What makes an estuary

And many marine organisms, including most commercially-important species of fish, depend on estuaries at some point during their development. Because they are biologically productive, estuaries provide ideal areas for migratory birds to rest and refuel during their long journeys. Because many species of fish and wildlife rely on the sheltered waters of estuaries as protected spawning places, estuaries are often called the "nurseries of the sea.

Estuaries have important commercial value and their resources provide economic benefits for tourism, fisheries and recreational activities. The protected coastal waters of estuaries also support important public infrastructure, serving as harbors and ports vital for shipping and transportation. The economy of many coastal areas is based primarily on the natural beauty and bounty of estuaries. When those natural resources are imperiled, so too are the livelihoods of those who live and work in estuarine watersheds.

Over half the U. Estuaries also perform other valuable services. Water draining from uplands carries sediments, nutrients and other pollutants to estuaries. As the water flows through wetlands such as swamps and salt marshes, much of the sediments and pollutants are filtered out.

This filtration process creates cleaner and clearer water, which benefits both people and marine life. Salt marsh grasses and other estuarine plants also help prevent erosion and stabilize shorelines. The wetlands that fringe estuaries perform extremely valuable functions that protect our waterways and communities. Wetlands act as natural buffers between the land and the water, guarding against erosion, flood waters and storm surges.

As water runs off the land, wetlands absorb and filter out pollution before it can make its way into the Bay. Estuaries have important commercial value. Many major shipping ports are located in estuaries.

Estuaries are important for people, too. Estuaries support many popular educational and family activities, including boating, fishing, swimming, jet skiing, photography and bird-watching. Like the Chesapeake, the San Francisco Bay was only filled with water during the last ice age. As glaciers retreated, land around the bay experienced post-glacial rebound —without the massive weight of the glacier on top of it, the land gained elevation. The Pacific Ocean rushed in through the Golden Gate to flood the downwarped valley.

When a lagoon or bay is protected from the ocean by a sandbar or barrier island , it is called a bar-built estuary 3. The Outer Banks, a series of narrow barrier islands in North Carolina and Virginia, create sandy, bar-built estuaries. The Outer Banks protect the region's coast from waves and wind brought by Atlantic Ocean hurricane s.

The islands and sandbars also protect the delicate, brackish ecosystem s created by the outflow of many rivers, such as the Roanoke and Pamlico. For these reasons, engineer s monitor the shifting sandbars of the Outer Banks, and constantly work to maintain them.

Fjord estuaries 4 are a type of estuary created by glaciers. Fjord estuaries occur when glaciers carve out a deep, steep valley. Glaciers retreat and the ocean rushes into fill the narrow, deep depression. Puget Sound is a series of fjord estuaries in the U.

Like fjords found in Alaska and Scandinavia, the fjord estuaries of Puget Sound are very deep, very cold, and very narrow. Unlike many of those fjords, Puget Sound's fjord estuaries also have inflows from local rivers and streams. Many of these streams are seasonal, and fjord estuaries remain mostly salty. Freshwater Estuaries Some estuaries not located near oceans.

These freshwater estuaries are created when a river flows into a freshwater lake. Although freshwater estuaries are not brackish, the chemical composition of lake and river water is distinct. River water is warmer and less dense than lake water. The mixing of the two freshwater systems contributes to lake turnover —the mixing of the waters of a lake.

Freshwater estuaries are not affected by tides, but large bodies of water do experience predictable standing wave s called seiche s. Seiches, sometimes nicknamed sloshes, rhythmically move back and forth across a lake.

The Great Lakes, in the United States and Canada, experience seiches and have many freshwater estuaries. Estuary Ecosystems Many plant and animal species thrive in estuaries. The calm waters provide a safe area for small fish, shellfish, migrating birds and shore animals. The waters are rich in nutrients such as plankton and bacteria. Decomposing plant matter, called detritus , provides food for many species. The estuarine crocodile, for example, is an apex predator of tropical Australian and Southeast Asian estuaries.

The estuarine crocodile is the largest reptile in the world. A specimen caught in the Philippines in measured 6. Like most apex predators, estuarine crocodiles eat almost anything. This means the estuary must support a wide variety of food web s. Estuarine crocodiles do not usually consume producer s—sea grasses, seaweeds, mushrooms, and plankton in the estuary. However, they do prey on consumers in the second trophic level , which rely on these plants and other photosynthetic organisms for food: insects, mollusks, birds, and fruit bats.

Estuarine crocodiles also prey on consumers at the third trophic level, such as boars and snakes and, rarely, people. Estuarine crocodiles are ideally adapted to the brackish water of river estuaries. They can survive equally well in freshwater and saltwater ecosystems.

During the rainy season, estuarine crocodiles live in freshwater rivers and streams. They feed on fish such as barramundi, and terrestrial species such as kangaroos and monkeys.

During the dry season, estuarine crocodiles swim to river mouths and even out to sea. Fish remain the main component of their diet. Some estuarine crocodiles have even been known to attack and consume sharks. Estuarine crocodiles have also adapted to seasonally vanish ing estuaries. The reptiles can go months without eating. Estuarine crocodiles can simply not eat when the estuary shrinks and food becomes scarce. Estuaries and People Estuaries are excellent sites for community living.

They provide freshwater for drinking and hygiene. Access to both rivers and oceans helps the development of trade and communication. In fact, the earliest civilization s in the world developed around estuaries. Ur was a sophisticated urban area , with a population of more than 60, at its height. Its estuary was the most important port on the Persian Gulf. All ships carrying trade goods from places such as India and the Arabian Peninsula had to pass through Ur. The estuary's wetland s and flood plain s provided a rich source of wild game and allowed for the development of irrigation and agriculture.

Today, Ur is an archaeological site well inland from the Persian Gulf coast. The landscape has changed, and the estuary of the Euphrates is more than kilometers miles away. These urban areas have undergone rapid change, and put their estuaries at environmental risk through land reclamation, pollution, and overfishing. Land Reclamation Communities have filled in the edges of estuaries for housing and industry since the times of Ur.