1150 Coastal lagoons
Description and ecological characteristics
Coastal lagoons are areas of shallow, coastal salt water, wholly or partially separated from the sea by sandbanks, shingle or, less frequently, rocks. Lagoons show a wide range of geographical and ecological variation; five main sub-types have been identified in the UK, on the basis of their physiography, as meeting the definition of the Annex I habitat type.
- Isolated lagoons. These are separated completely from the sea or estuary by a barrier of rock or sediment. Seawater enters by limited groundwater seepage or by over-topping of the sea barrier. Salinity is variable but often low. Isolated lagoons are often transient features with a limited life-span due to natural processes of infilling and coastal erosion.
- Percolation lagoons. These are normally separated from the sea by shingle banks. Seawater enters by percolating through the shingle or occasionally by over-topping the bank (e.g. in storms). The water level shows some variation with tidal changes, and salinity may vary. Since percolation lagoons are normally formed by natural processes of sediment transport, they are relatively transient features, which may be eroded and swept away over a period of years or decades or may become infilled by movement of the shingle bank.
- Silled lagoons. Water in silled lagoons is retained at all states of the tide by a barrier of rock (the ‘sill’). There is usually little tidal rise-and-fall. Seawater input is regular (i.e. on most tides) and although salinity may be seasonally variable, it is usually high, except where the level of the sill is near to high tide level. These lagoons are restricted to the north and west of Scotland and may occur as sedimentary basins or in bedrock (where they are called ‘oban’). Muddy areas are dominated by filamentous green algae, amongst which may be colonies of rare charophytes, such as foxtail stonewort Lamprothamnium papulosum. There may be beds of tasselweed Ruppia spp. and, in the deeper most stable lagoons, eelgrass Zostera marina.
- Sluiced lagoons. Sluiced lagoons are formed where the natural movement of water between the lagoon and the sea is modified by artificial structures, such as a culvert under a road or valved sluices. Communities present in sluiced lagoons vary according to the type of substrate and salinity, but may resemble those of silled lagoons.
- Lagoonal inlets. Seawater enters lagoonal inlets on each tide and salinity is usually high, particularly at the seaward part of the inlet. Larger examples of this sub-type may have a number of different basins, separated by sills, and demonstrate a complete gradient from full salinity through brackish to freshwater. This salinity gradient significantly increases the habitat and species diversity of the sites in which it occurs.
The water in lagoons can vary in salinity from brackish (owing to dilution of seawater by freshwater) to hypersaline (i.e. more salty than seawater as a result of evaporation). The plant and animal communities of lagoons vary according to the physical characteristics and salinity regime of the lagoon, and consequently there are significant differences between sites. Although, compared to other marine habitats, there is usually only a limited range of species present, they are especially adapted to the varying salinity regimes of lagoons and some are unique to lagoon habitats. The vegetation may include beds of eelgrass Zostera spp., tasselweed Ruppia spp., and pondweeds Potamogeton spp., or stoneworts such as foxtail stonewort Lamprothamnium papulosum. In more rocky lagoons, communities of fucoid wracks Fucus spp., sugar kelp Laminaria saccharina, and red and green algae are also found. The fauna is often characterised by mysid shrimps and other small crustaceans, worms that burrow into the sediment, gastropod molluscs, and some fish species. Species that are particularly found in lagoons and consequently have restricted distributions in the UK include starlet sea anemone Nematostella vectensis, lagoon sandworm Armandia cirrhosa, lagoon sand-shrimp Gammarus insensibilis and foxtail stonewort L. papulosum.
European status and distribution
Coastal lagoons are a scarce habitat in the EU and have a restricted distribution on the Atlantic coast.
UK status and distribution
Coastal lagoons are a relatively uncommon habitat in the UK. Some of the lagoon sub-types have a very restricted distribution, e.g. silled lagoons are found mainly in the Outer Hebrides (where they provide the highest density of lagoonal habitats in the UK), and soft-sediment lagoons occur mainly on the east coast of England.
