Estuaries - Where Rivers Meet the Sea

An estuary is a partially enclosed body of water where incoming seawater is mixed with fresh water coming from the land. Examples of estuaries include bays, sounds, salt marshes, mangrove forests, mud flats, swamps, inlets, and sloughs.

Estuaries provide a transition from fresh water to salt water. A small disturbance in the habitat can have serious repercussions. Because of the difference in density between fresh and salt water, salt water will move into the estuary along the bottom, while fresh water will flow downstream to the ocean along the surface. This causes a layered condition.

Life in an Estuary
Life in an estuary is an interesting and diverse mix of land and sea creatures, and some animals. Mammals, birds, fish, reptiles, shellfish, and plants all interact in this ecosystem to create some very complex food webs. Birds are very common in estuaries because of the abundance of fish, worms, crabs and clams. The feeding is easy in an estuary. Inside the soil, sediment sand and mud are lots of microscopic bacteria. These lower level creatures thrive because of the plentiful decaying plant matter. Plants thrive because of the nutrient rich soil and the available water.

Many marine organisms depend on estuaries at some point during their development. Some fish only use estuaries at certain times of the year, while others use the natural protection for the laying of eggs. Most commercially valuable fish and shellfish spawn, nurse, or feed in estuaries.

Influence of Tides
Tidal StreamLike other coastal communities, estuaries are dramatically influenced by tides. During the day time, when the tide is out, many aquatic creatures retreat into protective postures. Clams can close their shells, worms stay underground, while other creatures sleep. The change in temperature, the exposure to air and the vulnerability of being active during daytime are all reasons why some creatures are only active at night. Of course, some animals, like birds, are active during the low tide daytime because the supply of food is easier to get to.

At night when the tide returns the estuary comes alive. The returning sea water floods and submerges creeks, salt marshes, mud flats, mangroves, and estuaries. The water brings protection from predators. Many estuary creatures become active only at night.

A Water Filtration System
Rivers often contain lots of sediment, nutrients and pollutants. Estuaries remove sediments and nutrients before they reach the ocean. Otherwise, valuable top soil and nutrients would be flushed into the open seas where they could not be used again.

Flood & Storm Control
Estuaries act as natural buffers between the land and the ocean. Porous, resilient salt marsh soils and grasses absorb flood waters and dissipate storm surges. Like barrier islands, they protect the mainland and people from the brunt of heavy storms. Estuaries help protect human lives, upland animals, and real estate.

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