Description and ecological characteristics
Estuaries are habitat complexes which comprise an interdependent mosaic of subtidal and intertidal habitats, which are closely associated with surrounding terrestrial habitats. Many of these habitats, such as 1140 Mudflats and sandflats not covered by sea water at low tide, saltmarshes, 1110 Sandbanks which are slightly covered by sea water all the time and 1170 Reefs, are identified as Annex I habitat types in their own right.
Estuaries are defined as the downstream part of a river valley, subject to the tide and extending from the limit of brackish water. There is a gradient of salinity from freshwater in the river to increasingly marine conditions towards the open sea. The input of sediment from the river, the shelter of the estuary from wave action, and the often low current flows typically lead to the presence of extensive intertidal sediment flats and sediment-filled subtidal channels. There is usually only a limited extent of rocky habitat. In contrast, marine inlets where seawater is not significantly diluted by freshwater are considered as Annex I type 1160 Large shallow inlets and bays.
The structure of estuaries is largely determined by geomorphological and hydrographic factors. There are four main sub-types:
- Coastal plain estuaries. These estuaries have formed where pre-existing valleys were flooded at the end of the last glaciation. They are usually less than 30 m deep, with a large width-to-depth ratio. This is the main sub-type of estuary, by area, in the UK.
- Bar-built estuaries. These characteristically have a sediment bar across their mouths and are partially drowned river valleys that have subsequently been inundated. Bar-built estuaries tend to be small but are widespread around the UK coast.
- Complex estuaries. These have been formed by a variety of physical influences, which include glaciation, river erosion, sea-level change and geological constraints from hard rock outcrops. There are few examples of this sub-type of estuary in the UK.
- Ria estuaries. Rias are drowned river valleys, characteristically found in south-west Britain. The estuarine part of these systems is usually restricted to the upper reaches. The outer parts of these systems are little diluted by freshwater and typically conform to Annex I type 1160 Large shallow inlets and bays.
The intertidal and subtidal sediments of estuaries support biological communities that vary according to the type of sediment and salinity gradients within the estuary, together with geographic location and the strength of tidal streams. The parts of estuaries furthest away from the open sea are usually characterised by soft sediments and the salinity is more strongly influenced by riverine freshwater input. Here the sediment-living animal communities are typically dominated by oligochaete worms, with few other invertebrates. Where rock occurs, there may be communities characteristic of brackish flowing water, consisting of green unicellular algae, sparse fucoid seaweeds, and species of barnacle and hydroid. The silt content of the sediment decreases towards the mouth of the estuary, and the water gradually becomes more saline. Here the animal communities of the sediments are dominated by species such as ragworms, bivalves and sandhopper-like crustaceans. In the outer estuary, closer to the open sea, the substrate is often composed of fine sandy sediment, and supports more marine communities of bivalves, polychaete worms and amphipod crustaceans. Where rock occurs, a range of species more characteristic of the open coast is found. The upper reaches of estuaries often support saltmarsh at the top of the shore, whilst nearer the estuary mouth this may be replaced by sand dune systems.
In addition to the sedentary subtidal and intertidal communities, the water column of estuaries is an important conduit for free-living species, such as fish, and juvenile stages of benthic plants and animals. In particular, it is the means by which migratory fish species make the transition between the marine and freshwater environments.
European status and distribution
Estuaries are widespread throughout the Atlantic coasts of Europe. Approximately one-quarter of the area of estuaries in north-western Europe occurs in the UK.
UK status and distribution
The UK has over 90 estuaries. They are widely distributed around the coast but there are few examples in some areas, such as Northern Ireland and western Scotland.
Click here view UK distribution of this species
Alde, Ore and Butley Estuaries
This estuary, made up of three rivers, is the only bar-built estuary in the UK with a shingle bar. This bar has been extending rapidly along the coast since 1530, pushing the mouth of the estuary progressively south-westwards. The eastwards-running Alde River originally entered the sea at Aldeburgh, but now turns south along the inner side of the Orfordness shingle spit. It is relatively wide and shallow, with extensive intertidal mudflats on both sides of the channel in its upper reaches and saltmarsh accreting along its fringes. The Alde subsequently becomes the south-west flowing River Ore, which is narrower and deeper with stronger currents. The smaller Butley River, which has extensive areas of saltmarsh and a reedbed community bordering intertidal mudflats, flows into the Ore shortly after the latter divides around Havergate Island. The mouth of the River Ore is still moving south as the Orfordness shingle spit continues to grow through longshore drift from the north. There is a range of littoral sediment and rock biotopes (the latter on sea defences) that are of high diversity and species richness for estuaries in eastern England. Water quality is excellent throughout. The area is relatively natural, being largely undeveloped by man and with very limited industrial activity. The estuary contains large areas of shallow water over subtidal sediments, and extensive mudflats and saltmarshes exposed at low water. Its diverse and species-rich intertidal sand and mudflat biotopes grade naturally along many lengths of the shore into vegetated or dynamic shingle habitat, saltmarsh, grassland and reedbed.
