What Lives in an Estuary?
25 September 1998
Estuaries are coastal areas where sea water mixes with fresh water, primarily from rivers. Nutrients carried downstream are trapped in the estuary, creating fertile ground for the growth of a wide variety of diverse plant life. This, in turn, supports the habitats of many different animals. Because estuaries are protected from the wave action of the open sea, they are also perfect nursery grounds for a large variety of important shell and fin fish.
North Carolina has more square miles of estuarine waters than any state on the east coast. Our estuaries vary considerably – from broad, shallow sounds like the Albemarle and the Pamlico, to narrow bodies of water, such as the Currituck Sound. Different water levels, types of basins, tidal patterns, salt levels, temperature, and types of sediment make each estuary unique and define the types of habitats and organisms found there.
The coastal marsh is just one habitat found in North Carolina’s estuarine system.
Coastal marshes are valuable, productive wetlands that are sometimes flooded, depending on the tides. There are four kinds of coastal marsh habitats in our estuaries: low salt marshes, high salt marshes, brackish marshes and freshwater marshes.
The low salt marsh is the foundation of the estuarine food web and as such is relied upon in some way by most living things in the estuary. Salt marsh cordgrass is one of the most important plant species in this habitat. Cordgrass is especially suited to the drastically changing salinities, temperatures and water levels in the low salt marsh. It stabilizes the fine particles of clay and silt that are deposited by the estuary’s slow current by creating a network of roots and stems that hold the soft soil together. Cordgrass also protects the mainland from erosion by slowing down the force from incoming waves and tides.
Cordgrass is important to the animals in the low salt marsh as well. It is one of the most important producers in the estuarine food web. When the plant dies back each fall and is washed into the estuary, its tissues are broken down by bacteria into detritus. Detritus is a primary food source for many aquatic animals.
Several types of snails live among the cordgrass, including marsh periwinkles. They eat algae that grows on its stalks, and they use the stalks as an escape from the rising tide. Ribbed mussels, oysters and fiddler crabs are found in the low salt marsh habitat. Birds and mammals also forage in the low salt marsh. Egrets, herons, willets and osprey are just a few of the birds that rely on the low salt marsh for food. Racoons, meadow mice and even bats venture into the marsh for a meal.
Fewer animals use the high salt marsh for food. High salt marshes are the areas where daily tidal flooding in the low salt marsh has created a build-up of fine sediment. Soil elevation increases and shrubs gradually move into the upland edge of these zones. Salt meadow cordgrass, salt grass, sea ox-eye, black needlerush and glasswort become more common. Marsh elder and wax myrtle are among the shrubs that are prevalent in this area. Insect-eating birds like the red winged blackbird, meadowlark and seaside sparrow nest in the high marsh. Other common animals here include the sand fiddler crab, marsh crab and meadow marsh.