Estuaries are wetlands that form where a river flows into the sea. They may be described as "partially enclosed areas of water, receiving freshwater from rivers and open to the saline waters of the sea". Estuaries are dynamic and complex places, constantly undergoing changes in salinity, temperature and turbidity as a result of fresh and saltwater mixing.
estuarine facts
- Because their sediments are incredibly rich in nutrients, estuaries can support large numbers of animals, especially invertebrates.
- The abundance of invertebrates in mud makes estuaries important habitats for fish.
- Britain's estuaries are of national and international importance for migrant and over-wintering waders and wildfowl.
- Few plants can tolerate estuarine conditions, but eelgrass and algae have been able to colonise successfully and play a vital role in trapping sediment, preventing erosion and providing food.
- Mudflats are those glistening expanses of mud uncovered by receding tides in estuaries, and sheltered bays.
- Saltmarsh often forms naturally as a boundary to estuaries, where salt-tolerant plants can root and give stability to the soft floor.
Mud is inhabited by millions of worms - there can be more than ten million microscopic nematode worms in a square metre of mud!
Other worms, such as the harbour ragworm Hediste diversicolor and lugworm Arenicola marina, are more obvious. ‘Star-like’ markings are visible around the hole from where the rag worm emerges, and those familiar twirled casts are left by the lugworm from its ‘U’ shaped burrowing.
threats
Estuaries, mudflats and saltmarsh can look muddy and dirty, and for this reason development is often seen as an "improvement". As well as large scale development, the UK's estuaries are threatened by ‘piece-meal’, bit-by-bit developments, at an alarming rate.
Organic effluent such as domestic sewage is a serious problem - excessive quantities will deplete oxygen, causing changes to the composition of plants and animals.
Industrial waste, with its large load of heavy metals, can be toxic at low concentrations. Animals that filter food from the water tend to accumulate these. When the organisms are eaten by predators further up the food chain (including humans), the concentration will be magnified.
It is generally accepted that the consequence of global warming will be a rise in sea level which will affect low-lying estuarine areas. If allowed to respond, estuaries would adapt slowly. However. much of the coastline is now "fixed" by man-made coastal defences along the saltmarsh edge. One solution is to actually remove parts of the existing sea defences and let the coast retreat naturally, allowing habitats to re-establish, in "Managed Retreat".