Carmarthen Bay and Estuaries/ Bae Caerfyrddin ac Aberoedd
East Wales, Extra-Regio, West Wales and The Valleys
Carmarthen Bay and Estuaries provides an example of a large estuarine site on the south coast of Wales, encompassing the estuaries of the Rivers Loughor, Tâf and Tywi (coastal plain estuaries) and the Gwendraeth (a bar-built estuary). These four estuaries form a single functional unit around the Burry Inlet, with important interchanges of sediment and biota. The estuaries of this site support a range of subtidal and intertidal sediments that grade from sand at the mouth to mudflats in the upper estuary. The fauna of the sediments varies, but includes communities with polychaete and oligochaete worms and areas with extensive cockle beds and other bivalve molluscs. This site has a range of undisturbed transitions to coastal habitats.
Dornoch Firth and Morrich More
Extra-Regio, Highlands and Islands
Dornoch Firth is the most northerly large, complex estuary in the UK. The estuary is fed by the Kyle of Sutherland and is virtually unaffected by industrial development. There is a complete transition from riverine to fully marine conditions and associated communities. Inland, and in sheltered bays, sediments are generally muddy. Gravelly patches occur in the central section of the Firth. Wide sandy beaches dominate the large bays at the mouth of the Firth, and areas of saltmarsh occur around the shores. Sublittoral sediments are predominantly medium sands with a low organic content. Several of the associated coastal habitats have been proposed as Annex I interests in their own right.
Drigg is an example of a small, bar-built estuary on the north-west coast of England. It is fed by three rivers (the Irt, Mite and Esk) which discharge through a mouth that has been narrowed by large sand and shingle spits. The sediments within the estuary are largely muddy within the Rivers Irt and Mite, while those of the Esk are more sandy, particularly towards the mouth. There is a substantial freshwater influence in the upper reaches of all three rivers, with good development of associated animal communities. Within the site are some of the least-disturbed transitions to terrestrial habitats of any estuary found in the UK.
This is a large estuarine site in south-east England, and is a typical, undeveloped, coastal plain estuarine system with associated open coast mudflats and sandbanks. The site comprises the major estuaries of the Colne, Blackwater, Crouch and Roach rivers and is important as an extensive area of contiguous estuarine habitat. Essex Estuaries contains a very wide range of characteristic marine and estuarine sediment communities and some diverse and unusual marine communities in the lower reaches, including rich sponge communities on mixed, tide-swept substrates. Sublittoral areas have a very rich invertebrate fauna, including the reef-building worm Sabellaria spinulosa, the brittlestar Ophiothrix fragilis, crustaceans and ascidians. The site also has large areas of saltmarsh and other important coastal habitats.
Firth of Tay and Eden Estuary
Eastern Scotland, Extra-Regio
The Firth of Tay and the Eden estuary are two high-quality estuarine areas. The two estuaries have been proposed within a single site because they are integral components of a large, geomorphologically complex area that incorporates a mosaic of estuarine and coastal habitats. The Tay is the least-modified of the large east coast estuaries in Scotland, while the Eden estuary represents a smaller ‘pocket’ estuary. The inner parts of the estuaries are largely sheltered from wave action, while outer areas, particularly of the Tay, are exposed to strong tidal streams, giving rise to a complex pattern of erosion and deposition of the sandbank feature at the firths’ mouth. The sediments within the site support biotopes that reflect the gradients of exposure and salinity, and are typical of estuaries on the east coast of the UK. The abundance, distribution and composition of the associated plant and animal communities are ecologically representative of northern North Sea estuaries.