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Bae Cemlyn/ Cemlyn Bay
West Wales and The Valleys
Cemlyn lagoon on the north coast of Anglesey, north Wales, is considered to be the best example of a saline coastal lagoon in Wales. The lagoon is separated from the sea by a shingle bank with a narrow channel at the western end, across which a sluice system was built in the 1930s. Seawater exchange occurs mainly through the sluice and by percolation through the shingle bank, although in extreme storms coinciding with spring tides waves break over the top of the shingle bank. Cemlyn lagoon supports a relatively diverse set of species, several of which are specific to lagoons, including the bryozoan Conopeum seurati, the lagoon cockle Cerastoderma glaucum and the lagoonal mud-snail Ventrosia ventrosa. Cemlyn lagoon is also the only site in Wales where the lagoonal isopod Idotea chelipes has been recorded. A number of uncommon plant species are found within the lagoon, including the brackish water-crowfoot Ranunculus baudotii and beaked tasselweed Ruppia maritima.
Benacre to Easton Bavents Lagoons
Benacre to Easton Bavents Lagoons is a series of percolation lagoons on the east coast of England. The lagoons (the Denes, Benacre Broad, Covehithe Broad and Easton Broad) have formed behind shingle barriers and are a feature of a geomorphologically dynamic system. Sea water enters the lagoons by percolation through the barriers, or by overtopping them during storms and high spring tides. The lagoons show a wide range of salinities, from nearly fully saline in South Pool, the Denes, to extremely low salinity at Easton Broad. This range of salinity has resulted in a series of lagoonal vegetation types, including beds of narrow-leaved eelgrass Zostera angustifolia in fully saline or hypersaline conditions, beds of spiral tasselweed Ruppia cirrhosa in brackish water, and dense beds of common reed Phragmites australis in freshwater. The site supports a number of specialist lagoonal species.
Chesil and the Fleet
Dorset and Somerset
The Fleet, on the south coast of England, is the largest example of a lagoonal habitat in England and has features of both lagoonal inlets and percolation lagoons. It is bordered by the fossil shingle barrier beach structure of Chesil Beach, through which sea water percolates into the lagoon, but most of its water exchange occurs through the narrow channel that links it to Portland Harbour. A low freshwater input produces fully saline conditions throughout most of the Fleet, with reduced salinity occurring only in the west. The lagoon is extremely sheltered from wave action and has weak tidal streams, except in the eastern narrows and entrance channel. The tidal range is much smaller and temperature range far greater than on the open coast. The lagoon supports extensive populations of two species of eelgrass Zostera and three species of tasselweed Ruppia, including the rare spiral tasselweed R. cirrhosa, and a diverse fauna that includes a number of nationally rare and scarce species.
Loch nam Madadh
Extra-Regio, Highlands and Islands
Rock-bound silled lagoons in Europe are virtually restricted to the Outer Hebrides in Scotland, where they are known as oban. Loch nam Madadh (Loch Maddy) is one of two sites selected in North Uist to reflect this distribution. Loch nam Madadh lagoons form the most extensive and diverse saline lagoon system in the UK. The only comparable site is Obain Loch Euphoirt (Loch Eport), which has also been selected. There are 14 lagoons in the Loch nam Madadh complex. These connect with the extensive system of freshwater lochs and lochans in the North Uist hinterland and the fjardic sea loch of Loch nam Madadh itself. There is a wide range of types, from large, complex lagoons with several sills and basins to small, shallow single lagoons. Together they encompass the full transition from freshwater to marine conditions, through a series of basins and sills, and have an exceptionally wide range of habitat types with associated characteristic communities. Most have one or more basins floored with soft, peaty mud, and there are usually boulders and cobbles around the margins. At the entrances to some of the lagoons there are rock and boulder waterfalls. Others have rock and coarse sediment rapids that flood at all states of the tide, and yet others have a percolation barrier. Within the basins of Loch an Dùin and Loch an Strumore there are beds of dwarf eelgrass Zostera noltei, small patches of eelgrass Z. marina, large quantities of the scarce green alga Cladophora battersii, and the scarce foxtail stonewort Lamprothamnion papulosum. Other lagoons in the complex have tidal rapids with kelp Laminaria spp. and sea-oak Halidrys siliquosa, which supports extensive epiphytic growths of sponges, anemones and ascidians. Others have intertidal rapids dominated by fucoid algae, and in these lagoons, the fan-worm Sabella pavonina is found, associated with areas of coarse sediment. Shallow peaty mud in the basins has large numbers of the burrowing amphipod Corophium volutator and lugworm Arenicola marina, and beds of mussels Mytilus edulis and the green alga Codium sp. are found on hard substrates near the entrance channels.
