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.