East Yorkshire and Northern Lincolnshire, Extra-Regio, Lincolnshire
The Humber is the second-largest coastal plain estuary in the UK, and the largest coastal plain estuary on the east coast of Britain. It is a muddy, macro-tidal estuary, fed by the Rivers Ouse, Trent and Hull, Ancholme and Graveney. Suspended sediment concentrations are high, and are derived from a variety of sources, including marine sediments and eroding boulder clay along the Holderness coast. This is the northernmost of the English east coast estuaries whose structure and function is intimately linked with soft eroding shorelines. Habitats within the Humber Estuary include 1330 Atlantic salt meadows and a range of sand dune types in the outer estuary, together with subtidal sandbanks (H1110 Sandbanks which are slightly covered by sea water all the time), extensive intertidal mudflats (H1140 Mudflats and sandflats not covered by seawater at low tide), glasswort beds (H1310 Salicornia and other annuals colonising mud and sand), and 1150 coastal lagoons. As salinity declines upstream, reedbeds and brackish saltmarsh communities fringe the estuary. These are best-represented at the confluence of the Rivers Ouse and Trent at Blacktoft Sands. Upstream from the Humber Bridge, the navigation channel undergoes major shifts from north to south banks, for reasons that have yet to be fully explained. This section of the estuary is also noteworthy for extensive mud and sand bars, which in places form semi-permanent islands. Significant fish species include 1099 river lamprey Lampetra fluviatilis and 1095 sea lamprey Petromyzon marinus which breed in the River Derwent, a tributary of the River Ouse.
Cumbria, Extra-Regio, Lancashire
Morecambe Bay in north-west England is the confluence of four principal estuaries, the Leven, Kent, Lune and Wyre (the latter lies just outside the site boundary), together with other smaller examples such as the Keer. Collectively these form the largest single area of continuous intertidal mudflats and sandflats in the UK and the best example of muddy sandflats on the west coast. The estuaries are macro-tidal with a spring tidal range of 9 m. The significant tidal prisms of the estuaries result in the Bay being riven by large low-water channel systems. The Kent, Leven and Lune estuaries have been modified variously by railway embankments, flood embankments and training walls but support extensive intertidal areas. Although cobble ‘skears’ and shingle beaches occur at their mouths, the estuaries consist predominantly of fine sands and muddy sands. The estuaries support dense invertebrate communities, their composition reflecting the salinity and sediment regimes within each estuary. Extensive saltmarshes and glasswort Salicornia spp. beds are present in the Lune estuary, contrasting with the fringing saltmarshes and more open intertidal flats of the Leven and Kent estuaries. Most of the saltmarshes are grazed, a characteristic feature of north-west England. In the upper levels of the saltmarshes there are still important transitions from saltmarsh to freshwater and grassland vegetation. Water quality is generally good.
Pembrokeshire Marine/ Sir Benfro Forol
Extra-Regio, West Wales and The Valleys
Pembrokeshire Marine includes the Daugleddau estuary, a ria estuary in south-west Wales, formed in the upper reaches of one of the best examples of a ria in the UK. Associated with the wide range of environmental conditions, particularly seabed substrates, tidal streams and salinity gradients, there is a wide diversity of communities and species. The species-richness of sediment communities throughout Milford Haven and the Daugleddau is high. Tide-swept sponge communities on shell/cobble substrates and bedrock in the upper reaches of the Daugleddau are exceptional in their diversity. The site also includes smaller estuaries entering the Daugleddau and Milford Haven, and wide intertidal mudflats with rich and productive invertebrate annelid and mollusc communities, occurring in ‘pills’ (creeks).
Pen Llŷn a'r Sarnau/ Lleyn Peninsula and the Sarnau
East Wales, Extra-Regio, West Wales and The Valleys
Pen Llyn a’r Sarnau has representative examples of bar-built estuaries in north-west Wales, and includes the Glaslyn/Dwyryd, Mawddach and Dyfi estuaries. There is a continuous gradient between the clean sands near the entrance to the sea and the mud or muddy sands in the sheltered extremes of the estuaries. The intertidal sandflats support communities of burrowing invertebrates, including dense populations of polychaete worms, crustaceans, bivalve molluscs and gastropod molluscs. Saltmarsh fringing the shores of the estuaries, and the saltmarsh creeks and pools, are important habitat features for juvenile fish.
