denison-Pender Shoal is submerged, African Banks is a sand cay and Remire Reef is a drying reef, and all are uninhabited. Remire Island, which is an uplifted sand cay, has a coconut plantation. African Banks is visited by people with charter yachts, and the island has been exploited by poachers for nesting birds. In 1995, there was one coconut tree, but, other than that, the island is treeless. A derelict automatic lighthouse is at the north-eastern end. Alphonse Atoll + Islands Alphonse Atoll , Bijoutier and St.

Francois Atoll together are the Alphonse Group and are in the Outer Islands Group of the Seychelles . Alphonse is three kilometres north of St. Francois and has a population of less than 300 people. The island is 1.74 square kilometres with a diameter of three kilometres. Including the lagoon in the middle of the atoll, the island is eight square kilometres. The small atolls were first named on the Portuguese charts as San Francisco after 1562. In 1730, the commander of a French frigate gave the islands his name, which was Alphonse de Pontevez. Today, Alphonse is famous for some of the best fly fishing in the world. The island is a one hour flight south of Mahe . The Natural Habitat The isolation of Alphonse attracts migratory birds. The Seychelles Bird Records Committee has found more bird species on Alphonse that on any island south of the granitics other than Aldabra. Conservation on the group of atolls is managed by Island Conservation Society.

Aldabra Atoll + Islands Aldabra is a raised coral atoll on the western side of the archipelago, closer to Africa, and has been basically untouched by humans including the Persians and Arabs who found the island before the Portuguese arrived in 1511. For this reason, it is truly a natural wonder because it has been virtually unchanged for thousands of years. UNESCO World Heritage Site The atoll is made up of four large islands made of coral that surrounds a shallow lagoon. There is a coral reef that encircles these four islands. The island is not easy to access and for this reason it still has the largest population of giant tortoises in the world. The main importance of this untouched ecosystem is for the study of evolutionary and ecological processes. The extraordinary beauty of this atoll is derived from the diversity of the ocean and land, which join to make a spectacular array of formations and colours. Location and Geology of Nikumaroro - TIGHAR Location and Geology of Nikumaroro From TIGHAR Need to locate Howard's original illustrations, if possible.-- MXM. Description Nikumaroro , formerly known as Gardner Island, is a typical Central Pacific coral atoll.

It is located just south of the equator, due north of Western Samoa and the Tokelau Islands, at 04º 30' S, 174º 30' W (Fig. 1). The atoll is 7 km long by 2.5 km wide, elongated northwest-southeast, having grown on the crest of a submarine volcanic cone with a similar orientation that rises out of 5000m of water. <embed src='I23jungleDM.jpg> Fig. 1: Nikumaroro is an island in the Phoenix Group . It is located just south of the equator, in a line due north from Samoa, via the Tokelau Islands. The island forms part of a volcanic chain that includes Carondelet Reef to the south and a substantial seamount to the north with dimensions similar to Nikumaroro’s. Other islands in the Phoenix group, such as Orona , Manra , McKean , and Kanton , lie on parallel volcanic chains. The atoll is made up of a central lagoon , a surrounding terrestrial rim, an extensive ocean reef flat, and a steep drop-off into the ocean abyss (Fig. 2). Fig.

2: Nikumaroro is an elongated atoll, 7km long and 2.5km wide, formed on the crest of a submarine volcano, rising through 5000m of water. It has a central lagoon served by two passages on the leeward (southeastern) side. The lagoon is relatively shallow, on average no more than 5 metres deep, and is studded with coral heads and patch reefs. Most of the lagoon shore lacks beach; a few short stretches of sand arc around the northwest end. Commonly, heavily vegetated coral shelves drop directly into the lagoon, or into a swampy foreshore of calcareous ooze, a mixture of bird droppings and coral erosion products. Water clarity varies greatly depending on weather conditions. When turbid it is virtually opaque; when calm it can be fairly clear. It is usually a bright aquamarine-turquoise in color. The terrestrial rim is built on Holocene, and perhaps Pleistocene, beachrock ramparts that have provided a solid foundation for the overlying sediment. This sediment mantle has been built up by deposition from multiple overwash events that happen during severe storms.

A mineral exploration survey report in 1978 described the island as "a series of cemented coral rubble platforms." The rim is continuous along the windward, northeastern side, but the continuity of the southwestern, leeward side is interrupted by two passages. The northern passage – Tatiman Passage – is a semi-navigable channel, while the southern passage – Baureke – is a usually non-navigable overflow/overwash channel. On both sides of the island, though more noticeably to windward, there are ridges or berms of coral rubble just behind the beach in many places, the results of storm surges. The highest point of the terrestrial rim, near the northwest end, is elevated only about 6-metres above sea level. The prevailing wind is from the east-northeast, as is the set of the swells (Figure 3). The windward (northeast) shore is a long sandy beach about 20 to 40 meters wide, varying with the tide, behind its. The southwest side of the island, normally in the lee, takes the brunt of periodic serious storms from the southwest and NW. Its shoreline is variable both in width and character.

