Estuaries

ESTUARY Severn Estuary Credit - Leigh PrestonESTUARY Severn Estuary Credit - Leigh Preston

An estuary is a transition zone where freshwater from river bodies meets the saline (salty) water from the sea. Freshwater is lighter than saltwater, so in estuaries freshwater tends to float upon the saltwater, and while the saline current edges its way up the estuary it brings nutrients with it depositing them on the estuary bottom.

What are they?

An estuary is a transition zone where freshwater from river bodies meets the saline (salty) water from the sea. Freshwater is lighter than saltwater, so in estuaries freshwater tends to float upon the saltwater, and while the saline current edges its way up the estuary it brings nutrients with it depositing them on the estuary bottom.

The saline concentration reduces with distance from the sea. Plant species are not able to adapt to changes in varying salinity and therefore are only found in areas with a consistent salt concentration.

Life in estuaries is dependent on the tide. Low tide reveals invertebrate rich mudflats, while the high tide brings with it nutrients in the form of zooplankton and oxygen, providing a rich habitat to a great diversity of species.

Where are they found?

Estuaries can be found widely around the coast of the UK - anywhere that rivers meet the sea. However, there are fewer occurrences in Northern Ireland and Western Scotland. There are approximately 90 estuaries in the UK.

The UK has a large proportion of Europe’s estuaries, with a quarter of the estuaries found in north-western Europe.

Why are they important?

The varying salinity and the change in tides creates an assortment of habitats in estuaries with sandbanks forming near the estuary mouth, salt marshes at the furthest reach of the sea and mudflats exposed when the tide recedes. This rich diversity of habitats makes estuaries one of the most productive ecosystems in the world.

The sandbanks provide habitats for nationally important invertebrates, while also being a nesting site for birds.

When the tide is out mudflats are revealed, these are host to a great range of invertebrates burrowed into the mud and this provides a rich feeding site for waterfowl and wading birds.

Salt marshes often provide grazing grounds for cattle and sheep making them economically important. In addition, the grassland provides feeding sites for wading bird species including snipe, redshank, lapwing and curlew.

Estuaries are also important migratory routes for Atlantic salmon, swimming from the sea through the estuary to the freshwater rivers to spawn, and later in life for migration from nursery site to feeding grounds in the Atlantic Sea.

Are they threatened?

The ecosystems within estuaries are under threat from pollution including sewage, agricultural-, industrial- and domestic waste, which can be directly toxic to plant species or affect them by increasing the algae concentration in the water and out-competing established species.

Alteration of estuaries to improve shipping channels disturbs and alters plant communities, which can have a ripple-up affect on other species that are dependent on the plant community, for example for food.

What are The Wildlife Trusts doing to help? What can I do to help?

Visit wildlifetrusts.org/livingseas to find out how you can help our marine conservation work in the UK.

Depending on where you live, local Wildlife Trust volunteers help out with everything from recording marine wildlife sightings to beach cleans and educational work. Visit our Living Seas pages online or contact your local Trust to find out more.