Loch of Stenness
Highlands and Islands
Loch of Stenness has a single basin characteristic of Coastal lagoons in Orkney, and has features of both silled lagoons and lagoonal inlets. It is the largest brackish lagoon in the UK and is of particular importance on account of its large size, stability, reduced salinity regime and northern location. There is a salinity gradient in the lagoon, and communities representing sheltered marine, brackish and freshwater conditions are found. Loch of Stenness is predominantly sedimentary: the lagoon basin is floored by soft mud, while round the shoreline muddy sediments with sand and gravel grade into pebbles, cobbles and boulders. The soft sublittoral mud supports mats of filamentous green algae and large numbers of burrowing worms, and the bivalve Mya arenaria and the snail Hydrobia ulvae may be dominant. Littoral boulders are dominated by filamentous green algae or fucoid algae, with Fucus ceranoides, characteristic of brackish conditions, abundant in places. Submerged boulders in more saline areas of the lagoon support clumps of mussels Mytilus edulis, the brown alga Fucus serratus and species of foliose red and filamentous green algae. In the inner parts of the loch, where salinity is reduced, the cobbles and gravel support stands of beaked tasselweed Ruppia maritima, F. ceranoides and filamentous green algae. Extensive stands of pondweed Potamogeton spp. are present in areas of particularly low salinity.
Loch Roag Lagoons
Extra-Regio, Highlands and Islands
Loch Roag Lagoons is a complex of silled lagoons illustrating the range of variation from freshwater to marine conditions on the Atlantic coast of the Hebrides. Tòb Valasay has a complex salinity regime, determined by the balance between fully marine water introduced over the sill and freshwater from run-off and small inlet streams. Its basins contain a diverse range of habitats, including rocky outcrops, boulders and muddy sand, with softer mud in the eastern basin, and boulders, cobbles and shell-gravel in the narrows. A range of communities are present, including beds of eelgrass Zostera spp. and tasselweed Ruppia spp., turfs of marine algae and stands of large brown algae. Loch Shader is a smaller lagoon that is mainly brackish in character. It has soft, sheltered mud and sand sediments, with some boulders and cobbles on the shore and in shallow water. These substrates support a characteristic range of species. The narrows consists of a bedrock and boulder sill, supporting a more diverse community with a variety of species, including kelp Laminaria spp., anemones and sponges.
North Norfolk Coast
Note: not a marine feature as occur landward of Highest Astronomical Tide This site encompasses a number of small percolation lagoons on the east coast of England; together with Orfordness - Shingle Street and Benacre to Easton Bavents, it forms a significant part of the percolation lagoon resource concentrated in this part of the UK. The most notable of the lagoons at this site are Blakeney Spit Pools, a lagoon system of six small pools between a shingle ridge and saltmarsh. The bottom of each pool is shingle overlain by soft mud. The fauna of the lagoons includes a nationally rare species, the lagoonal mysid shrimp Paramysis nouveli.