In some places shelves of coral, often heavily overgrown, drop directly to the reef flat, while in other places there are beaches, but these are for the most part narrower, steeper, and rockier than on the windward side, typically made up of finger-sized coral rubble mixed with sand. There are substantial stretches of sandy beach toward the northwest end, however. Outboard of the terrestrial rim is an extensive reef flat, varying from 200 to 250 metres wide. From the toe of the beach, the reef surface comprises the typical geomorphic succession of features (Fig. 3). The inner section consists partly of discontinuous ridges of beachrock, which presumably date from the early Holocene (5000 - 10,000 y.B.P.). Fig. 3: Generalised geomorphology of Nikumaroro, showing the principal reef, rim and lagoon features. Either in association with the beachrock outcrops, or as a result of its absence, a “boat channel ” runs along the inboard edge of the reef flat. Where it is actually present, the boat channel is often no more than a series of discontinuous channels, troughs and pools (Fig. 4).

The walls of these pools are usually steep-sided, and they are commonly less than 1 metre deep. This zone is on average 130m wide. Fig. 4: Photo of the “boat channel”, which on Nikumaroro, where it is actually present, is a non-navigable series of discontinuous channels, troughs and pools, not usually more than 1-metre deep. They form temporary sediment traps. Seaward of the boat channel is a more-or-less flat coralline pavement that often has a pocked surface and is criss-crossed with fractures and tension joints. The surface is slippery with the growth of the coralline algae Porolithon, which must be washed on a daily basis by seawater to maintain it. The pavement averages 70m in width. The outer edge of the reef comprises the “spur and groove” zone (Fig. 5). The groove consists of a steep walled, narrow canyon, typically several metres deep, that starts at the reef edge, and extends in a direction perpendicular to the reef edge, back towards the shore as a gradually shallowing and tapering slot.

Grooves form in the high-energy surf zone and act as gutters to channel water draining from the reef surface. Grooves are commonly spaced at 20 to 50 metre intervals and can stretch back into the reef pavement up to 50m. Fig. 5: Photo of a groove tapering back into the pavement surface from the reef edge. They act as water chutes funnelling water up onto the reef, as well as drainage channel for water returning from the intervening spur surface back to the ocean. The intervening spurs are tabular to slightly convex rather than rounded as the name implies, but have sufficient profile to allow water to be shed from the reef surface. The swash is channelled back out to the ocean via the grooves. Like the reef flat, the spur surfaces and even the groove walls are covered with slippery Porolithon algae. Large boulders are present on the reef surface in the spur and groove zone. These have been broken off and thrown up by the hydraulic hammering of surges underneath reef edge overhangs. Beyond the reef edge is the steep drop-off zone. The average gradient of the upper submarine slope down into the abyss is about 40º.

However, the surface is far from smooth, with ledges and precipices stepping down that reflect Pleistocene sea level still-stands. There is usually a prominent submarine terrace at -50m, but others occur at -10 and -15m. At the nadir of the last glaciation, sea level stood at -130m. Below this depth, the flanks of the volcano are likely to be covered with a mantle of tumbled material, from sand and gravel to boulders and blocks. The flanks of the volcano are not smooth surfaces but are fluted, with wide-spaced ridges and deep valleys. Transport of sediment into the abyss is down the floors of these valleys. The atoll is subject to semi-diurnal (equal-amplitude, twice daily) tides. There is also a marked spring-neap tidal dichotomy. Vegetation In 1937, Harry Maude commented that "the profuse vegetation on Gardner Island gave it the appearance, from the sea, of possessing several low hills". The same can be said today. Nikumaroro retains significant stands of the indigenous tree called “buka” in I Kiribati (Pisonia grandis).

Bukas are impressive trees, up to 20 meters high with a canopy extending in a radius up to 8 meters or so from the thick, gray-barked trunk. Major buka stands are in the northwest, with a fringe running down the narrow land behind the windward shore. Substantial buka stands have been cleared behind the lee shore and on the southeast end (compare Figures N-3 and N-4 [Based on airphotos, show sequence of clearing]). According to Maude , in 1937 the island supported "some fine groves of 'kanawa' trees (Cordia subcordata). The fine-grained kanawa wood was much sought after for the construction of furniture and containers; it will feature importantly in subsequent chapters. A major grove occupied the area known as "kanawa point", and others may have grown on the northeast side of the lagoon on the land called Taraea. Kanawa seems to have been completely cleared from the island, neither we nor a botanical survey party that visited the island in 1978 noted any specimens. Bukas are today being seriously crowded by introduced coconut palms.

Even where the bukas have not been cleared to make way for coconut plantations, feral coconuts have grown up along the fringes of the buka stands and compete with the indigenous trees for light and nutrient. “Te mao,” (Scaevola sp.) is a major competitor as well, and a major impediment to human travel around the island. Mao presents a tangled jungle of interwoven stalks, each up to about 5 cm. in diameter, making up interwoven bushes up to 3 meters high. When fresh, mao is easily cut with a bush knife, but when dry it defies non-mechanized human attack. Since Nikumaroro experiences periodic dry conditions, most mao bushes are made up of dry stalks masked by a deceptive “skin” of green ones. mao can be virtually impossible to cut through, yielding only to a chain saw. Mao tends to be concentrated along the ocean and lagoon shores, but can extend 50 or more meters inland, so on narrower parts of the island its coverage is virtually continuous. [[RenTe ren, -- Tournefortia argentea, is also common along the shores, though far more scattered than mao.

