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.
Salt Marsh
Mangrove Forest
Mud Flats
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 Like other coastal communities, estuaries are dramatically influenced by tides. During the day time, when the tide is out, many aquatic creatures retreat into protective pastures. 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.
Salt Marshes Just like marshes further inland, salt marshes are also wetlands. Salt water marshes occur in places where the land meets the sea, such as barrier islands and other coastal areas. They are exposed to water at different times of the day or year.
Sometimes the marsh has little water, sometimes it is dry, and sometimes it is very full of water almost like a pond. Their salinity, or salt content, varies depending upon whether they are located right on the ocean or further upstream in the estuary or sound. The water level and salinity level determine which plants and animals make their homes there.
Salt marshes are extremely productive. Microscopic organisms like bacteria, fungi, and algae make their home in the decaying marsh grasses. These decaying plants and micro-organisms are then eaten by fish, worms, and crustaceans, furthering the cycle of decomposition. The plentiful insects provide food for birds and fish.
The vegetation of the salt marsh also provides shelter from predators, especially for young animals who use the salt marsh as a nursery.
Mud Flats Mud flats (also known as tidal flats) are found next to salt marshes. Twice each day, water flows in and out with the tides, filling or draining the flat.
The mud flat receives nutrients from the tidal flow and the nearby marsh, particularly as it decays. This means that mud flats have a rich plant and animal life.
Phytoplankton and zooplankton are abundant. So are mud snails. Animals like oysters and clams that filter-feed live in mud flats because of the availability of plankton. Fish and crabs move through the flats at high tide. Birds and predatory animals visit tidal flats at specific times for their catch.
About one inch below the surface of the mud, burrowing animals such as clams, worms, and crustaceans can be found. Because there is little or no oxygen, they breathe through tubes to get oxygen from the surface.
Mangrove forests thrive near the mouths of large rivers where river deltas provide lots of sediment (sand and mud). Mangrove roots collect sediments and slow the water's flow, helping to protect the coastline and preventing erosion.
Mangrove Trees A mangrove is a tropical maritime tree or shrub of the genus Rhizophora. Mangroves have special aerial roots and salt-filtering tap roots that enable them to thrive in brackish water (brackish water is salty, but not as salty as sea water).
Mangroves need to keep their trunk and leaves above the water line. Yet they also need to be firmly attached to the ground so they are not moved by waves.
There are three types of mangrove roots that help in this process: 1. Support roots which directly pierce the soil. 2. Level-growing roots which twist upward and downwards, with the upward twists emerging on the water surface. 3. Level-growing roots whose downward twists (sub-roots) appear on the water surface.