Obain Loch Euphoirt
Highlands and Islands
Rock-bound silled lagoons in Europe are virtually restricted to the Outer Hebrides in Scotland, where they are known as oban. Obain Loch Euphoirt (Loch Eport) is one of two sites selected in North Uist to reflect this centre of distribution. Obain Loch Euphoirt lagoons forms one of the most extensive and diverse systems of Coastal lagoons in the UK. The only comparable site is Loch nam Madadh, which has also been selected. The site is a complex of four individual lagoons, which together support the complete range of physical conditions and communities characteristic of this part of Scotland. Loch Obisary is a silled lagoon and is unique amongst the brackish basins of the UK on account of its size, depth (over 40 m), permanent hydrographic stratification and the range of communities it supports. There is a zonation of species within the loch. Several algal species are restricted to within 200 m of the sea water entrance channel. Some, such as Phyllophora pseudoceranoides, are abundant on shallow rock close to the inlet and dominate deeper water throughout the loch. This deeper rock also supports marine ascidians, and there are beds of the freshwater species fennel pondweed Potamogeton pectinatus in shallow water with a greater freshwater influence. Oban nam Fiadh is an extensive system of sluiced lagoons consisting of a series of basins separated by channels and sills with a full transition from marine to freshwater conditions. There is a transition from plant communities typical of fresh or brackish waters, occurring in the inner basin, through distinctive brackish communities including the nationally scarce foxtail stonewort Lamprothamnium papulosum, to marine communities with fucoid algae at the entrance to Loch Euphoirt. Beds of eelgrass Zostera spp. and tasselweed Ruppia spp. occur, supporting the opisthobranch mollusc Akera bullata. The smaller Oban Sponish is a silled lagoon separated from Loch Euphoirt by a boulder sill. There are two basins. Of particular note in the inner basin is a brackish/marine community of a dense bed of eelgrass Zostera marina and tasselweed Ruppia spp. supporting the opisthobranch mollusc Akera bullata. The outer basin supports communities more typical of those found in very sheltered marine conditions. A fourth lagoon in this site is the small brackish silled lagoon, Baigh Uaine, west of Loch Euphoirt. It has a mud floor that supports a dense bed of tasselweed Ruppia spp. The rocky intertidal zone and the sill support characteristic marine fucoid algae.
Orfordness - Shingle Street
Note: not a marine feature as occur landward of Highest Astronomical Tide Orfordness – Shingle Street encompasses a series of percolation lagoons on the east coast of England, and, together with Benacre to Easton Bavents and The Wash and North Norfolk Coast, forms a significant part of the percolation lagoon resource concentrated in this part of the UK. The lagoons at this site have developed in the shingle bank adjacent to the shore at the mouth of the Ore estuary. The salinity of the lagoons is maintained by percolation through the shingle, although at high tides sea water can overtop the shingle bank. The fauna of these lagoons includes typical lagoon species, such as the cockle Cerastoderma glaucum, the ostracod Cyprideis torosa and the gastropods Littorina saxatilis tenebrosa and Hydrobia ventrosa. The nationally rare starlet sea anemone Nematostella vectensis is also found at the site.
Pen Llŷn a'r Sarnau/ Lleyn Peninsula and the Sarnau
East Wales, Extra-Regio, West Wales and The Valleys
Morfa Gwyllt lagoon is a small percolation lagoon that consists of a depression in a shingle bar across the mouth of the Afon Dysynni in mid Wales. This is the only example of a percolation lagoon in Wales. The substrate is a mosaic of medium sand over/amongst shingle, with muddier patches within the deeper pockets, and scattered larger pebbles. Three lagoonal specialists have been found at this site: the amphipod Sphaeroma hookeri, the bryozoan Conopeum seurati and the alga Chaetomorpha linum.
Solent and Isle of Wight Lagoons
Hampshire and Isle of Wight
The Solent on the south coast of England encompasses a series of Coastal lagoons, including percolation, isolated and sluiced lagoons. The site includes a number of lagoons in the marshes in the Keyhaven – Pennington area, at Farlington Marshes in Chichester Harbour, behind the sea-wall at Bembridge Harbour and at Gilkicker, near Gosport. The lagoons show a range of salinities and substrates, ranging from soft mud to muddy sand with a high proportion of shingle, which support a diverse fauna including large populations of three notable species: the nationally rare foxtail stonewort Lamprothamnium papulosum, the nationally scarce lagoon sand shrimp Gammarus insensibilis, and the nationally scarce starlet sea anemone Nematostella vectensis. The lagoons in Keyhaven – Pennington Marshes are part of a network of ditches and ponds within the saltmarsh behind a sea-wall. Farlington Marshes is an isolated lagoon in marsh pasture that, although separated from the sea by a sea-wall, receives sea water during spring tides. The lagoon holds a well-developed low-medium salinity insect-dominated fauna. Gilkicker Lagoon is a sluiced lagoon with marked seasonal salinity fluctuation and supports a high species diversity. The lagoons at Bembridge Harbour have formed in a depression behind the sea-wall and sea water enters by percolation. Species diversity in these lagoons is high and the fauna includes very high densities of N. vectensis.
