An estuary is a semienclosed area where saltwater and fresh water meet and mix. They are some of the most productive environments on Earth, and thus major cities (such as New York, London, and Tokyo) are located on them. Estuaries can be classified into four groups: Drowned river valley, bar-built, tectonic, and fjords.


Drowned River Valley
Drowned River Valleys (also known as Costal Plain estuaries) were formed at the end of the last Ice Age when the sea level rose, flooding the mouths of rivers that emptied into the ocean. They are the most common type of estuary and can found anywhere a river flows into an ocean. Chesapeake Bay is a drowned river valley. Drowned river valleys receive their fresh water from the river flowing into the ocean.

Bar-Built Estuaries Bar-Built Estuaries are formed when sand bars gradually enclose an area of shoreline and cut off the area from the sea. These are found on the Gulf Coast, the North Carolina coast, and the North Sea cost along Germany and The Netherlands. They receive their fresh water from rainfall.

Tectonic Estuaries
These estuaries are formed when one tectonic plate subsided, separating the other from the ocean. They also receive their fresh water from the rain. This one's hard to explain, but I drew a picture for you:

                                  (  -  -   )
            Sleepy Raincloud --> (    _      )
                                      |  | 
                                   |      |
                                       | |  |  
                                   | |     |
  Ocean Surface                       |   |
 Salt water --->
                   |      North American Plate
   Pacific        ||                ^
    Plate         ||                |
                  ||                |
      |           ||
      |           ||
      V           ||

Fjords are formed when glaciers cut deep valleys along a coast. They're found in Norway, (who didn't see that one coming?), Alaska, and the South island of New Zealand, among other places. They get their fresh water from the melting of the glacier.

Physical Characteristics

Estuaries are prone to large changes in salt concentration due to the rising and falling of the tides. Since seawater is denser than freshwater, the sea water forms a salt wedge along the bottom, making the the surface less salty than the bottom. The Coriolis Effect causes the the water coming in from the river to curve to the right in the Northern Hemisphere and to the left in the Southern.

Estuaries must also deal with the sediment being carried down the river. The substrate (bottom) of most estuaries is mud. Mud (being a mixture of silt and clay) can not easily be flowed through (that was a horrible sentence), and as a result the substrate is usually devoid of Oxygen. The sediment is thus inhabited by anaerobic bacteria (the kind that dont need O2 to have a good time, or live for that matter) that create hydrogen sulfide (H2S) which gives the substrate an odor of Rotten Eggs.


Organisms in estuaries must find ways to cope with the salinity changes. As a result there are few Stenohaline (tolerating a small range of salinity) species than there are Euryhaline (tolerating a wide variety of salinities) species.

Osmosis is the process in which water moves from a high concentration to a low concentration across a membrane. If the organisms don't keep this in check, the will either shrivel up and die, or explode due to the rush of water coming into their small marine bodies. Estuary organisms who deal with salinity come in two varieties: Osmoconformers and Osmoregulators. Osmoconformers deal with salinity by changing their own body fluids to match with the salinity of the surrounding water. Osmoregulators on the other hand, pump excess water out (they pee alot) to maintain their blood salinity.

Organisms must also adapt to the mud. There is nothing to hold on to (mud isnt a very firm thing) so most burrow in, or live in permanent tubes in the mud. Since the mud is low in O2, many organisms have a special hemoglobin (its a protein that binds Oxygen to blood cells) to survive. Some organisms, such as clams, can go days without Oxygen.


Open Water
Open water is the area of an Estuary out in the open water (duh.). These areas have large numbers of fish. Many fishes and shrimps (they're words) use the open water area as a nursury area, making it an important economic area.

Mud flats
The mud flats are the bottom of estuaries that become exposed at low tides. Plant life is not very obvious in these parts, although a few seaweeds survive. The algae are the main primary producers and often undergo large blooms in which they go through a sort of hyper-production mode. Most animal life is of the deposit feeder variety. They go through the sediment and eat the organic matter that they find. At low tide, birds predate on the animals they can find, and at high tide fishes take their place.

Salt Marshes and Mangrove Forests
These areas extend inland from the mud flats. In the temperate and subartic areas, Salt Marshes emerge. In the tropics and subtropics, salt marshes are replaced by Mangrove forests (also called mangals). These areas are subject to the same extremes in saltiness, temperature, and tides as the mud flats. In salt marshes, the vegetation is mostly grasses. In mangrove forests (mangals) the grasses are replaced by mangroves (flowering land plants adapted to dealing with salt water.)

The other two communities are Seagrass Beds and Oyster Reefs. Seagrass beds form when large numbers of seagrasses stabilize the sediments and give shelter to various organisms. Oyster reefs are formed when oysters grow on top of successive generations of oysters, giving shelter to various organisms as well.


  • Marine Biology, 3rd edition. Peter Castro and Michael Huber.
  • A month of Marine Science notes.