Estuaries have unique ecosystems combining aspects of the sea and of rivers. Life has adapted to environments that change with the tides and with changing amounts of rain in the rivers. They are valuable for their own sake, their role in protecting the coast, and their significance to ocean processes.
An estuary has at least one main channel and a number of intertidal mud flats that become covered and uncovered by the tides. One characteristic of estuaries is sharply defined degrees of salinity, which also move with the tides. This is a harsh environment since most organisms adapt to a certain level of salinity and water level. The constantly changing environment of estuaries requires organisms to be able to adapt to these circumstance, and many have. Estuaries are in fact extremely productive and biologically diverse.
There are three main types of estuaries. Salt wedge estuaries have virtually no mixing. Rivers discharge small quantities of sediment and there is little tidal movement. Well mixed estuaries show a great deal of mixing of sea and river water. Partially mixed estuaries show a combination of the two.
Where the tides are small freshwater river organism are found close to the mouth, as are marine organisms. Freshwater lifeforms in particularly cannot usually tolerate any salinity and in well mixed estuaries they do not penetrate very close to where the river meets the sea.
While each estuary is different they generally show a series of similar habitats. Moving from the sea to the upper regions you first have the pelagic zone. This is affected by the river but is in some ways close to a marine environment. Here the dominant life forms are zooplankton, phytoplankton and fish with a few larger predators such as sharks, dolphins and seals making occasional appearances. There are often plenty of nutrients from coastal run off but the turbulence of the water limits the amount of light, and hence the productivity of the phytoplankton.
Then you have the mudflats or sandbars. Here benthic (seabed) life predominates since the flats are not covered by water all the time. The primary producers are benthic algae and further up the food chain you have large numbers of molluscs, crustaceans, worms, flatfish and shorebirds. The visibility and appeal of birds means that these are popular regions for nature lovers. Mudflats also are important to the seafood industries.
Still predominantly under water are seagrass communities, some of which have large stands of seaweeds as well as seagrass. These are filled with invertebrates including snails, bivalves, more crustaceans and worms. Seagrass zones also function as the nurseries for many large fish including sharks and salmon. In some places the grass is grazed by large and often endangered herbivores such as turtles and dugongs.
Further up are saltmarshes with marshgrasses or mangroves and these have a vital role between the sea and the land. They protect the coastline, collect nutrients and are extremely productive ecosystems in their own right. Saltmarshes are visited by terrestrial animals, including mammals and reptiles. Fluctuating temperatures and salinity make it one of the most difficult environments for species to live in and the plants are highly specialised.
Although estuaries are vital ecosystems this hasn’t always been recognised and human activity has impacted them perhaps more than any other marine environment. They trap pollutants from both the rivers (for example the run-off from factories) and from the seas (for example from oil spills).
Saltmarshes have regularly been drained to make way for developments, partly because they were seen as little more than soggy wasteland. It is gradually being recognised that estuaries are some of the most productive natural environments on the planet and play an important role in global ecosystems, although rather too late in many cases.