The ‘Dorn’ is a silled lagoon on the eastern side of Strangford Lough in Northern Ireland. The Dorn, from the Gaelic word for ‘narrow channel’, refers specifically to the channel which connects several exceptionally sheltered bays to the main area of the lough. Near the mouth, rock barriers or sills hold back water as the tide falls, creating saltwater rapids, unique in Ireland. In the area of the Dorn rapids, abundant growths of sea anemones, sponges and ascidians clothe the rock and boulders. Several of the animals found in the area of the rapids normally occur in relatively deep water. These include the featherstar Antedon bifida, purple sun-star Solaster endeca, sting winkle Ocenebra erinacea, king scallop Pecten maximus and light-bulb sea-squirt Clavelina lepadiformis. The main trough of the Dorn supports a dense forest of sugar kelp Laminaria saccharina and sea-oak Halidrys siliquosa. The gravelly-sand bottom has unusually dense colonies of peacock worm Sabella pavonina and sand gaper Mya arenaria, with occasional native oysters Ostrea edulis and P. maximus. The channel immediately above the sill has fast tidal streams without turbulence, enabling sponges to grow to exceptional proportions. The sheltered marine ‘ponds’ feeding the Dorn feature beds of common eelgrass Zostera marina and the green alga Codium fragile ssp. tomentosoides.
Extra-Regio, Highlands and Islands
The Vadills, at the head of Brindister Voe in Shetland, is an outstanding example of a complex lagoon system. The complex comprises a number of shallow basins and has examples of both lagoonal inlets and silled lagoons. This is an extremely sheltered, undisturbed and natural environment. There is a gradation of habitats within the system, from brackish to fully marine conditions, from still to fast-flowing water, and from soft, flocculent, peaty mud through coarse sediments including maerl Lithothamnion corallioides to bedrock and boulders. There is a correspondingly wide range of communities, with a high diversity of species. Such diversity is unusual, given the complex’s northern location and relatively small size. The site supports several unusual species and communities. These include the holothurians Leptopentacta elongata and Leptosynapta inhaerens, present in dense populations, areas of the free-living fucoid alga Ascophyllum nodosum ecad mackaii, for which this is the only known location in Shetland, and the brittlestar Ophiura affinis, which is unusual in such shallow water. There are small areas of extremely sheltered littoral sediment, which support filamentous green and brown algae, and several beds of beaked tasselweed Ruppia maritima. Marlee Loch supports a bed of eelgrass Zostera marina. Shallow rock supports sugar kelp Laminaria saccharina, whilst L. hyperborea and sea-oak Halidrys siliquosa occur in the channels, where tidal streams are faster.
SACs where this Annex I habitat is a qualifying feature, but not a primary reason for site selection
- Humber Estuary East Yorkshire and Northern Lincolnshire, Extra-Regio, Lincolnshire
- Morecambe Bay Cumbria, Extra-Regio, Lancashire
- Pembrokeshire Marine/ Sir Benfro Forol Extra-Regio, West Wales and The Valleys
- Solent Maritime Extra-Regio, Hampshire and Isle of Wight, Surrey, East and West Sussex
- South Uist Machair Highlands and Islands
- Sullom Voe Highlands and Islands
- The Wash and North Norfolk Coast East Anglia, Lincolnshire
Many designated sites are on private land: the listing of a site in these pages does not imply any right of public access.