Definition

Distribution and conservation importance

Saline lagoons are defined as areas of shallow, coastal saline water, wholly or partially separated from the sea by sandbanks, shingle or, less frequently, rocks (Brown et al 1997). Five main sub- types have been identified in the UK, on the basis of their physiography, as meeting the definition of the habitat type, and these sub-types have been used in identifying and designating statutory sites including SACs (Brown et al 1997) and SSSIs (JNCC 1996):

Isolated lagoons: these are separated completely from the sea by a barrier of rock or sediment. Seawater enters by limited ground water 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. Isolated lagoons may have less water exchange than percolation lagoons and consequently a more impoverished biota.

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 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 and frequent 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 >obs=). Muddy areas are dominated by filamentous green algae, amongst which may be colonies or rare charophytes, such as foxtail stonewort Lamprothamnium papulosum. Beds of tasselweeds, Ruppia spp. and, in the deeper, most stable lagoon, eelgrass Zostera marina may be present.

Sluiced lagoons: sluiced lagoons develop where the natural movement of water between the lagoon and the sea is modified by human mechanical interference such as the construction of culverts under a road or valved sluices. Communities present in sluiced lagoons vary according to the substrate type 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 fresh water. 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 (due to dilution of seawater by fresh water) to hypersaline (i.e. more salty than sea water as a result of evaporation). Lagoons can contain a variety of substrata but most commonly include soft sediments. The plant and animal communities of lagoons vary according to the physical characteristics and salinity regime of the lagoon, and therefore there are significant differences between sites. Although a limited range of species may be present compared with other marine habitats, many of these species are specifically adapted to conditions of varying salinity and some are found only in lagoonal habitats. The vegetation may include beds of eelgrasses Zostera spp., tasselweeds Ruppia spp, the alga Chaetomorpha, 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 attached green algae are also found. The fauna is often characterised by mysid shrimps and other small crustaceans, worms which burrow into the sediment, prosobranch and 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 Lamprothamnium papulosum. Lagoons may also provide important habitat for waterfowl, marshland birds and seabirds.

Lagoons are localised in Europe and on the Atlantic coast have a restricted distribution. They are listed as a priority Annex I habitat in the Habitats Directive. The habitat type is complex, and a wide range of physical types and origins can be included in the broad definition. The habitat is also relatively uncommon in the UK (and consequently is listed as a priority habitat under the UK's Biodiversity Action Plan (UK Biodiversity Group 1999)). However, in the UK there is a range of geographical and ecological variation in the habitat type, and some of the types of lagoon found in the UK are rare elsewhere in Europe. Therefore, a high proportion of the sites identified as meeting the definition of the habitat type have been selected as Special Areas of Conservation (SACs). Only sites on natural substrata have been selected as SACs. Sites that are entirely artificial in origin, e.g. some docks, have been excluded from the definition, even though, in some cases the communities present may be similar to those of more natural sites. Ten sites (incorporating over 60 individual lagoon sites) have been identified as SACs for lagoons in the UK (see linked figure and the table below). As a priority habitat there are likely to be numerous lagoon SACs in other EU Member States but no information was available at the time of writing on sites selected.

Location of candidate Special Areas of Conservation for marine interests.

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