Estuaries and Coastal Wetlands where Freshwater and Saltwater Mix

Coastal zones comprise a variety of life zones. Coastal wetlands provide habitat for a variety of plant and animal species. Two typical life zones found in a coastal zone include an estuary and a coastal wetland. Estuaries are enclosed areas of coastal water where ocean water mixes with freshwater from inland rivers and streams, forming brackish water. Coastal wetlands include large extensions of land from which water drains directly into the oceans, and are the life zones for varied plant and animal species. They also serve as popular recreation areas. Wetlands help maintain the quality of coastal water by filtering and settling out pollutants and nutrients and help protect coastal land from erosion and flooding.


An estuary is a partially enclosed body of water typically found where rivers and streams meet the coastal sea. Estuaries form a transition region between the ocean and river environments. The mixture of both salt water and fresh water (brackish water) provide high levels of nutrients, making estuaries one of the most productive ecosystems in the world. Estuaries contribute to maintain healthy ocean environments by filtering out sediments and pollutants flowing along rivers and streams. The salinity of water varies among estuaries and is subject to change depending on weather, tides and other factors. Many animal species rely on them for food and as places to nest and breed.

Estuaries serve as transition areas that connect the ocean and land habitats. They are subject to both marine influences, including  waves, tides and the inflow of saline water; and river influences, including fresh water flows and sedimentation. The daily tides greatly influence the estuary environment and are dependent upon factors, including coastland shape, local winds, water depth, and water flow restrictions. In addition to affect the abundance and distribution of plants and animals, the daily tides affect all types of commercial and recreational transportation, as well.

Estuaries provide nesting and feeding habitats for a variety of aquatic plants and animals. Most fish and shellfish consumed in the United States, including herring, salmon, and oysters stem from estuaries. Animal species, including birds, fish, amphibians, insects, among others rely on estuaries to live, feed and reproduce. Oysters reside permanently in estuaries, horseshoe crabs complete part of their cycle in estuaries and migratory birds use them as stopovers. Fish, such as the Atlantic menhaden, the American shad and striped bass, which live most of their times in the ocean, return to estuaries to spawn.


More than estuaries and salt marshes, wetlands include all watersheds and the regions of land from which water drains directly into the oceans. Wetlands are areas saturated with water either permanently or seasonally. Wetlands can be of four kinds, including marshes, swamps, fens and bogs. Wetlands play a major role in water purification, flood control and shoreline stability. Wetlands are considered as the most biologically diverse of all ecosystems. Wetlands provide nurseries, shelter, spawning grounds, and food for a wide variety of fish and bird species, among other wildlife.

Wetlands occur naturally in every continent of the world except Antarctica. The largest wetlands are found in the Amazon River basin and the West Siberian Lowland. Another large wetland is the Pantanal, comprising Bolivia, Brazil, and Paraguay in South America. Coastal wetlands aid at the maintenance of coastal water quality by filtering out pollutants, such as pesticides. Wetlands are particularly important because they help protect coastal land from flooding and damage caused by storms. Wetlands unique soil (hydric soil) supports aquatic plants, including cattails, water-lilies and sedges. The water found in wetlands can be salt water fresh water or brackish water.

In the United States, more than half of commercially harvested fish stems from coastal wetlands. The abundance of commercially harvested shrimp, oysters, blue crab, among other species is directly related to the quality of wetlands. Wetlands also provide feeding and breeding habitat for approximately 85% of freshwater birds and other migratory birds. Nearly 50% of the nation’s threatened and endangered species rely on coastal habitats for survival.  Wetlands help improve the quality of water by filtering and detoxifying residential and agricultural wastes. They also protect coastal areas against storm and wave damage.

According to NOAA Coastal Services Center, wetlands function as natural sponges that trap and slowly release water, lowering flood heights due to storms. Energy produced by waves that otherwise produce inland damage is absorbed by coastal wetlands. Wetlands reduce pollution by storing and filtering urban runoff, as well as retaining or removing nutrients and sediment, thus reducing the costs of building, operating and maintaining water treatment plants. Wetlands contribute to a wide diversity of wild life and provide a suitable means for breeding, nesting and migration stopovers. In the U.S., wetlands support the life cycle of 75% of commercially harvested fish and shellfish and up to 90% of recreational fish catch.