Plymouth Sound and Estuaries
Cornwall and Isles of Scilly, Devon, Extra-Regio
Plymouth Sound and Estuaries is representative of ria estuaries in south-west England. The Rivers Tamar and Lynher are linked at their mouths. The upper parts of the Tamar and Lynher include a very well-developed estuarine salinity gradient. As a consequence, they exhibit one of the finest examples in the UK of changing estuarine communities with changing salinity regime. Rocky reefs in low salinity estuarine conditions far inland on the Tamar are very unusual and support species such as the hydroid Cordylophora caspia. The Tamar is one of few estuaries where zonation of rocky habitats (intertidal and subtidal) can be observed along an estuarine gradient.
Severn Estuary/ Môr Hafren
Dorset and Somerset, East Wales, Extra-Regio, Gloucestershire, Wiltshire and Bristol/Bath area
Habitat occurrence description not yet available.
Extra-Regio, Hampshire and Isle of Wight, Surrey, East and West Sussex
The Solent encompasses a major estuarine system on the south coast of England with four coastal plain estuaries (Yar, Medina, King’s Quay Shore, Hamble) and four bar-built estuaries (Newtown Harbour, Beaulieu, Langstone Harbour, Chichester Harbour). The site is the only one in the series to contain more than one physiographic sub-type of estuary and is the only cluster site. The Solent and its inlets are unique in Britain and Europe for their hydrographic regime of four tides each day, and for the complexity of the marine and estuarine habitats present within the area. Sediment habitats within the estuaries include extensive estuarine flats, often with intertidal areas supporting eelgrass Zostera spp. and green algae, sand and shingle spits, and natural shoreline transitions. The mudflats range from low and variable salinity in the upper reaches of the estuaries to very sheltered almost fully marine muds in Chichester and Langstone Harbours. Unusual features include the presence of very rare sponges in the Yar estuary and a sandy ‘reef’ of the polychaete Sabellaria spinulosa on the steep eastern side of the entrance to Chichester Harbour.
Cumbria, Extra-Regio, South Western Scotland
The Solway is a large, complex estuary on the west coast of Britain. It is one of the least-industrialised and most natural large estuaries in Europe. Tidal streams in the estuary are moderately strong and levels of wave energy can be high. There is considerable seasonal fluctuation in water temperature, owing to the shallow nature of the estuary. The sediment habitats present, mainly dynamic sandflats and subtidal sediment banks, are separated by six main river channels, which are continually changing their patterns of erosion and accretion. The sublittoral sediment communities are typically sparse in the inner estuary, owing to the mobility of the sediment coupled with low and variable salinity. Communities become richer towards the outer estuary, where there are less extreme environmental conditions and more varied substrates. The dominant species of bivalve molluscs, polychaete worms, crustaceans and echinoderms vary, depending on location within the estuary.
Northumberland and Tyne and Wear
The Tweed Estuary is a complex estuary, which discharges into the North Sea. It is a long narrow estuary, which is still largely natural and undisturbed, with its water quality classified as excellent throughout. It supports a wide range of habitats compared with other estuaries in north-east England. At its mouth there are substantial sandbanks and some areas of rocky shore. Further upstream, large areas of estuarine boulders and cobbles overlie sediment flats and extend into subtidal areas of the channel. Sheltered estuarine mud and sandflats occur away from the fast-flowing river channel. A wide range of littoral sediments occurs within the estuary. These range from exposed east-facing sandy shores at the estuary mouth, including its sheltering sand-spit, to muddy gravels where the river is actively eroding the banks. The most exposed sandy shores are subject both to wave action and, in places, the scouring action of the outflowing river; their mobile infauna (crustaceans and a few polychaetes) and ephemeral algae reflect these conditions. Species and habitat diversity rises with increasing shelter, until increasingly low-salinity estuarine conditions upstream lead to naturally low infaunal diversity, dominated by characteristic species that are tolerant of brackish-water conditions. Fish species include the rare anadromous 1102 allis shad Alosa alosa, which runs in the estuary, migratory 1106 Atlantic salmon Salmo salar, and occasional records of 1099 river lamprey Lampetra fluviatilis and 1095 sea lamprey Petromyzon marinus.
SACs where this Annex I habitat is a qualifying feature, but not a primary reason for site selection
- Dee Estuary/ Aber Dyfrdwy Cheshire, East Wales, Extra-Regio, Merseyside, West Wales and The Valleys
- Fal and Helford Cornwall and Isles of Scilly, Extra-Regio
- Glannau Môn: Cors heli / Anglesey Coast: Saltmarsh West Wales and The Valleys
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