Te ren is a small tree, on Nikumaroro growing to a height of perhaps six or seven meters. Other plants recorded on the island include a small tree called "non" (Morinda citrifolia), a bush known as kaura (Sida fallax) and a creeper called boi (Portulaca sp.) . Fauna The only mammals on the island today (as far as we know) are rats, which are quite numerous. Feral dogs and cats have been reported; we observed one not-too-long dead cat corpse in 1989, and occasionally have heard what sound like cat cries in the forest, but have never seen a live representative. Dogs were subjected to a deliberate eradication program in the 1980s, which seems to have been successful. Indigenous terrestrial animals fall into three roughly defined practical categories: crabs, birds, and insects/arachnids. Crab varieties include Coconut or Robber Crabs (Birgus latro), other hermit crabs (Coenobitidae), and smaller shore crabs (Gelasimus) .

Large numbers of very small hermit crabs certainly represent juvenile stages of Birgus latro , but there are many larger hermit crabs as well. Common bird species include snowy tern (kiakia in I Kiribati ), Frigate Birds (Boobie, Gannet), and Red-Tailed Tropic Birds . Insects and arachnids include ants, bees, and many web-spinning spiders. A single scorpion was noted in 1997. Offshore the fauna is much more extensive and varied. Fish include Grouper , Trevally , Barracuda , Blacktip Sharks and Whitetip Sharks , Gray Reef Sharks , and a wide variety of other reef and deepwater species. Sea turtles are common, and come ashore in some numbers along the windward side to lay eggs. Small reef sharks are common in the lagoon. Because coconut crabs may have played roles in the fate of Earhart and Noonan, some detail about their character and behavior is in order. Also referred to as robber crabs, Birgus latro is closely related to the hermit crab, and in fact goes through a hermit phase.

Adults are about a meter long from head to tail and weigh about 17 kg. Each has a set of two large pincers that constitute its primary tool for acquiring and preparing food. The food most often commented on is the coconut, Birgus is reputed to be able to tear the husks off nuts, pierce their hard inner shells, and then crack them open to extract the meat, but we can find no published support for this reputation. There is abundant evidence, however, including our own observations, that coconut crabs are omnivores, and readily consume meat. We observed them eating rats, delicately peeling the flesh out of the skin, which was discarded. Whether the rats were carrion or were caught alive is something we cannot say, and on which the literature appears to be silent. Coconut crabs are solitary, though they may be so numerous in some places that they appear to be gregarious. They tend to move along "paths of least resistance" between the shore and interior areas, particularly during the breeding season (roughly December to February). After copulating on land, females make their way to the shore, where they release their larvae into the water.

The larvae go through a planktonic stage and then come ashore as very small crablets. In this phase they begin carrying and hiding in the abandoned shells of univalves. This juvenile shell carrying phase continues until a crab is about two years old, with a carapace about two to three centimeters across. At this point the crab abandons its shell carrying practice. By the age of five years the crab is about ten centimeters across, and they continue to grow, apparently, until death. Coconut crabs have been known to live for over forty years. Coconut crabs dig burrows in which to moult. Burrows may be complicated tunnel systems ending in chambers, usually not over 50 cm. below the surface. Crabs have been observed to remain in their burrows for up to sixteen weeks, shedding the old exoskeleton and hardening the new one. Coconut crabs abound on Nikumaroro .

Niue Islanders who worked on the island in the late 19th century referred to it as Motu Aonga--the land of the coconut crabs, and in late 1937 a party of I Kiribati and British explorers built fires in a ring around its campsite--"as protection against the giant robber crabs, who stalked about in the half-light or hung to the branches staring balefully at us." Beachrock Ramparts Nikmaroro is a apparently a bit different from other atolls in the area on account of its beachrock. These ramparts, apparently formed in the early Holocene (10ky), have given the island a terrestrial rockmass core (above current sea level) that has trapped sediment, either aeolian or overwash. These sediments in turn become more beachrock as Ca-saturated groundwater from the lagoon passes through them, cementing the particles together. When an atoll is able to do this, it seems to acquire a stable core, while others that do not have beachrock, don't (e.g. Carondelet Reef ).

Beachrock cores to atolls would be a fascinating study as they offer a protection to the atoll that is otherwise absent ("rampart" is the name I have given the eroded beachrock outcrops that are above current MHWS). They would be what could protect an atoll from sea level rise - colonise them that do have ramparts, abandon them that don't. Geodata Consulting Ltd. Waiheke Island New Zealand Oceandots Synopsis

Question: What name is given to an island of coral that surrounds a lagoon?
Answer: Atoll