A lagoon is a shallow body of water protected from a larger body of water (usually the ocean) by sandbars, barrier islands, or coral reefs. Lagoons are often called estuaries, sounds, bays, or even lakes.

Coastal Lagoons

Lagoons sheltered by sandbars or barrier islands are called coastal lagoons. Coastal lagoons form along coastal plains—flat or gently sloping landscapes. They form in areas with small tidal ranges. Coastal lagoons are created as a shallow basin near the shore gradually erodes, and the ocean seeps 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 wetlands. 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 sounds: Currituck Sound, Albemarle Sound, and Pamlico Sound. These areas are sheltered from storm surges 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. Engineers 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 mouths 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 harbors. 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 marshes and mudflats. 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 altered 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 dredged the lagoon to create a series of islands and canals. 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 subsided—Venice sank. Venice's lower elevation made it increasingly vulnerable to strong seasonal tides from the Adriatic.

Artesian wells 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 lagoons 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 islands. 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 exoskeletons. 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.

lagoon
The tropical lagoons of Bora Bora are protected from harsh ocean waves by atolls and barrier islands.

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.

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.

alter
Verb

to change.

Noun

an underground layer of rock or earth which holds groundwater.

artesian well
Noun

type of confined aquifer that flows to the Earth's surface without the need for pumping.

Noun

a coral reef or string of coral islands that surrounds a lagoon.

atoll lagoon
Noun

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.

Noun

a dip or depression in the surface of the land or ocean floor.

Noun

all the different kinds of living organisms within a given area.

brackish water
Noun

salty water, usually a mixture of seawater and freshwater.

canal
Noun

artificial waterway.

Noun

gradual changes in all the interconnected weather elements on our planet.

coastal erosion
Noun

wearing away of earth or sand on the beach by natural or man-made methods.

coastal lagoon
Noun

shallow body of water between the coast and a series of sandbars or barrier islands.

Noun

low, flat land lying next to the ocean.

coral reef
Noun

rocky ocean features made up of millions of coral skeletons.

dredge
Verb

to remove sand, silt, or other material from the bottom of a body of water.

Noun

community and interactions of living and nonliving things in an area.

engineer
Noun

person who plans the building of things, such as structures (construction engineer) or substances (chemical engineer).

erode
Verb

to wear away.

exoskeleton
Noun

the hard external shell or covering of some animals.

fortify
Verb

to strengthen.

Noun

increase in the average temperature of the Earth's air and oceans.

Noun

part of a body of water deep enough for ships to dock.

hurricane season
Noun

time of year when the risk of hurricanes is greatest. Atlantic hurricane season runs from June 1 to November 30.

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.

leach
Verb

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.

Noun

wetland area usually covered by a shallow layer of seawater or freshwater.

Noun

place where a river empties its water. Usually rivers enter another body of water at their mouths.

mudflat
Noun

coastal wetland formed as rivers or tides deposit sediment.

Outer Banks
Noun

barrier islands off the coast of the U.S. state of North Carolina.

pristine
Adjective

pure or unpolluted.

salinity
Noun

saltiness.

sandbar
Noun

underwater or low-lying mound of sand formed by tides, waves, or currents.

Noun

increase in the average reach of the ocean. The current sea level rise is 1.8 millimeters (.07 inch) per year.

seep
Verb

to slowly flow through a border.

sound
Noun

body of water, larger than a bay, partially surrounded by land.

Noun

abnormal rise in sea level accompanying a hurricane or other intense storm. Also called a storm tide.

subside
Verb

to return to a lower level.

tidal range
Noun

the difference in height between an area's high tide and low tide.

tourism
Noun

the industry (including food, hotels, and entertainment) of traveling for pleasure.

volcanic island
Noun

land formed by a volcano rising from the ocean floor.

Noun

area of land covered by shallow water or saturated by water.