Coastal saline lagoons
What are they?
Saline lagoons are bodies of saline water, either natural or artificial, that are partially (but not completely) separated from the adjacent sea.
Typically, they lie adjacent to the sea and are separated from it by a shingle or sandy beach or sand dune system through which sea water either percolates or infiltrates through a narrow channel; these are known as 'true' lagoons.
Other types of lagoon may be created as ponded waters in depressions on soft sedimentary shores or in depressions separated from the sea by a rocky shore or artificial construction, such as a sea wall. Old coastal gravel pits frequently become brackish through sea water percolation (often via drainage ditches) and these also qualify as saline lagoons. In the wider sense, brackish drainage ditches should also be regarded as linear saline lagoons.
Sea water exchange in saline lagoons may occur through a natural or artificial channel or by percolation either through or over the sand/shingle barrier. Saline lagoons retain a proportion of their sea water at low tide and, depending on the extent of sea water infiltration and the amount of fresh water input from ground and surface water sources, they may develop as brackish, fully saline or hyper-saline water bodies. Salinity levels often vary within a lagoon; for example, a lagoon may be fully saline adjacent to its connection with the sea but largely fresh water adjacent to a stream inflow at the other end.
Saline lagoons generally contain soft sediments, which support filamentous green and brown algae and often charophytes and tassel weeds. They are important for their invertebrate fauna, which can be divided into essentially freshwater species, brackish/marine species and lagoon specialists. In addition, the habitat is important for birds, particularly waders, wildfowl and some seabirds such as gulls and cormorants.
The situation in the South East
|Extent in England|| app. 5200 ha
|Extent in the SE region|| app. 401 ha
|Percentage UK resource in the SE||8%
|Extent covered by SSSI designation||unknown|
Rate of change
|County||1998 extent (ha)||2008 extent (ha)|
||85 (Hampire and Isle of Wight)|
|Isle of Wight||30
|| (incuded in Hampshire figure)
2008 data taken from regional or national BAP habitat inventory.Please note that some of the changes listed here are due to improvements in mapping and habitat definition.
The processes that lead to the natural development of some types of lagoons are generally inhibited by human coastal activities and it is likely that that the formation of new lagoons will not keep pace with the process of lagoon loss.
Current factors affecting this habitat type are:
- Transience. Many lagoons, particularly in England and Wales, are naturally transient; salinity regimes change as succession leads to freshwater conditions and eventually to vegetation such as fen carr. Some formerly saline sites are now freshwater. The bar-built sedimentary barriers of 'typical' coastal lagoons naturally tend to move landwards with time. Lagoons behind them will eventually be in-filled as bar sediments approach the shore.
- Pollution. In particular, nutrient enrichment leading to eutrophication can have major detrimental effects. This may result from direct inputs to the lagoon or from water supply to the lagoon.
- Artificial control of water (sea and fresh) to lagoons. This can have profound influences on the habitat.
- Land claim. Many lagoons are often seen as candidates for infilling or land claim as part of coastal development.
- Coastal defences. Some coastal defence works can prevent the movement of sediments along the shore and lead to a gradual loss of the natural coastal structures within which many coastal lagoons are located.
- Rising sea levels. The impact of coastal defences will be compounded by the effects of sea level rise. One study in 1992 estimated that about 120ha of coastal lagoons in England (10% of the existing resource in England) would be lost over the subsequent 20 years, mainly as a consequence of sea level rise.
- Lack of new habitat. Sea level rise may present opportunities for creation of new lagoon habitat where sea water inundates freshwater areas, including sites that were once coastal lagoons
Vision for Coastal Saline Lagoons
The South East Biodiversity Forum’s vision for this habitat is that there should be:
- No further loss of existing habitat
- Good management, including visitor management, on all extant sites
- No damage to site integrity from activities arising outside the sites, eg. inadequately managed public access
- Re-creation of sand dunes on appropriate sites to restore some past losses, including the linking up of fragmented sites
- Greater public appreciation of sand dunes and their specialist wildlife, including greater awareness of the impacts of human pressures, such as dog-walking, mountain-biking, dumping of waste
- Creation of alternative green space around important sand dunes under pressure for increasing new housing