Saline lagoons are natural or artificial pools of salt water, partially separated from the adjacent sea. They retain sea water at low tide which may be diluted by freshwater input, or concentrated by evaporation. Lagoonal plants and animals have
to be able to adapt to these changes. Lagoons have a restricted distribution in Europe and are relatively uncommon in
the UK. England supports four of the five lagoon types found in the UK. Those formed behind shingle ridges (percolation lagoons) are particularly rare elsewhere in Europe. Of the 177 lagoonal sites in England (covering around 1,200 ha) just over half are within existing SSSIs. By area, around 11% of these SSSI’s are in unfavourable condition.

The largest saline lagoon in England, The Fleet in Dorset (480 ha), shows many of the characteristics common to other saline lagoons. It is separated from the sea by a shingle bank through which some seawater is exchanged by percolation. Most seawater exchange takes place through a narrow channel at the eastern end of the lagoon. Here salinity is close to that of seawater and reduces to brackish conditions towards the western end, where freshwater influence is greatest. Daily, seasonal and spatial changes in salinity are tough to cope with, often leading to low numbers of species, but those that survive include specialists found nowhere else but saline lagoons35. The latter include a variety of priority UK BAP, Red Data Book and protected species such as several stoneworts (species of aquatic plants), the lagoon sand shrimp, and the starlet sea anemone. Because these species rely on lagoons, their survival is threatened by habitat loss. Lagoons are an important habitat for certain birds, particularly the avocet.

Many saline lagoons in England, particularly those associated with mobile sediments, may be regarded as short-lived (on a geological timescale). The natural process of loss and re-establishment is an important feature in maintaining a mosaic of coastal habitats, but makes it difficult for saline lagoon plants and animals to move from one lagoon to another. Animals and plants can disperse through seawater, but shingle barriers often have a filtering effect on larvae. Alternatively they may be transferred by, for example, birds moving between lagoons. How dispersal takes place is still something of mystery, but chance events may well account for the variation in the communities present in apparently similar lagoons.

The existence and quality of saline lagoons are threatened by a number of coastal activities, not least the interference with coastal processes that  have moved, destroyed and recreated saline lagoons over time. Some coastal defence works prevent sediment movement, leading to a gradual loss of coastal structures in which lagoons occur, or interfere with crucial water exchange. Conversely, some lagoons have been formed, or protected, by coastal flood defence activities. Sea level rise, although likely to lead to a loss of saline lagoons, may also provide opportunities for creating new lagoons where the sea inundates freshwater areas or low-lying land. It is important to ensure that such opportunities are promoted in shoreline management planning. Communities of plants and animals in lagoons depend on a delicate balance of seawater and freshwater supply. If these are disrupted, the water quality in the lagoon changes and the characteristic plants and animals will not survive. Water quality is another key aspect. Because of their restricted water exchange lagoons are particularly susceptible to build up of nutrients, or other chemical pollutants. Lagoons are often naturally rich in nutrients, since their limited connection with the open sea encourages the retention of dissolved nutrients.

However, relatively low inputs of more nutrients from surrounding land, or the adjacent sea, can cause eutrophication (nutrient enrichment leading to biological change, often undesirable, such as increased growth of algae). This can reduce the light availability to submerged seagrasses and stoneworts, and increased phytoplankton blooms which can produce toxins, or result in oxygen depletion when they decay. When nutrients enter a lagoon or estuarine system, they can be stored in the sediment. Then starts a cycle of algal growth, based on nutrients from the sediments, algal death, and decomposition putting the nutrients back into the sediments. In such cases the impacts of poor water quality can continue for some time, or become self-perpetuating. Once a certain level of nutrient is reached, detrimental impacts can continue without any further increase in nutrient inputs. The box below gives an example from Australia; concern has been expressed that a similar process may be occurring in The Fleet. It is important, therefore, to identify water quality impacts within lagoons as early as possible, and take a precautionary approach to interpreting and acting on suspected impacts.